GAS FLOW SILENCER

Abstract: A device (a silencer) for silencing a flow of gas, such as an exhaust gas, comprises at least one acoustic chamber (1, 2, 3) through-flowed by gas, at least one inlet pipe (6) for leading gas into the device and at least one pipe or passage (4, 12, 13) interconnecting two chambers or a chamber and an exterior environment or chamber and is designed with such features, including such cross-sectional area transitions between pipes or passages (4, 6, 12, 13) and the chambers (1, 2, 3) that the sound attenuation achieved by the device is high while the pressure drop across the silencer is low and that high attenuation at low characteristic frequencies of flow systems comprising the device are obtained. One or more diffusers (10a, 12a, 13a) for diffusing at least part of the gas flow, e.g., at the inlet to the chambers may be comprised in the device. One or more monolithic bodies or catalysers may be comprised in the device. Devices with curved or helical passages (Fig. 5) allowing a low natural frequency and embodiments having resonance chambers (Fig. 10a, b) attenuating at selected frequencies are disclosed. The device may be used for vehicles, including ships or boats and for stationary installations, such as power plants or stationary engines. A method for designing and/or dimensioning the device for a given desired sound attenuation over a given frequency spectrum at given spatial restrictions is disclosed.

Description: GAS FLOW SILENCER The present invention relates to a method for designing and/or dimensioning a device for silencing a flow of gas such as exhaust gasses originating from a combustion device, a method for silencing such a flow and a number of devices for silencing such gasses, a vehicle comprising one or more such devices and a stationary power generating installation comprising one or more such devices. While a number of silencer designs are known, most of these are not particularly beneficial with respect to flow dynamic properties. As a result of intensive studies of the flow dynamic behaviour ana requirements of silencer systems, the invention provides both basic physical principles to be complied with by silencer designs m order to obtain hitherto unattainable combinations of effective noise damping, low back pressure (pressure drop across the silencer device) and small size -and specific novel mechanical design features, such as physical conformations of the passages or bodies involved m the flow path - which cooperate with a suitable overall design to provide superior combinations of performance results. In addition, the invention provides an efficient computerized or computer-aided method for designing and producing silencers, which method is suitable for arriving at suitable, flow- dynamically balanced or optimized silencer designs with design efforts and design time consumptions that are realistic and competitive (with even the most powerful presently available supercomputer systems, the solution of the discretized Euler equations and, in case of viscous computations, the discretized Navier-Stokes equations which are the governing equations for the flow in silencer systems is not realistically possible within an acceptable time span, in particular not for unsteady, turbulent flows. In particular m case of such flows, solutions of the above-mentioned equations requires and unacceptable number of floating operations, not possible within acceptable time spans; according to the invention, it has been found that a rational utilization of a few relatively simple, but carefully selected algebraic equations, combined with empirically determined data, will provide a guide to near optimal or optimal solutions with minimum computer requirements which are met even by normal commercial personal computers. While it is contemplated that the method will to a large extent be used to design and produce silencer systems of the novel types mentioned above, it also has a very important utility as a method for a rational and economical design and production of silencer systems of types known per se . It is well known withm the art to silence such a flow by directing the flow into an inlet passage to a container, through one or more chambers in said container intercommunicating by means of passages, through a diffuser associated with one of said passages and into an outlet passage from said container. The design and/or dimensioning of such known devices has been based on experience, empirical iterations, partial application of acoustic theory, and traditional solutions. This method of design and/or dimensioning has traditionally resulted m attenuation of the sound intensity of the exhaust flow to a degree that has been acceptable m the past. However, this known method has not consistently been able to provide sound intensity attenuation in general, and m particular for special applications, that comply with the increasing low acceptance of noise m modern society. For applications m the mass production industry of for instance gasoline and diesel engines, the great number of modifications and design changes based on experiments and empirical methods allowed by the economical resources available therefor has resulted in some relatively acceptable silencing devices. However, as the method is to a great extent based on trial and error, it has not been possible to consistently translate the success in one case to a general principle for achieving success in apparently similar cases, not to speak of rather different cases. In the case of tailor-made solutions for one-off installations or very small production series, application of the traditional method has not been able to provide optimal solutions except m exceptional cases where the element of luck has been a factor. This is owing to the fact that the economical and practical possibilities for carrying out experiments and consequent design and/or dimensioning modifications and changes are not at hand. Furthermore, the large number of parameters and considerations having implications for the sound attenuation m a silencing device have m the past prevented those skilled m the art from designing and dimensioning such a device simply and reliably m such a manner that a desired sound attenuation with an acceptable loss of pressure through the device and acceptable overall dimension consistently were achieved. One main object of the invention is to provide a method for simply and reliably designing and/or dimensioning a device comprising certain elements and for silencing a flow of gasses originating from a flow system, e.g., a combustion device, wherein the same general mathematical expressions are applied m connection with the particular given parameters regarding at least the space constraints and the desired attenuation of noise sources and, in many cases also the acceptable pressure loss across the device, the sound spectra to be attenuated, and the flow system. The method of the invention is typically applied to a device of the type comprising one or more passages leading the flow into and/or out of one or more chambers of the device and one or more diffusers diffusing at least a part of the gas flow through one or more of the passages, the geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned mainly on the basis of the number of changes m the cross sectional area of the gas flow, the values of the individual changes m cross sectional area, the volume of each of the one or more chambers and the length of each o said one or more passages. Hereby, a consistent compliance with the desired attenuation of the sound spectra has been achieved for the said given particular parameters while the overall dimensions of the device are minimized. One aspect of the invention relates to a device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising: a casing, - at least one acoustic chamber contained in the casing, said chamber being through-flowed by gas, at least one inlet pipe for leading gas into one of said at least one acoustic chamber, at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber, optionally one or more monolithic bodies comprised m each of one or more of said at least one acoustic chamber, said device showing at least two through-flowed transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area a L and a relatively higher cross-sectional area A the device fulfilling the following criteria: I) the average sound attenuation ΔdB conferred by each transition of cross-sectional area, approximated by the following expression: A. n ,_, a. n being the total number of transitions of cross -sectional area of the device, A being the relatively higher cross- sectional area at the i ' h transition of cross -sectional area of the gas flow, a 1 being the relatively lower cross- sectional area at the 1 ' th change of cross-sectional area of the gas flow, k being an empirically determined constant of the value 6.25 dB, is at least 2.9 dB when the device comprises no more than two acoustic chambers, 1.8 dB when the device comprises three acoustic chambers, - 1.1 dB when the device comprises 4 or more acoustic chambers, (n) the pressure loss over each acoustic chamber expressed as the dimensionless parameter ζ 3 ' , defined as the ratio between the static pressure loss over the chamber and the dynamic pressure at a location m said passage: _ 4P, ζ, :Pu Δp- j being the static pressure loss over the ]'th chamber, exclusive of the static pressure loss over a monolith optionally comprised in the j ' h chamber, p being the density of the gas at said location, u being a velocity of the gas at said location, preferably the mean gas velocity, is at the most 1.5. Another aspect of the invention relates to a device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising: a casing, - at least one acoustic chamber contained in the casing, said chamber being through-flowed by gas, at least one inlet pipe for leading gas into one of said at least one acoustic chamber, at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber, optionally one or more monolithic bodies comprised in each of one or more of said at least one acoustic chamber, said device showing at least two transitions of cross-sectional area for the flow of the gas between a relatively lower cross- sectional area a! and a relatively higher cross-sectional area ,, at least one passage selected from said at least one passage being curved, wherein the following applies to at least one selected chamber selected from said at least one acoustic chamber: the mean cross-sectional area A of each of said selected chamber is at least four times the largest of: - the sum of all cross-sectional areas of passages leading gas to the selected chamber, ai, and the sum of all cross-sectionial areas of passages leading gas from the selected chamber, a_, the mean cross-sectional area, A 1 , being defined as the mean value of all cross-sectional areas along a mean trajectory for sound waves travelling from across the selected chamber, the volume of the at least one selected chamber, V 1 , is at least 8^( , + a 2 ) 12 j , a and a being defined as stated above, the cross-sectional area, A , and the volume, V-, , including any elements belonging to sound absorptive material inside the selected chamber and any other parts being in acoustic communication with the selected chamber. A further aspect of the invention relates to a device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising: - a casing, at least two acoustic chambers contained in the casing, said chambers being through-flowed by gas, - at least one inlet pipe for leading gas into one of said at least two acoustic chambers, at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least two acoustic chambers to another of the at least two acoustic chambers or to an exterior environment or an exterior chamber, optionally one or more monolithic bodies comprised in each of one or more of said at least two acoustic chambers, said device showing at least two through-flowed transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area a λ and a relatively higher cross-sectional area A x , said inlet pipe being continued by a flow deflecting element causing the gas to flow with a radial component and for causing pressure recovery to the flow upstream of a first one of said at least two acoustic chambers, wherem said passage comprises at least one diffuser for leading the gas flow into a second one of said at least two acoustic chambers, the diffuser of said passage being of a different type than a radial diffuser. Sound absorptive material is preferably provided withm at least one of the at least one acoustic chamber. Sound absorptive material may be provided withm each of the at least one acoustic chamber. Preferably, the local natural frequency, f e , of at least one system comprising the gas of two consecutive acoustic chambers AC-, and AC-,+ι and the gas of the passage interconnecting said two acoustic chambers, approximated by the following expression: V and V\ + 1 being the volumes of the chambers AC-, and AC 1 + ι respectively (the volume V\«ι being set to infinite when the chamber AC : is connected to an exterior environment or an exterior chamber in a downstream direction) , a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L being the length of the passage, and c being the local sound velocity, is at the most 0.75 times a characteristic frequency of the flow system. For many applications of the device, it is preferred that the local natural frequency, f e , is at the most 0.5 times the characteristic frequency of the flow system, such as 0. times or 0.3 times or even 0.25 times, such as 0,2 times, 0.15 or 0.1 or even lower. Preferably the value ζ-, ' of each acoustic chamber is at the most 1.0. The value of ζ 1 ' of at least one acoustic chamber is preferably at the most 0.75, or even lower such as 0.5, 0.25, 0.2 or even lower. Special designs of the device allow for a value less than or equal to 0, as will be discussed below. The average sound attenuation AdB conferred by the transitions of cross-sectional area, approximated by the expression given under (I) m claim 1 is preferably at least 3.7 dB when the device comprises no more than two acoustic chambers, 2.9 dB when the device comprises three acoustic chambers, 1.9 dB when the device comprises 4 or more acoustic chambers, or even at least: 5.6 dB when the device comprises no more than two acoustic chambers, 4.8 dB when the device comprises three acoustic chambers, 3.7 dB when the device comprises 4 or more acoustic chambers . In the device according to the invention curvatures, preventing flow separation, are preferably applied to at least part of the contour of the outlet and/or inlet of a pipe or passage of the device, said pipe or passage being the inlet pipe or its extension mto the apparatus and/or the outlet pipe or its extension into the apparatus and/or a passage connecting two chambers. Thereby vena contracta phenomena may be eliminated or at least reduced, thereby reducing the pressure loss over the device . A device according to the invention may comprise one or more radial diffusers and/or one or more axial diffusers and/or one or more circular conical diffusers and/or one or more annular diffusers and/or a plurality of conical diffusers arranged on a substantially cylindrical surface and/or one or more diffusers for reversing the direction of flow and/or one or more double diversion diffusers, at least some of the above-mentioned diffuser types being known per se from the cited prior art. Any other diffuser types known per se may be applied. Each of the at least one acoustic chamber may be substantially cylindrical, and one or more outlets from said at least one diffuser may be located substantially at the axial centre of the chamber associated with said diffuser. When the chamber is substantially cylindrical it defines a cylinder axis. Preferably, one or more outlets from said least one diffuser are located at a distance from the cylinder axis of approximately two thirds of the radius of the acoustic chamber, so as to obtain fixation of pressure nodes. This principle, known per se, is described in detail in European patent 0 683 849. In a device comprising m acoustic chambers, m being at least 2, the following preferably applies to each of at least m-1 of the acoustic chambers: - the inlet passage leading gas to the chamber is provided with a diffuser or a diffuser element, a curvature or curvatures, preventing flow separation, are applied to at least part of the contour of all passages leading gas out of the chamber, the pressure loss across the chamber expressed as ζ-, ' is less than or equal to 1.5, - the approximated average sound attenuation AdB conferred by each transition of cross-sectional area is at least: 2.9 dB for m=2, 1.8 dB for m=3, 1.1 dB for m>3, and wherein the following applies to each of at least m-2 of the passages interconnecting two consecutive acoustic chambers or interconnecting an acoustic chamber and the exterior environment or chamber: the approximated local natural frequency, f e , is at the most 0.75 times a characteristic frequency of the flow system. The following may apply to each of at least m-1 of the acoustic chambers : - ' the pressure loss across the chamber expressed as ζ-, ' is less than or equal to 1.0, the approximated average sound attenuation AdB conferred by each transition of cross-sectional area is at least: 3.7 dB for m=2, 2.9 dB for m=3, - 1.8 dB for m>3. For some applications the following may apply to each of at least m-1 of the acoustic chambers: - the pressure loss across the chamber expressed as ζ i ' is less than or equal to 1.0, the approximated average sound attenuation AdB conferred by each transition of cross-sectional area is at least: - 5.6 dB for m=2, 4.8 dB for m=3, 3.7 dB for m>3, and wherein the following applies to each of at least m-2 of the passages interconnecting two consecutive acoustic chambers or interconnecting an acoustic chamber and the exterior environment or chamber: the approximated local natural frequency, f e , is at the most 0.5 times a characteristic frequency of the flow system. It should be understood that the above performance criteria may be fulfilled by all chambers and all passages of the device by a device accordmg to the invention. The sound level of self-generated noise of each one of said at least one acoustic chamber at maximum gas flow rate is preferably less than 5 dB (A) higher than the self-generated noise of a circular cylindrical reference chamber through- flowed at said gas flow rate, the cross-sectional area of the inlet passage leading gas into said acoustic chamber being a r the cross-sectional area of the passage leading gas from said acoustic chamber being a 2 , said reference chamber: being of the same volume as each of said at least one acoustic chamber, having a length equal to its diameter, having flat end caps, being provided with centrally positioned holes m its flat end caps, having a first end cap which is connected to a cylindrical inlet pipe of a cross-sectional area which is approximately equal to ai, the terminating surface of said cylindrical inlet pipe being aligned with said first end cap, having a second end cap which is connected to a cylindrical outlet pipe of a cross-sectional area which is approximately equal to a 2 , said cylindrical outlet pipe having a rounded inner edge at its interconnection with said second end cap and being aligned with said second end cap. The sound level of self-generated noise of each one of said at least one acoustic chamber at maximum gas flow rate may be less than 4 dB (A) higher than the self-generated noise of the reference chamber, or even less than 3 dB (A) , such as 2 dB (A) or 1 dB (A) . A sound level of self-generated noise which is less than the self-generated noise of the reference chamber may even be achieved with a device according to the mvention. In a device according to the invention, the distance between an inlet to a chamber and the inlet to a passage is preferably so large that substantially no unstable flow occurs m the chamber . According to the invention, the generatrix of at least part of at least one curved passage selected from said at least one passage may be wound m a peripheral direction, at least part of the curved passage having a plane spiral form. The generatrix of at least a part of at least one curved passage selected from said at least one passage may be wound in a peripheral direction, said part of said curved passage extending m a longitudinal direction, so as to form a screwlike helical form. Thereby, all three dimensions of space are utilized in order to achieve a relatively long passage or relatively long passages. By winding the connecting passage, i.e. by utilizing the third spatial dimension, the length of the passage may be significantly increased, the natural frequency of the silencing device thus being decreased, cf . equation (1) . The flow of the passage may constitute a flow cross sectional area increase in the flow direction. Thus, a diffusing effect may be obtained for static pressure recovery. The cross sectional area increase may be two- or three-dimensional. The passage may have any cross sectional shape, such as rectangular, circular, ellipsoidal or any other shape. The curved part of the passage may extend radially over an angle between 0° and 90°, or over an angle between 90° and 180°, or over an angle between 180° and 270°, or over an angle between 270° and 360°, or over an angle between 360° and 720°, or over an angle of 720° or more. The device according to the invention may comprise at least two acoustic chambers, wherein the curved passage interconnects two chambers, a first of which surrounds a second one, the second chamber thus being 'embedded' in the first one. The generatrix of the curved part of the passage may extends along a surface of revolution, so at define itself a surface of revolution. The surface of revolution may have any shape, e.g., conical . At least one monolithic body or a monolith such as a catalyser or a particle filter, may be positioned upstream or downstream of an inlet passage and/or an outlet passage of the one or more chambers, in some embodiments the monolithic body may be positioned substantially immediately upstream or substantially immediately downstream of said inlet passage and/or said outlet passage of the one or more chambers. The monolithic body may be of an annular form. In the present context, the term "monolithic body" or "monolith" designates, as is customary in the art, a body of an overall or macroscopic monolithic appearance, often a cylindrical body, which has a structure allowing an overall axial gas flow through the body. The term "monolithic" does not rule out that the body could be made from a plurality of segments joined or arranged together. The structure allowing an overall axial gas flow through the body will depend on the construction and material of the monolith; two typical relevant monolith types are: a m noiith made from a corrugated foil wound up cylmdri- cally so that the corrugations provide axial gas flow channels, and a monolith made of a particulate ceramic material, e.g., silicon carbide particles sintered together, and having a honeycomb structure comprising axial channels constituted by a plurality of coextendmg throughgomg passages separated by common passage walls, the passages being closed at the inlet and the outlet end, alternately, Thus, m a filter body of this kind, the gas travels mto the passages open at the inlet side, through the walls mto the passages open at the outlet side and then out of the filter body. Monoliths are sometimes inserted mto silencers so as to combine silencing with gas purification, either m catalytic processes, mechanical filtering, or m both. In most cases such monoliths are placed withm one or more chambers of the silencer. Monoliths can provide significant silencing at medium and high frequencies, but less silencing at low frequencies. Obviously, monoliths cause added pressure drop to the piping system. In case the purification relies solely on catalysis, the monolith is usually made as a honeycomb structure with straight channels, termed a through-flow monolith. The walls are thin, so that the open frontal area is typically 70-90%, depending mainly on the material (ceramic, metal, etc.) . Alternatively, a monolith may be made as a wall-flow monolith, i.e. the channels are perforated and partly blocked, so that the gas flow is forced to pass through those perforations, describing a tortuous pathway through the monolith. Such a monolith is used either for pure filtering or for combined filtering and catalytic treatment of the gas. Sometimes the open frontal area becomes less than 70%. Wall-flow monoliths cause pressure drops which is substantially higher than pressure drops of through-flow monoliths. The silencing effects of monoliths can roughly be described as follows : 1. The flow withm the thin channels causes high viscous friction which dampens mainly medium and high frequencies. 2. The porosities of the channels provide an aggregate acoustic volume which adds to the volume of the chamber which the monolith is placed. 3. At the entrance and at the exit of the monolith there is an effective change cross-sectional area which causes sound reflection, the same way as occurs at flow entrances and flow exits connecting silencer chambers to passages. However, the relative change m cross sectional area is normally much smaller in the case of monoliths, in particular in connection with through-flow monoliths. Since monoliths are usually fixed to the casing by an annular ring element, the effective change cross section usually is somewhat bigger than what is given by the frontal area percentage of the monolith as such. This percentage is referred to as the effective frontal area percentage. On the basis of this understanding, monoliths are handled in the following way as elements of silencers designed and dimensioned according to the invention: when the effective frontal area percentage of the monolith placed in a chamber is bigger than around 50%, the porosity of the monolith is regarded as an extension to the chamber volume, when the effective frontal area percentage of the monolith is smaller than around 50%, the monolith is treated as a connecting passage with an effective cross section area roughly equaling the sum of cross sectional areas of all channels within the monolith, - the pressure drop across the monolith is added to the pressure drop of the silencer without the monolith, i.e. a silencer having the same dimensions and geometry, but without the monolith. This means that when dimensioning a silencer for a given total sound attenuation, SDB, and for a given pressure drop, SDP, the pressure drop across the monolith should be subtracted from SDP to create the residual pressure drop at disposal for silencer design. For given flow velocities and gas temperatures, approximate pressure drops across monoliths can be calculated on the basis of formula and experimentally based constants given in literature. More precise predictions can be made on the basis of data provided by manufacturers, or in rather simple laboratory experiments. A number of interesting embodiments of the aspects of the device, as well as a flow system incorporating the device and embodiments of the method for designing or dimensioning the device appear from claims 63-107. It should be understood that steps (m)-(vιι) of the iterative procedure outlined in claim 96 and/or 98 can be performed m any desired order. At least one pipe or passage may be annular, constituted by an inner cylinder and by an outer cylinder. The annular pipes or passages may be provided with means, such as e.g. walls, for segmentatmg the annular passage into a number of sub-passages having a rectangular or circular cross sectional outline or any other cross sectional outline. Thereby, rotating stall phenomena may be eliminated or at least reduced. At least one of the at least one pipe or passage which is annular may be a passage connecting two chambers. The annular passage may diffuse at least part of the gas flow directed therethrough. The at least one pipe or passage bemg annular may thus constitute a flow cross sectional area increase m the flow direction. By applying an annular passage constituting a cross sectional area mcrease it is possible to achieve a relatively large cross sectional area increase over a relatively short longitudinal distance while avoiding flow separation in the passage. Thus, a relatively large pressure recovery may be achieved over a relatively short distance which is important, e.g., for applications where the available space is limited, e.g., m vehicles such as trucks. The annular passage may comprise a constant flow area part and an outlet diffuser part. The constant flow area part contributes to the length of the connecting passage. The inner cylinder may extend mto said first chamber by a cylinder of substantially the same diameter as said inner cylinder, and said outer cylinder may be connected to a flow- guid g body with a curvature, so as to obtain optimal flow conditions through the device. Sound absorptive material is preferably contained withm said cylinder and/or withm a continuation cylinder extending mto said first chamber and/or withm a continuation cylinder extending mto said second chamber. Obviously, one aim of providing sound absorptive material is to reduce the sound level of the gas flow. Though in preferred embodiments, the sound absorptive material is comprised withm said cylinders, it may additionally/alternatively be comprised at the outer periphery of the surrounding casing. Preferably, at least some of the sound absorptive material communicates with the gas flow, e.g., through a perforated wall. Thus, at least part of the continuation cylinder may be perforated. It is preferred to apply said cylinders which at least partly separate the sound absorptive material from the gas flow in order to avoid that the sound absorptive material is being damaged by the gas flow. At locations of cross-sectional increase or decrease or in the vicinity of such locations, the walls are preferably non- perforated so as to avoid damaging of the sound absorptive material and/or so as to avoid un-desired flow perturbations which may increase pressure loss or generate turbulence. In a preferred embodiment, the outflow from said connecting passage passes mto an annular passage inside said second chamber, said annular passage being made up of at least a perforated portion of an inner cylinder and an outer, perforated cylinder, both said cylinders separating sound absorptive materials from gas flow withm said second chamber. The outflow from the connecting passage may pass directly mto an annular passage. In order to obtain optimal flow conditions m the device, unstable flow conditions m the devices according to the invention should be avoided. Thus, for example, the distance between the inlet to the first chamber and the inlet to the annular passage should be so large that essentially no unstable flow occurs m the first chamber. With the aim of preventing unstable flow in the first chamber and/or allowing for a rather long passage, the distance may be at least 2% larger than the distance below which unstable flow would occur. Preferably, said distance should be at least 5% larger than the distance below which unstable flow would occur, normally at least 10% larger. When the total length of the device is limited, it is not desired that the distance is more than 50% larger than the distance below which unstable flow would occur, however for some applications the distance may exceed said 50%. The device according to the invention is adapted for being mounted to a flow system, e.g., the exhaust system of a vehicle comprising an internal displacement engine and/or a turbo machine or they may suited for bemg mounted to the exhaust system of a stationary power generating installation comprising an internal displacement engine and/or a turbo machine. The above mentioned vehicle may be any vehicle, such as e.g., a diesel engine powered truck, a bus, a car or a railway locomotive, a petrol, a gasoline or a gas engine powered truck, bus, car or any other moveable engine driven device. The vehicle may also be any ship or boat having a combustion device. The stationary power generating installation may be a power station having one or more gas turbines driven by flow originating from suitable combustion means, such as, e.g., one or more boiler, fuel engines or other combustion means. One major benefit of a device according to the invention is a significant reduction of pressure loss over the device compared to known devices. The reduction of pressure loss over the device reduces the fuel consumption of the combustion device and increases the power generated by the combustion device at a given fuel consumption. The pressure drop may be expressed as the dimensionless parameter ζ being defined as the ratio between the pressure loss over the device and the dynamic pressure at an appropriate location the device or adjacent to the device, i.e: where: Δp is the pressure drop over the device, p is the density of the gas at said location, u is a velocity of the gas at said location, preferably the mean gas velocity. An appropriate location could be, e.g., the mlet pipe, the outlet pipe, a location upstream of the mlet pipe, a location downstream of the outlet pipe, or any appropriate position inside the device where the flow velocity corresponds to the gas flow rate origination from the combustion device. As will be illustrated m the example below the invention provides a device for silencing a gas flow, the device having a substantially lower ζ-value. In most embodiments of the invention, ζ will be lower than 10. Typically, it will be between 0.5 and 4. The pressure drop across a silencer of a given type is typically roughly proportional to the number of chambers. Therefore, when analyzing pressure drops, it is expedient to focus the ζ-value per chamber, ζ , defined as: where : P! is the static pressure at a suitable location of the chamber mlet passage (s), p 2 is the static pressure at a suitable location m the chamber outlet passage (s), Ap m ιs the static pressure drop across a monolith, optionally comprised m the chamber, - , is the mean flow velocity in the outlet passage (s), the suitable location bemg, e.g., halfway between passage mlet and outlet in case of a passage connecting two chambers, substantially immediately upstream of the first chamber m case of an mlet passage to the silencer (unless when the mlet passage extends mto the silencer and shows a decrease of cross-sectional area for the gas flow; m that case p, is the static pressure taken at the most upstream position where the mean flow velocity reaches a maximum) , and substantially immediately downstream of the last chamber in case of an outlet passage of the silencer. One reason for subtracting the pressure drop across a possible monolith is that such an element normally contributes only to a small extent to low frequent sound attenuation while causing a substantial pressure drop addition to the pressure drop across the chamber. Provided the monolith is inserted in an appropriate way, the pressure drop across a silencing chamber having a monolith comprised therein may be expressed as the sum of the pressure drop across the monolith and the pressure drop across the chamber when havmg no monolith comprised therein. In a preferred embodiment of the invention, ζ x will be below 1.5. In further embodiments it will be lower than 1.0, or lower than 0.5, or even lower than 0. Negative values can be achieved when a diffuser is fitted onto the chamber mlet passage, and the flow cross-sectional area of an outlet passage is substantially larger than the flow cross-sectional area of an mlet passage, so that chamber pressure drop is rated against a rather small dynamic pressure prevailing in the outlet passage. In embodiments of the invention, λ is usually below 1.0, when flow cross-sectional areas of passages are of equal or almost equal size. The combustion device/means mentioned the present text may be an internal combustion engine, such as a diesel, petrol, gasoline or gas engine, e.g. a two or four stroke piston engine, a Wankel engine comprised, gas turbine connected to a boiler or any other appropriate combustion or energy extracting device, e.g., the combustion system of a stationary power generating installation, such as power station. The device accordmg to the invention may preferably be applied for silencing a flow of gasses originating from a combustion means, - the device being of the type comprising a firsc container with one or more compartments or chambers each havmg one or more let passages and one or more outlet passages, and at least one diffuser associated with one or more let passages and/or one or more outlet passages, - the flow of gasses being directed into said one or more let passages and out of said one or more outlet passages and at least partly through said at least one diffuser, - the method comprising applying the following expressions for designing and/or dimensioning the geometric configuration and arrangement and the relative dimensions of the one or more chambers, the one or more let passages, the one or more outlet passages and the at least one diffuser: and AdB = k dB \og- X ) a where f„ is the local natural frequency of a system of two volumes interconnected by a passage, c is the sound velocity, a is the representative cross sectional area of the passage, L is the length of the passage, V, is one volume, V 2 is the other volume, V 2 being set to infinite when said passage interconnects a chamber with an exterior environment, e.g., the atmosphere, ΔdB is the sound attenuation provided by each chamber inlet and outlet, k is a constant, and - A is the representative cross sectional area of a chamber upstream or downstream relative to the respective passage with respect to the flow direction. The invention also relates to a method for silencing a flow of gasses, the method comprising: directing the flow through a device comprising one or more passages leading the flow mto and/or out of one or more chambers of the device and - diffusing at least a part of the flow through one or more diffusers, - the geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned mainly on the basis of the number of changes m the cross sectional area of the gas flow, the values of the individual changes m cross sectional area, the volume of each of the one or more chambers and the length of each of said one or more passages. The invention further relates to a device for silencing a gas flow directed therethrough - the device comprising one or more passages for leading the flow into and/or out of one or more chambers of the device and one or more diffusers for diffusing at least a part of the gas flow through one or more of the passages, - the geometric configuration and arrangement and the relative dimensions of the one or more chambers and the one or more passages being designed and/or dimensioned mainly on the basis of the number of changes m the cross sectional area of the gas flow, the values of the individual changes m cross sectional area, the volume of each of the one or more chambers and the length of each of said one or more passages. According to a further aspect of the invention, a vehicle comprising an internal displacement engine and/or a turbo machine and a device according to the invention is provided, the device being comprised in the exhaust system of the vehicle . The invention further relates to a stationary power generating installation comprising an internal displacement engine and/or a turbo machine and a device according to the invention, the device being comprised m the exhaust system of the power generating installation. In the following, embodiments of the silencing device according to the invention and of the method according to the invention for designing and/or dimensioning a silencing device will be explained with reference to the drawings, m which Fig. 1 shows a representative graph illustrating silencer performance versus unattenuated noise source spectrum, Fig. 2 shows a representative graph illustrating an example of an mfrasound noise spectrum, shown as linear (unweighted) and subjected to the so-called A-weightmg, respectively, Fig. 3a is a diagrammatic representation in longitudinal section and cross sections along lme I-I (Fig. 3b) and II-II (Fig. 3c) of a silencer comprising elements associated with the method for designing and/or dimensioning according to the invention, Fig. 4 is a partial (lower half not shown m section) longitudinal section of a silencing device according to the invention, the design of which has been performed using the method of the mvention, Fig. 5 is a longitudinal section of a somewhat generalized silencer illustrating key variables in understanding the principle of a coil-shaped, helical or curved passage and the sometimes critical distance between a chamber mlet and a chamber outlet, Fig. 6 is a longitudinal section of an interesting embodiment of a silencing device of the invention, suitable, e.g. for a truck, Fig. 7 is a longitudinal section of another interesting embodiment of a silencing device of the invention, suitable, e.g., for a truck, Fig. 8 shows sections of an interesting embodiment of a two- chamber silencing device according to the invention with a helical connecting passage between the chambers, Fig. 9 show^ an interesting compact embodiment of a two-chamber silencing device according to the invention with a helical connecting passage between the chambers and incorporating a monolith, e.g., a catalyser, Fig. 10 shows two examples of resonators added to a mam silencer according to the invention, and Fig. 11 shows various principles of curved/helical forms and embodiments . In Fig. 1, Si shows an unattenuated noise source spectrum, e.g., after subjection to the so-called A-weightmg, whereas S2 shows the spectrum of the attenuation caused by a silencer (which is to be subtracted from Si to obtain the resulting attenuated spectrum) . According to the invention, silencers are preferably designed such a way that the lowest natural frequency, f e , for passages of the silencer, is lower than a characteristic frequency, frhar/ of the unattenuated noise source. In most cases, f chaι is the frequency at which the A- weighted spectrum has its maximum Pl, P2 being a secondary peak. A-weightmg is commonly adopted to take mto account the frequency-dependent sensitivity of the human ear. Most noise limit standards are based on dB (A) . Sometimes A-weightmg can be considered to cause exaggerated suppression of low frequencies. An important example of this occurs when the noise limit to be met is located inside a building at a distance from the noise source. In such cases sound is attenuated, both due to distance and due to insulation caused by walls, windows etc, so that the A-weightmg underrates the low frequencies (which are not particularly attenuated m this situation) . It may then be more appropriate to use dB (C) -weighting instead for the unattenuated sound source . These years increasing concern is being raised about the possible disturbance to humans caused by mfrasound, i.e. sound of lower frequencies than can be heard, roughly below 20 Hz. Although the matter is controversial, increasing medical evidence is being accumulated to support the assumption that disturbances may be real and not only imagined by overly concerned people. Drafts have been made of how to evaluate mfrasound in practice, but no commonly accepted standards have yet been agreed upon. Since the present invention is particularly efficient in providing silencers with strong low-frequent attenuation to gas flow noise, it has an interesting potential for combating mfrasound. Fig. 2 illustrates how, as a first suggestion, the simple linear, i.e. non-weighted, unattenuated spectrum SL, can be used to more realistically assess the mfrasound contribution having a maximum peak PL which is with the mfrasound frequency range. When changing over from A-weightmg (which suppresses mfrasound) , the maximum peak is seen to shift from PA, which occurs in the audible frequency range, to the peak PL occurring at the lower (mfrasound) frequency. Whatever weighting will be agreed upon for mfrasound, some characteristic frequency can be established as an input for the designing and dimensioning procedure of the invention. In general, extending the silencing range down into the mfrasound range calls for bigger chambers and larger connecting passages. As with silencers for conventional audible sound alone, the various measures of the present invention offer, also for mfrasound attenuation, a better trade-off between the degree of attenuation, the pressure drop, and the space requirement for the silencer, as compared to conventional technique . Depending on the particular circumstances, various combinations of parameters may be given as the basis of the design and dimensioning of a silencer. The following combination of data can often be considered given for a substantially cylindrical silencer for the exhaust gasses from a piston engine (see Fig. 3) : SDB (dB) = Total sound attenuation (damping) requirement . SDP (mm WG) = Total pressure loss allowable through the silencer. f ιgn (Hz) = Ignition frequency or firing frequency for the engine. øD (mm) = Desired diameter of the outer casing corresponding to an area A (mm 2 ) . ød (mm) = Diameter of pipes corresponding to areas a (mm 2 ) . Q (mVs) = Flow rate of exhaust gasses. c (m/s) = Sound velocity. The silencer 5 illustrated Fig. 3 is substantially cylindrical and comprises an mlet pipe 6 leading exhaust gasses from a piston engine (not shown) mto the silencer 5, an outer casing 7 and an outlet pipe 4 leading the silenced gasses out of the silencer 5 to the atmosphere. The interior space defined by the outer casing 7 is subdivided mto three chambers 1, 2 and 3 having axial lengths L } , L 2 and Li, respectively, by means of partitions 8 and 9. A radial diffuser 10 with outlet 10a is arranged as the outlet of an mlet passage (pipe) 11 to the first chamber 1. An axial diffuser 12 (that is, a passage of a type characteristically different from the radial diffuser 10) consisting of a series of pipes 12 having an axial length L 12 approx . equal to 0.5 times L 2 and having outlets 12a with increasing diameter m the flow direction constitutes the passage from chamber 1 to chamber 2. A radial diffuser 13 with outlet 13a is arranged as the outlet of a passage (pipe) 14 having a length L 23 approx. equal to 0.5 times L^ and leading from chamber 2 to chamber 3. Sound damping material B such as mineral wool is arranged in the chambers adjacent to the pipes 11, 12 and 13. SDB is often arrived at by means of a separate, conventional acoustical calculation based on the measured unattenuated noise at a certain distance from the outlet from the exhaust system correlated with a desired maximum noise level at another pomt in space . f c h a r, which is often identical to f ιgn m the case of a piston engine, is determined by the rpm of the engine, the number of cylinders and the type of engine process (two-stroke or four- stroke) . If the engine is coupled to the general power grid by means of a generator, the rpm will be given thereby. During start-up of such engines or in propulsion engines for ships, road vehicles and so on, the rpm is variable, and therefore the value of f lgn in such cases must be estimated suitably low based on a rough estimate or on more detailed considerations, for instance based on known acoustical-statistical calculations. In some cases m connection with V-type cylinder arrangements, a frequency around half the value of the ignition frequency may be preponderant which may motivate utilizing this frequency as a basic par meter for the dimensioning and design according to the mvention of the respective silencer according to the invention. Q and c can be calculated from the given mass flow and temperature of the exhaust gasses. SDP and øD are typically "semi-fixed" parameters. Often it is very desirable to limit each of them to a maximum value, but if SDB already is fixed then SDP and øD may not be determined freely. The smaller øD is chosen, the larger the sound attenuation will be. Therefore, it will often be a question of combining the design and dimensioning of the silencer with considerations regarding the interrelationship between silencing and øD including individually adapting the silencer structure to the geometrical constraints given by erection requirements, available space etc. A typical procedure when carrying out the method of designing and/or dimensioning according to the invention is: The number of chambers m the silencer is determined as n = I SDB/12.5 dB | For instance: n = | 40/12.5 | = 3 Hereafter, the types of intermediate pipes (passages) and of diffusers are decided. A combination of different types may be chosen for maximum repression of chamber resonances. So as to further hinder or avoid resonances, rather different chamber lengths Lι~L 3 may be chosen. The outlets 10a, 12a and 13a of the diffusers 10, 12 and 13, respectively, are located at or near the axial centre of the respective chambers and at or near 2/3 of the radius corresponding to the pressure nodes of the respective chambers. The relation A/a of the cross sectional areas may be tentatively chosen to be 10. Typically, different chamber lengths are chosen, for instance: The local natural frequency for the system: The two last chambers 2 and 3 with associated pipes 12 and 14, respectively, are dimensioned such that: as A/a = 10 so that D/d 3 = VϊfJ and therefore: wherefrom L 3 is determined. L, and L? are determined herefrom so that the total length of the silencer 5 is determmed. The other cross sectional areas of the connecting tubes 12 and 11 are determined such that the relations between the corresponding areas A and a also are approx. 10, a for the pipes 12 being the total cross sectional area of the pipes 12. Now the total sound attenuation may be calculated as the sum of all Δ dB according to equation (2) . The total pressure loss may now be calculated by using a combination of known elementary formulas and a detailed knowledge of the efficiency of different diffuser types, consideration being given to the mlet flow profiles to the diffusers, the chosen detailed geometry and so on. If one or more of the calculated values for f e , degree of silencing or pressure drop differ from the desired values then one or more of øD, ød or L are adjusted and the calculations indicated above are repeated. In cases where maximum compliance of a silencer with the given requirements is desired, the dimensioning indicated above is supplemented by an adjustment so that the "peaks" and "troughs" are adapted to the requirements. This is done by varying the dimensions (chamber lengths etc) and calculating the damping spectrum by means of impedance analysis and constants involved herein, the constants being determined by separate theoretical and empirical investigations. Referring now to Fig. 4, a practical example of designing and dimensioning a silencer for a six-cylmder, four stroke engine is illustrated. The parameters of interest are given at the top of Table I, it being furthermore given that the total sound attenuation SDB desired is 35 dB . Tabl e I 220 HP at 1800 rpm Firing frequency 90 Hz 6 cylinders in R- 4 stroke Natural form aspiration 475mm G max. allowable backpressure Exhaust gas 227 g/s 0.477793m3/s massflow Exhaust gas 470 Celcius temperature Density 0.475101 kg/m3 Sound 546.5786 m/s vel 1.031818 g/s/HP Drum diameter 250 mm Drum length 2600mm Drum 2 mm thickness Chamber # Total Length [mm] 867 867 867 867 Stud diameter 114.3 114.3 114.3 114.3 114.3 [mm] Stud thickness [mm] Velocity in 50 50 50 50 50 [m/s] dyn press in 60.6 60.6 60.6 60.6 60.6 [mmWG] Diffusor data Rl [mm] 23 23 23 R2 [mm] 108 108 108 S[mm] 25 25 25 Outer diameter 197 197 197 [mm] Area out [mm2] 14283 14283 14283 Area in [mm2] 9555 9555 9555 Area 1.49 1.49 1.49 ratio Zeta diffusor -0.23 -0.23 -0.23 Zeta 0 0.05 1.05 1.05 1.05 inlet Press drop 0 -11 49.5 49.5 63.6 151.7 [mmWG] dB 0 4.4 8.7 8.7 4.4 26.1 local frequency 90 90 45 [Hz] % F ignition 50 The length of the drum (outer casing) is ideally desired to be 2600 mm but may be slightly larger if necessary. The stud diameter minus stud thickness gives the internal diameter of the passage tubes and thereby the areas a. The drum diameter minus the drum thickness gives the area A. In the initial calculation step, Table I, the number of chambers is chosen to be three and the chamber lengths are chosen equal as are the pipe diameters (areas a) . The tail pipe (outlet from the drum) is also involved m the dimensioning, the volume of the "chamber" constituted by the atmosphere being infinite. The length of the tail pipe is initially equal to the lengths of the other passage pipes. The static pressure drop across a diffuser is negative and may be calculated as: Vj being the mean flow velocity m the mlet passage to a chamber, and η d being a diffuser efficiency of a value which is typically 0.5-0.8. In case of chambers being designed m a fluid-dynamically proper way and not being excessively long, such as, e.g., the chambers of the device of Fig. 4, the pressure drop withm the chamber is approximately zero, i.e.: p2 and p3 bemg the static pressure at the locations indicated Fig.4. From the chamber mlet to the outlet passage the static pressure drop is: 1 2 P 3 - P 4 = Q + k e )- p v 2 p 3 and p 4 being the static pressure at the locations indicated m Fig. 4, k e being a constant, the value of k e typically being 0.1 when the transition to the mlet of the passage (s) leading gas from the chamber is rounded off. Pressure drops passages of arbitrary cross section form may be calculated as: p 4 and p 5 being the static pressure at the locations indicated in Fig. 4, a being the cross-sectional area of the passage at a representative position and P being the perimeter of the passage at the same position. For a circular pipe (as Fig. 4), the denominator becomes the diameter, λ depends on pipe surface roughness and Reynolds number; a typical value is 0.02. For various types of diffusers, pipe roughnesses, etc. appropriate values of the constants can be found in handbooks of flow dynamics. The total pressure drop is calculated as 151.66 mmWG, i.e. well below the maximum back pressure of 475 mmWG allowable for the engine. The total sound attenuation is 26.13 dB, i.e. too small. The local natural frequency of the chamber system 1-2 and the chamber system 2-3 is too high as it should be much nearer half the firing frequency, i.e. 45 Hz. In Table II, the diameters of the tubes between chambers 1 and 2 and between chamber 2 and 3 are reduced by the same amount so as to increase the sound attenuation and lower the natural frequency of the two said chamber systems accordance with the expressions for Δ dB and f e . Table II Chamber 0 1 2 3 0 Total # Length 867 867 867 867 [mm] Stud diameter 114.3 114.3 70 70 114.3 [mm] Stud thickness 2 2 2 2 2 [mm] Velocity in [m/s] 50 50 139.7 139.7 50 dyn press in 60.6 60.6 472.5 472.5 60.6 [mmWG] Diffusor data Rl [mm] 23 13 13 R2 [mm] 108 64 64 S[mm] 25 15 15 Outer diameter 197 197 197 [mm] Area out [mm2 ] 14283 8779 8779 Area in 9555 3421 3421 [mm2] Area 1.49 2.57 2.57 ratio Zeta -0.23 -0.47 -0.47 diffusor Zeta 0 0.05 1.05 1.05 1.05 inlet Press drop [mmWG] 0 -11 274.7 274.7 63.6 602 dB 0 4.4 14.3 14.3 4.4 37.3 local frequency 53.9 53.9 45 [Hz] % F 50 ignition The sound attenuation is now 37.28 dB and the natural frequencies of the two chamber systems are much closer to 45 Hz and are in fact acceptable, but the pressure drop of 602.04 mmWG is too high. In Table III the diameter of the second and third pipes is increased by different values while the diameter of the first pipe leading from the inlet of the silencer into the first chamber is decreased, resulting in a reduced pressure drop. Table III Chamber # 0 Total Length 867 867 867 867 [mm] Stud diameter [mm] 114.3 88.9 88.9 76.1 114.3 Stud thickness [mm] 2 2 2 2 2 Velocity in [m/s] 50 84.4 84.4 117 50 dyn press in [mmWG] 60.6 172.5 172.5 331.7 60.6 Diffusor data Rl [mm] 17 17 14 R2 [mm] 83 83 70 S [mm] 19 19 16 Outer diameter [mm] 197 197 197 Area out [mm2] 11165 11165 9555 Area in 5661 5661 4083 [mm2] Area 1.97 1.97 2.34 ratio Zeta -0.38 -0.38 -0.44 diffusor Zeta 0 0.05 1.05 1.05 1.05 inlet Press drop [mmWG] 0 54.3 114.9 201.1 63.6 433.8 dB 1.4208 5.8 11.6 13.3 4.4 36.4 48 local frequency 69.3 58.8 45 [Hz] % F 50 ignition The sound attenuation is still high enough (36.43 dB) and the pressure drop is low enough (433.80 mm WG) , but the local frequencies of the chamber systems are too high to satisfactorily attenuate the firing frequency of the engine, i.e. too far above 45 Hz. Table IV Chamber 0 1 2 3 0 0 Total # Length 600 1225 775 969 [mm] Stud diameter 114 .3 88.9 88.9 76.1 114.3 [mm] Stud thickness 2 2 2 2 2 [mm] Velocity in [m/s] 50 84.4 84.4 117 50 dyn press in 60 .6 172.5 172.5 331.7 60.6 [mmWG] Diffusor data Rl [mm] 17 17 14 R2 [mm] 83 83 70 S[mm] 19 19 16 Outer diameter 197 197 197 [mm] Area out [mm2] 11165 11165 9555 Area in [mm2] 5661 5661 4083 Area 1.97 1.97 2.34 ratio Zeta -0.38 -0.38 -0.44 diffusor Zeta 0 0.05 1.05 1.05 1.05 inlet Press drop [mmWG] 0 54.3 114.9 201.1 63.6 433.8 dB 1.4208 5.8 11.6 13.3 4.4 36.4 48 local frequency 60.4 59.4 45 [Hz] 50 ignition In Table IV the lengths of the chambers 1-3 are changed as indicated, but as the resulting local natural frequencies of the chamber systems still are not satisfactory it is necessary to lengthen the drum by 300 mm to 2900 mm as shown in Table V. Table V Chamber 0 1 2 3 0 Total # Length 650 1400 850 884 [mm] Stud diameter 114.3 88.9 88.9 76.1 114.3 [mm] Stud thickness 2 2 2 2 2 [mm] Velocity in [m/s] 50 84.4 84.4 117 50 dyn press in 60.6 172.5 172.5 331.7 60.6 [mmWG] Diffusor data Rl [mm] 17 17 14 R2 [mm] 83 83 70 S[mm] 19 19 16 Outer diameter 197 197 197 [mm] Area out [mm2] 11165 11165 9555 Area in 5661 5661 4083 [mm2] Area 1.97 1.97 2.34 ratio Zeta -0.38 -0.38 -0.44 diffusor Zeta 0 0.05 1.05 1.05 1.05 inlet Press drop [mmWG] 0 54.3 114.9 201.163.594 433.8 63 dB 1.4208 5.8 11.6 13.3 4.4 36.4 48 local frequency 53.9 53.8 45 [Hz] % F 50 ignition Now the local frequencies are close enough to 45 Hz to give a satisfactory attenuation of the firing frequency of the engine Even though further optimization will be achievable such further improvement will be relatively small and without much practical value m the actual situation. The example given above of an embodiment of a method according to the invention for designing and dimensioning a device for silencing a flow of exhaust gasses is directed to relatively simple and uncomplicated situations where the silencing, pressure loss and space constraints are not strict. The passages between the chambers may be prolonged by several means either alone or m combination. The pipes may be prolonged backwards nto the upstream chamber and forwards mto the downstream chamber, or the pipes may be prolonged by adopting a helical design for same. If the pipes either alone or combined with a diffuser are prolonged both upstream and downstream, the outlet of the upstream pipe in a chamber may be downstream of the mlet of the downstream pipe thereby twice reversing the direction of the ma flow in said chamber. A diffuser having an umbrella-like shape with the convex surface thereof facing downstream will also have the effect of prolonging the passage and reversing the flow. Various types and shapes of baffle plates and guiding plates and bodies may be employed. The design and dimensions of the diffusers are important for optimizing the pressure recuperation thereof and thereby minimizing the pressure loss through the silencer. Each diffuser may be a radial diffuser or an axial diffuser or a circular conical diffuser or an annular diffuser or a multiplicity of conical diffusers arranged on a cylindrical surface or a diffuser for reversing the direction of flow or a double diversion diffuser or any other kind of diffuser. All mlets of the passages should be suitably rounded so as to avoid vena contracta flow with associated vortices giving rise to pressure loss and noise that may be amplified in the passage by resonance. The outlets of the diffusers should, if possible, be located at the centre of the longitudinal direction of the chamber relative to the mam flow direction and/or at the pressure node of a transverse oscillation in the chamber. Hereby, the basic resonance of the chamber in the two directions is repressed. The arrangement of any sound absorbing material in the silencer is important. Particularly in large silencers it should not be too thick or compact so as to not decrease the acoustic volume of the chambers. Further embodiments of the mvention rely on supplementing series connected acoustic filters by one or more side-branch chambers, acting as resonators, and in some cases bemg provided with built-in sound absorptive material. Silencers relying solely on resonators are per se well-known in prior art. A resonator can provide substantial attenuation around a selected center-frequency, which is determined by the dimensions of the device. However, a single resonator chamber cannot provide the broad-band attenuation achieved by devices according to the invention. In principle, many resonators with different center-frequencies can make up for this deficiency, but silencers of such a configuration become very bulky. An attractive feature of resonators is that they produce very little pressure loss, since the gas flow does not pass through them. According to the mvention, resonators are only used as supplements to achieve improved attenuation at and around selected frequencies which may need further attenuation after achieving otherwise good attenuation by adopting the series- type connection of acoustic filters according to the mam principles of the invention. As an example, an unattenuated sound spectrum of an engine exhaust may have, as previously described, a major peak at the ignition frequency, which can be efficiently dampened by the mam method of the invention. But at twice this frequency a smaller peak may also be present, which can be targeted by a side-branch connected resonator. Sometimes a combination of geometric restrictions for the silencing arrangement can be such that addition of a resonator according to the invention represents good economy with both available space and with pressure drop m the exhaust. For example, a tail pipe 3, leading gas from a silencer, may be prescribed to be positioned at a certain distance WD from a wall, as shown m Fig. 10a. A resonator 4 is here shown to be arranged as an appendix to a mam silencer 1, i.e. as a separate container connected to a last chamber 2 of the mam silencer 1 by means of a connecting pipe 5. The resonator can be an empty chamber or contain a movable piston 6, as shown m Fig. 10a. In the first case the center frequency of the resonator will be fixed. In the second case the center frequency can be selected freely withm a variation band, by moving the piston, whereby the attenuation spectrum can be optimized. This last facility can be convenient if the exact sound spectrum of the engine is not known, as often the case in practice, when engines and silencers are manufactured by different suppliers. Fig. 10b shows an alternative embodiment of the invention, m which the resonator has been arranged as a direct extension of the last chamber of the mam silencer. In this case, sound absorptive material Ba (e.g., basaltic mineral wool) has been inserted into the resonator. The theory of resonators is well-known m acoustics and will therefore not be referred here, except for some mam characteristics, as a guide for applying resonators according to the invention: When the resonator is connected to the mam silencer by means of pipe, as in Fig. 10a, the peak in the added attenuation spectrum will be more narrow and sharp, as compared to a configuration m which there is a direct connection without a 'neck', as shown in Fig. 10b. In the first case the center frequency is determmed as the so-called Helmholz frequency, well-known m acoustics. When calculating this frequency, the acoustic 'stiffness' of the last chamber of the mam silencer should be added to the acoustic stiffness of the resonator chamber. The resulting Helmholz frequency thus becomes higher than as given by the resonator itself. Acoustic wave phenomena with the resonance chamber will also influence the Helmholz frequency and should be taken into account m an accurate calculation. The center frequency of a resonator without a connecting pipe is approximately equal to the frequency corresponding to a quarter of a wavelength being equal to the length of the full chamber. According to the invention, resonators can also be built mto the main silencer as an integrated feature of a chamber. Fig. 7, discussed m greater detail below, shows and example of this. Here, cylindrical cavity 70, contained withm the inner cylinder of the annular passage, functions as a resonator, providing added attenuation with a center frequency being approximately equal to the frequency corresponding to a quarter of a wavelength being equal to the length of the cylindrical cavity. Fig. 6 shows a preferred embodiment of a device according to the invention. Here, two chambers 1 and 2, are contained withm a casing 7 and are separated by partition walls 8a and b. An mlet passage pipe 6 passes the flow to chamber 1 via a radial diffuser 10. From chamber 2 the flow is passed to the outlet passage pipe 4 via an opening provided with a curvature 22 preventing flow separation. The two chambers are interconnected by an annular passage 12. As can be seen from the figure, the combination of the radial mlet diffuser and the annular passage effectively prevents sound waves from 'short cutting' chamber 1 m passing from the mlet passage 6 to the annular passage, even though the mlet to the annular passage is positioned at a not very long distance Dl from the mlet to the first chamber. I.e., sound energy effectively fills chamber 1. The flow path from mlet to outlet of the second chamber contains less change of direction. However, due to the bigger distance D2 the tendency for sound waves to 'shortcut' the second chamber is rather small. The mlet 12a to the annular passage 12 contains several features which contribute to make the mlet smooth, preventing pressure-loss associated vena contracta phenomena: Both cylinders 42 and 44 are extended by design parts mto the first chamber, thus providing guidance for the flow accelerating mto the annular channel. The inner cylinder 42 is extended to the left by a cylinder 41, which is full immediately upstream of the mlet to the annular passage, and is otherwise perforated and contains sound absorptive material Ba . The outer cylinder 44 is extended to the left by a conical cylinder 21 bemg connected to cylinder 44 by a curvature 20. The outermost diameter of the conical cylinder is big enough to provide sufficient guidance for the flow, i.e. the flow velocity at this diameter is much smaller than the flow velocity m the annular passage. On the other hand, the distance D3 between conical cylinder 21 and the casing 7 is not unnecessesarily small, since this would tend to acoustically isolate the right- hand, annular part of chamber 1. The outlet 12d from annular diffuser 12c passes the flow into an annular passage 30 inside chamber 2, constituted by an inner, perforated cylinder 43 and an outer, likewise perforated cylinder 46. Sound absorptive materials Bd and Be are placed inside cylinder 43 and outside cylinder 46, respectively. The size D4 of passage 30 is chosen as a compromize between opposing demands: On the one hand, the smaller the size of D4 is the more efficient sound absorption is achieved. On the other hand, D4 should not become so small that too strong, turbulent noise is generated, or that too strong fluid- mechanical forces, tending to abstract absorptive material, are generated. Annular flow between sound absorbing walls is a per se known as an acoustically efficient configuration. The length L 1 of connecting passage 12 is chosen to be long enough for the local natural frequency f e to become sufficiently low, as given by the method of the invention, cf. equation (1) . When the total length of the casing is given and is comparatively short, L12 is chosen by balancing of number of demands: Distance Dl can be made rather small, say in the order of half the diameter of the casing, depending upon a number of further geometric choices, among them the size of distance D3. Shortening distance D2 will cause a somewhat deteriorated acoustic function of chamber 2, but as a gradual function of D2. In general, it is desired that the level of the noise which may be generated due to turbulence should not, at any frequency, exceed the level of the noise created by the engine. For a given length L12, the annular passage type, the flow- friendly features of a smooth and guiding let and a diffuser outlet, allow for a comparatively low natural frequency f e and rather big, effective sound-reflective flow area ratios A/a, cf . equation (2) . From fluid-dynamic diffuser theory it is known that a maximum angle α of divergence, indicated in Fig. 6, exists which, when exceeded, leads to flow separation occurring inside the diffuser, i.e. a less than optimal operation which should be avoided. This angle is not very wide, so that for a large outlet to inlet flow area ratio of the diffuser, it tends to become long. However, for given sizes of inlet and outlet cross-sectional areas, an annular diffuser allows for a shorter diffuser length than does a conical diffuser. This means that, with an embodiment as shown in Fig. 6, even though a low flow velocity is wanted at outlet 12d, and a high flow velocity is wanted in the constant flow area part 12b of passage 12, the diffuser length Lι 2c can be chosen rather short, without flow separation occurring. For a given total length L12, this in turn means that the constant flow area part length L12b can be made rather long, so as to obtain a rather long L in equation (1) and so as to thereby obtain a rather low natural frequency, f e . The acoustically effective area a of the interconnecting passage 12 is a weighted mean of all cross-sectional areas occurring from inlet 12a to outlet 12d. Therefore, it is acoustically favourable that the smallest area, i.e. the cross- sectional area of constant-area part 12b, is comparatively long. The silencer embodiment shown in Fig. 6 contains a rather simple, central body 40 which is securely and accurately fixed to the outer parts of the silencer via a number of flow- aligned arms or sheets 47, 48, and 49. For example, in radial diffuser 10 there may be four arms or sheets 49, positioned at 90 degrees angle around the periphery. The total interface area between empty chamber volumes and volumes filled with sound absorptive material is big, providing a maximum of sound absorptive effect to assist sound reduction due to reflection at changes in cross-sectional area. In the embodiment shown Fig. 6, the flow occurring m annular passage 12 is coherent all around the periphery of the annularity, apart from the small interruptions provided by the arms or sheets 47 and 48. For manufacturing reasons it may be expedient to adopt various types of peripheral segmentations of the annular passage. Thus, the arms or sheets 47 and 48 may be substituted by deformations which may be formed be pressing operations made on inner parts 42 and 43 and/or on outer parts 44 and 45 constituting the annular passage 12. In bigger silencers, or when extreme outlet and/or mlet flow cross-sectional area ratios are wanted for the annular diffuser, it may be advisable to adopt a thorough peripheral segmentation of the annular passage, to prevent occurrence of fluid-mechanical instability of the rotating stall type, well- known from turbo machinery. This can be done by inserting radial partition walls mto the annular passage. An alternative is to split the annular passage mto a multiplicity of flow- parallel pipes arranged with the centerl es of all pipes situated on a cylinder with a centerlme coinciding with the centerlme of the silencer as such. These pipes can be circular, in which case diffusing outlet parts will be conical diffusers. Many other cross-sectional forms for pipes are also possible, e.g. squared cross-sections. Fig. 7 shows another preferred embodiment of the mvention. As m Fig. 6, an annular passage 12 connects two chambers 1 and 2. In Fig. 7 the casing is more elongated, as is e.g. typical of vertical silencers on trucks, i.e. it is rather long, and the diameter is rather small. The following differences compared to the embodiment of Fig. 6 are notable: Distance D 3 has vanished, so that conical cylinder 21 provides the partition wall 8b between chambers 1 and 2, together with rounded part 20 and central parts of the silencer. Still, the annular spacing 32 is not ' wasted' as an acoustic volume of the silencer, since it is a part of chamber 2. Perforated cylinders 41 and 50 provide inner delimiters of annular spaces 31 and 30, respectively. Whereas in Fig. 6 annular space 30 is delimited by both the inner and outer perforated cylinders 43 and 46, annular spaces 30 and 31 in Fig. 7 outwardly are delimited by the casing 7. Arms or sheets 51, 52, and 53 helps fix cylinders 41 and 50, together with filled-in sound absorptive materials Ba and Bd. Whereas in Fig. 6 the outflow from annular passage 12 passes directly into passage 30, the outflow in Fig. 7 passes a short distance D5 before entering annular passage 30. Cylindrical cavity 70 acts as a built-in resonator, as explained above. Figs. 8a-8e show a preferred embodiment of the invention in which a helical passage 12 connects two chambers 1 and 2, contained within a cylindrical casing 7 and separated by an inner, flat partition wall 8. The helical passage is delimited by casing 7, by an inner cylinder 42, and by helically formed sheets 60 and 61. Helical passage 12 is subdivided into a constant flow-area part 12b and a diffuser part 12c, in which the flow area gradually widens in the gas flow direction, as given by a gradually widening distance between sheets 60 and 61. Both chambers 1 and 2 are partly filled with sound absorption material Ba, contained behind perforated plates 41 and 43. These plates have been so formed and positioned that, together with the absorptive material they help guide the flow inside chambers 1 and 2 with low pressure drop and preventing unwanted flow swirling inside the chambers. Gas flow is led to the silencer via inlet pipe 6 and conical diffuser 10, which recovers dynamic pressure and helps further to prevent unwanted swirl inside chamber 1 by lowering the mlet flow velocity to the chamber. Here, the flow generally turns 90 degrees before entering helical connecting passage 12 at 12a. Here, a cylindrical rod 21 has been fitted onto inner cylinder 42 to improve mlet flow conditions, preventing vena contracta phenomena and mlet pressure losses. Inside passage 12 the flow first passes constant flow area part 12b and then diffuser part 12c in which dynamic pressure is recovered. The flow leaves passage 12 at outlet 12d, entering chamber 2. Inside this second chamber the general flow direction turns 90 degrees, both in plane AA and plane CC, before entering outlet pipe . From flow mlet 12a to flow outlet 12d of the helical passage 12, the flow in total turns 360 degrees inside the silencer casing. Thus, the length of the passage is approximately π times the casing diameter, contributing to a very low acoustical natural frequency f e , constituted by passage 12 and chambers 1 and 2. In Fig. 8, the length of casing 7 is only slightly m excess of the diameter. The embodiment thus demonstrates how, by adopting a helical passage between chambers according to the invention, it has become possible to achieve a much lower natural frequency than with a straight passage . Examples of particularly relevant applications of the embodiment of Fig. 8 are silencers for buses of trucks where there is space for a rather big silencer volume, given by a casing of a rather big diameter but of a short length. Even though the flow turns quite a lot inside the casing, the associated pressure loss is remarkably low. In spite of the embodiment being truly three-dimensional, the essentially 1- dimensional and dimensioning method of the invention applies. Naturally, an accurate description of the fluid-flow and acoustic properties of the silencer should be three- dimensional. However, this is also the case m silencers wherem the acoustic field and the flow field is substantially two-dimensional. It should also be pointed out, that m spite of the three-dimensional flow path through the embodiment of Fig. 8, it can be manufactured by rather simple members and by simple methods like sheet pressing, rolling, welding, etc. Many types of silencers with helical flow patterns inside casing are known from prior art. However, known silencer embodiments, helical flows have been desired for reasons differing from those of the present invention. Thus, for example, very efficient sound absorption has been achieved by adopting helical channels made by perforated cylinders m contact wirh absorptive material. Another reason for adopting helical internal flow m silencers has been to achieve a spark- arresting effect by increasing the residence time for exhaust gasses inside a silencer. The helical configuration of the invention allows the engineer to select the length of connecting channel 12 very freely and optimize this length according to the method of the invention. Thus, when a very low natural frequency is desired, even substantially more than 360 degrees turning mside the passage will be beneficial to select in some cases. An example of this could be a truck application, for which it is desired to attenuate mfra-sound created by the engine when running at low speed at engine start-up or at hauling operation of the truck. Attenuation of mfra-sound is further relevant m connection with gas turbine power stations. In other cases, less than 360 degrees flow turning in the helical passage can be appropriate, e.g. with higher ignition frequencies of engines, and when it becomes essential not to create too low resonant frequencies of the helical passage. To the silencer design engineer it obvious that the goals addressed by the embodiment shown in Fig. 8 can be achieved by many variations m design configuration. As an example, the flow-widening of the diffuser 12c can be achieved by varying the diameter of the inner cylinder 42. A helical passage can be fitted into a cubic casing, instead of a cylindrical casing. The wall 8, separating chamber 1 from chamber 2, can be a cylinder, and chamber 2 can be arranged essentially outside chamber 1, which is favourable from a shell noise emission point of view, since the sound level inside chamber 1 is higher than inside the downstream chamber 2. Figs. 9a and 9b show an embodiment of the invention in which a monolith, e.g. a catalyser, is built into a two-chamber silencer with a helical passage connecting the two chambers. The first chamber 1 is a flat, cylinder-like volume at the top of the silencer. The second chamber is divided into two parts, 2a and 2b, the division being caused by annular monolith 100. Sound absorbing material, Ba, is contained within a central cylinder 42, a perforated top sheet 41, and solid bottom plate 8. The helical passage 12 is delimited by the cylindrical silencer casing 7, by the top part of cylinder 42, and by two helical sheets 60 and 61. Usually a catalytical monolith has rather thin walls, in which case it only to a very limited extent causes an acoustical subdivision of the second chamber of the silencer. Thus, sub-chamber 2a, sub-chamber 2b, and the gas part (the porosities) of monolith 100 together constitute a single silencer chamber from an acoustical point of view. This second chamber is separated from the first chamber by plate 8, by cylinder 42, and by the walls of helical passage 12, i.e. sound can only pass from chamber 1 to the second chamber via passage 12. Gas enters chamber 1 from inlet pipe 6 and diffuser 10, turns approximately 90 degrees around the vertical silencer axis before entering helical passage 12 at its inlet 12a. At this inlet a cylindrical rod, 21, has been fitted onto helical sheet 60 prevent 'vena contracta' inlet flow. Within the helical passage the flow rotates a slightly more than 360 degrees around the silencer axis, simultaneously flowing a little downwards, before leaving the passage at 12d, passing into chamber 2a. Here the flow is distributed around the annular inlet face to the monolith, at the same time changing its direction from tangential to axial flow, given by axial flow channels within the monolith. After leaving the monolith, the axial flow changes back to tangential in chamber 2b, from which the flow leaves the silencer by pipe 4. Figs. 11a - j illustrate a number of variations of helical and spiral passage forms and embodiments incorporating helical passages, being special cases of curved passages. Fig. 11a shows a cross section of a cylindrical silencer with a tailpipe which is composed of a helical or spiral part and a short, straight part. The helical /spiral part is wound around the cylindrical casing. Fig. lib shows a screw-like helical generatrix form of a passage . Fig. lie shows a plane, spiral generatrix form of a passage. Fig. lid shows a conical, screw-like generatrix form for a passage, as an example of the winding radius varying along the longitudinal direction. Fig. lie shows a winding generatrix form for a passage, with an upstream part extending in one direction, and a downstream part extending in the opposite direction. This combination can be useful for a passage in a silencer for which the inlet and outlet passages of the silencer are placed at the same end of the casing. Figs, llf and g are sketches of an embodiment of the mvention in which a first chamber is contained withm a second chamber, the generatrix of the connecting passage being a plane spiral. In the figure the cross section of the passage is rather wide. Alternatively, the passage cross section (of area a) could be much more narrow, depending on the intended cross section area ratio A/a. Fig. llh shows two parallel, screw-like helical generatrix forms, which can be adopted for a helical passage being divided mto two parallel passage parts. Fig. Hi illustrates how a curved passage form can be adopted, both for providing a prolonged passage, and for reducing the change of flow direction withm a chamber. For example, this principle can be combined with a division mto two or more parallel passages (as illustrated above), m case of an mlet passage being arranged sidewise onto a silencer casing of a rather small diameter. In this way big pressure losses and swirl withm the first chamber can be avoided. Fig. 11] shows a detail of an embodiment of the mvention being a variation of the embodiment shown in Fig. 9. In addition to helical passage outlet 12d, a secondary outlet 12d' has been added in Fig. 11] . Thus, part of the flow leaves the passage through 12d and the rest through 12d' . Both flow parts leave the passage in a tangential direction withm the annular spacing above the monolith, 100. By adding outlet 12d' , the flow distribution to the face of the monolith can be improved. Further improvement in this respect can be achieved by adding further outlets to the passage. As in the preceding embodiment, the present embodiment demonstrates how a helical passage can be used to build in a long passage connecting two chambers of a rather short, cylindrical silencer. The tangential outflow at 12d ensures a very even flow distribution to the many, parallel channels of the monolith. Even though the total flow turning within the device is substantial, the pressure loss is rather small. The device is a very compact, flow- friendly silencer in which it has become possible to build in a monolith of a significant volume . The principles of the invention allow the engineer to adopt many forms of chambers and connecting passages, retaining efficient damping of all sound frequencies, and in particular it provides the engineer with tools for obtaining sufficient damping of low frequencies, even when the available space is narrow. Exhaust system layouts may, e.g., differ substantially from one truck or bus model to another. Sometimes a silencer is required to be of a relatively small outer diameter, but may be allowed to be rather long. In other cases other requirements may exist: The silencer can be allowed to be of a relatively large outer diameter while its length is restricted. Sometimes pipes leading exhaust gas to and from a silencer are required to enter or leave at opposite ends, sometimes at the same end of the silencer, sometimes in line with the silencer, sometimes at an angle, etc. In many cases, silencers are required to be of a cylindrical design, since this form is rather easy to manufacture. But other forms may also be appropriate, e.g. elliptical, squared or conical forms. Below some general design principles are outlined, which will provide engineers with tools for retaining optimal silencer performance complying with a broad array of geometric conditions. Following this, the general principles will then be exemplified by specific design cases. There will be two points of focus for optimal design and dimensioning: Chambers and connecting passages. In the following, reference is made to Fig. 5. Here, A p is a chamber cross-section area, measured perpendicular to the mam flow trajectory m the chamber. D is the straight line distance between the mlet to the chamber and the outlet from the chamber, s is a coordinate along the generatrix of a curved passage connecting two chambers. The passage length along the generatrix is L. D d is the direct straight lme distance in space between passage let and outlet, obviously, D d is shorter than L.Lj and L 2 are the lengths of the first and second chambers, respectively. Chambers should be of a sufficient size and of an appropriate form for sound reflections to exist m an effective way at transitions of cross-sectional area and for avoiding major eddies or swirl which could otherwise cause excessive pressure drop and undesired self-generated noise. If equation (1) is inspected m isolation, one might be led to the false conclusion that even with very small chambers it is possible to achieve an arbitrarily low natural frequency, provided the passage can be made arbitrarily low. This is, however, not true. The reason is that various phenomena, not included in the simple acoustic filter model underlying equation (1) will distort the damping effect. One problem is that small chambers may reduce or eliminate sound reflections at chamber inlets and chamber outlets. Another problem is that low-frequent resonance may be set up m connecting passages which may destroy low-frequent sound damping. Too small chambers will only serve as transfer elements which lead flow from one passage to another. Devices according to the invention may include such small chambers, e.g., for the purpose of smoothly deflect flow. But at least one, and often more, chambers m silencers should be sufficiently large to serve as effective elements m acoustic filters . Accordingly, such chambers should fulfill the following two criteria: (I) the mean cross section area A of a chamber should be at least 4 times the largest of the two cross section areas ai and a 2 , respectively, of passages leading gas to and from the chamber, respectively, (II) the chamber volume V should at least be of the size: Apart from this chambers may have many alternative forms for various applications. When determining A and V sound absorptive material should be regarded as empty space. The reason is that such material, for functioning properly as an absorbent, should not be packed too solidly or otherwise arranged in such a way that sound waves cannot enter the material rather freely. Thus, although the absorptive material is usually placed behind perforated plates for protection, the degree of perforation should not be too small . In cases of complicated geometry, the cross section area A of the chamber should be interpreted as the mean value along the mean trajectory for sound waves travelling from chamber inlet to chamber outlet. Usually, this trajectory more or less coincides with the flow trajectory. As a further guidance for interpreting A and V properly, it should be understood that all chamber parts in full acoustic communication with other parts of the chamber should be included. Thus, guide plates, various other chamber internals etc. should not cause a narrow restriction of A and V to such area and volume elements, respectively, which can be 'seen' directly from the trajectory through the chamber. In addition to the above-mentioned criteria, a third criterion can be added which, as shall be exemplified further below, in silencers according to the invention, is often applied to at least one chamber: (III) the flow trajectory within a chamber should turn significantly. Typically, this can be quantified as follows: (III ' ) the total change of flow direction from chamber inlet to chamber outlet should be at least 90 degrees. Thereby sound waves are prevented from shortcutting the chamber, shortcutting being a acoustic field phenomenon which may more or less hamper the acoustic filter effect, depending upon the geometry. Significant shortcutting will occur if a chamber outlet is placed close to, and aligned with, the inlet to the chamber. However, when the distance between inlet and outlet is big, the shortcutting effect becomes small. Regarding pressure losses across a silencer, application of criterion (III) is admittedly negative. However, when the chamber is designed properly from a fluid-dynamical point of view, the added pressure loss associated with flow turning inside the chamber can be kept moderate and may be justified because of improved sound damping. Usually, more deflections or turnings are justified when silencers are applied to engines that are only moderately sensitive to backpressure. Thus, more turnings inside chambers will, e.g., be applied to silencers for reciprocating engines than for silencers applied to gas turbines . Sometimes narrow space limitations for silencers and chambers create a desire for a short a straight-line distance between chamber mlet and outlet. The detailed geometry is then to a great extent determined as a balancing between abruptness of turning (preventing acoustic shortcutting) and prevention of flow instabilities. For a given type of geometry, the distance can (m design considerations or in experiments) be shortened down to a certain threshold, below which moderate turbulence will rather quickly turn mto major flow instability, with large eddies, flow separation, etc. This leads to a further mandatory criterion for chambers: (IV) For a chamber fulfilling criterion (III) and/or (III ) and for which overall silencer design and dimensioning demands as short a straight-line distance D as possible between chamber let and outlet, D should be selected to the value below which flow instability occurs the chamber, plus a reasonable safety margin, which can normally be set m the interval of 10 - 50° o said straight- lme distance. The safety margin allows for variation due to a number of factors which are difficult to control, such as: Varying gas temperature, manufacture tolerances, or non-linear and destabilizing effects of gas pulsation. Those familiar with modern fluid dynamic science will appreciate that flow instability to some extent escapes today's detailed physical understanding. Nevertheless, criterion (IV) can be handled by the practical engineer in systematic experiments. It should be noted that insertion of a diffuser to the chamber mlet lessens the instability-driving forces inside "the chamber and therefore eases the design task. Sometimes guide plates or similar devices can be fitted mto chambers to stabilize the flow and help it to turn or deflect m an ordered way, preventing instability. Guide plates should not be of such a size or placed in such a way that they will cause acoustical isolation of certain chamber elements or division of a chamber mto two or more sub-chambers. Apart from flow instability driven by strong flow turning inside a chamber, a second type of instability may occur in poorly designed silencer chambers: Major swirl following a round contour inside a chamber. In the case of a cylindrical chamber, the entire volume may be caused to rotate (swirl) around the axis of symmetry. Like all types of flow instability, this phenomenon may cause excessive pressure drop and self-generated noise. The two mam risk factors related to this such flow instability are : the degree of roundness of the chamber, - the extent of provoking fluid-dynamic forces, mainly due to tangential in- and outlets. The roundness can be lessened by modifying the chamber form or by fitting m plates or other components mto the chamber. Sometimes appropriately formed sound absorptive elements, m the form of perforated plates protecting mineral wool, may serve the double purpose of adding sound absorption and making a cylinder less round. In some cases radially extending plates can be used for preventing swirl. It is important that such elements do not pose major obstacles to the flow passing through the chamber. However, it is normally not difficult to avoid such a restriction. The reason is that swirl of the described type generally is associated with flow directions which deviate strongly from the mam trajectory of flow from mlet to outlet inside the chamber. This will be illustrated by an example, cf. the below description. A fifth mandatory chamber design criterion can now be formulated: (V) When there is a risk of major swirl occurring m a chamber, its geometry should be modified by inserting one or more elements which w ll pose one or more restrictions to such swirl. Such a modification of the geometry should be made in a way that it does not pose great obstacles to the flow from chamber mlet to chamber outlet. Sometimes the general geometrical conditions allow the silencer designer to select the length of connecting passages rather freely. In other circumstances there is a strong impetus to find ways of making the length of a chamber-connecting passage or of a tail-pipe longer than what seems possible with known silencer geometries. An example of this is when two or more chambers are to be fitted mto a rather short silencer casing, m which case it may seem difficult to avoid a short connecting passage, which leads to a rather high natural frequency and rather poor damping of low-frequent sound. If the designer disregards pressure drops, it is not difficult for him to achieve long passages by adopting one or more sharp turnings of the passage. However, this seldom leads to good performance and is in accordance w th the present invention. Flow-friendly, long passages can be created by subdividing the passage mto two or more, parallel passages, each having smaller transverse dimensions. For example, a circular pipe may be subdivided mto several smaller-diameter pipes, whose aggregate cross-sectional areas equal the cross sectional area of the single, larger-diameter pipe. Smaller transverse dimensions allow for sharper turnings, since flow- friendlmess is very much related to the ratio between radius of curvature and transverse dimension. As a general criterion, applicable to both smgle and multiple parallel silencer passages, the following may be prescribed: (VI) When passages diverge from straight forms, causing the general flow trajectory withm the passage to bend or turn, the passage form should be such that the radius of curvature of the trajectory nowhere along the trajectory, or at least almost nowhere, is smaller than the smallest transverse size of the passage, as measured perpendicular to the trajectory tangent at the pomt of inspection. Subdivision of passages is feasible in some circumstances. Drawbacks with this route to prolonged passages are: Increased manufacturing costs and added skm friction pressure losses in the passages. A way of increasing passage length, which deviates strongly from prior silencer art, but which can be very efficient, relies on utilizing the third dimension when selecting the form of a passage. In many cases, this can be done conveniently by selecting a helical form for the passage (as will be shown m detail below), but alternative forms are also possible. The general idea is that the prolonged passage fulfills one of the following two criteria, or both: (VII) the acoustically effective length L of the passage is at least 1.5 times the direct and straight distance in space between the passage mlet and outlet, (VIII) the acoustically effective length L of the passage is at least equal to: in case the passage connects two chambers: the sum of the lengths of the two chambers, - case the passage is a tail pipe from a silencer: the length of the chamber being connected to the atmosphere by the tail-pipe. Helical and other passage forms are m addition characterized in that: (IX) the mean trajectory of the passage is not contained within a plane surface. A necessary condition for a prolonged passage to provide effective low-frequent sound damping is that the chamber, or the chambers, to which the passage is connected, fulfills at least both above-mentioned criteria (I) and (II). Thus, helical or other prolonged passage forms should not be adopted m combination with inadequate chambers, as explained above. Criteria (VII) and (VIII) are based on the concept of acoustically effective length L'. In principle, L should be measured along the mean trajectory for acoustic energy propagation withm the passage. Usually this will coincide more or less with the mean flow trajectory. In most cases prolonged passages according to the invention are made with solid walls, not allowing acoustic energy to deviate from the passage through the walls, since this would hamper the acoustic function of the passage m combmation with the one or more chambers to which it is connected. However, to a limited extent perforations or other openings m the walls of the passage can be permitted, m particular when the passage is made longer than what is necessary to provide a sufficiently low natural frequency. Thus, a long passage could for instance have perforations along part of its length, combined with sound absorptive material being placed on the other side of the perforated part. The principle of using a helical passage form can also be applied to the tail pipe, i.e. the pipe leading exhaust gas from a silencer to the exterior environment, such as the atmosphere. For examle, a helical pipe can be wound round a cylindrical casing, ending up with a short straight end section of the tail pipe. The helical pipe may also be extended backwards, mto the last chamber of the silencer, to provide a smooth outflow in tangential direction. This configuration can be used to achieve a low natural frequency of the ta l pipe - last chamber system, even when the exhaust to the exterior environment is prescribed to be positioned close to the silencer. This is a frequent situation m trucks and busses with silencers placed close to a front wheel and with sidewise exhaust from the vehicle.

Claims: CLAIMS 1. A device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising: a casing, at least one acoustic chamber contained m the casing, said chamber being through-flowed by gas, at least one mlet pipe for leading gas into one of said at least one acoustic chamber, - at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber, optionally one or more monolithic bodies comprised in each of one or more of said at least one acoustic chamber, said device showing at least two through-flowed transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area a x and a relatively higher cross-sectional area A ι r the device fulfilling the following criteria: ' in) the average sound attenuation AdB conferred by each transition of cross-sectional area, approximated by the following expression: n X? a, n being the total number of transitions of cross-sectional area of the device, Ai being the relatively higher cross- sectional area at the i ' th transition of cross-sectional area of the gas flow, a x being the relatively lower cross- sectional area at the i ' th change of cross-sectional area of the gas flow, k being an empirically determined constant of the value 6.25 dB, is at least 2.9 dB when the device comprises no more than two acoustic chambers, 1.8 dB when the device comprises three acoustic chambers, 1.1 dB when the device comprises 4 or more acoustic chambers, (iv) the pressure loss over each acoustic chamber expressed as the di ensionless parameter ζ-, ' , defined as the ratio between the static pressure loss over the chamber and the dynamic pressure at a location in said passage: Δ j being the static pressure loss over the j ' th chamber, exclusive of the static pressure loss over a monolith optionally comprised in the j ' th chamber, p being the density of the gas at said location, u being a velocity of the gas at said location, preferably the mean gas velocity, is at the most 1.5. 2. A device for silencing a gas flow directed therethrough and being adapted for installation m a flow system, said device comprising: - a casing, at least one acoustic chamber contained m the casing, said chamber being through-flowed by gas, - at least one mlet pipe for leading gas mto one of said at least one acoustic chamber, at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber, optionally one or more monolithic bodies comprised m each of one or more of said at least one acoustic chamber, said device showing at least two transitions of cross-sectional area for the flow of the gas between a relatively lower cross- sectional area a x and a relatively higher cross-sectional area ,, at least one passage selected from said at least one passage being curved, wherem the following applies to at least one selected chamber selected from said at least one acoustic chamber: the mean cross-sectional area A-, of each of said selected chamber is at least four times the largest of: the sum of all cross-sectional areas of passages leading gas to the selected chamber, ai, and the sum of all cross-sectionial areas of passages leading gas from the selected chamber, a 2 , the mean cross-sectional area, A-,, being defined as the mean value of all cross-sectional areas along a mean trajectory for sound waves travelling from across the selected chamber, the volume of the at least one selected chamber, V-,, is at least and a 2 being defined as stated above, the cross-sectional area, A-,, and the volume, V-, , including any elements belonging to sound absorptive material inside the selected chamber and any other parts being in acoustic communication with the selected chamber. 3. A device for silencing a gas flow directed therethrough and being adapted for installation in a flow system, said device comprising: a casing, - at least two acoustic chambers contained in the casing, said chambers being through-flowed by gas, at least one inlet pipe for leading gas into one of said at least two acoustic chambers, at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least two acoustic chambers to another of the at least two acoustic chambers or to an exterior environment or an exterior chamber, optionally one or more monolithic bodies comprised each of one or more of said at least two acoustic chambers, said device showing at least two through-flowed transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area a, and a relatively higher cross-sectional area A said let pipe being continued by a flow deflecting element causing tne gas to flow with a radial component and for causing pressure recovery to the flow upstream of a first one of said at least two acoustic chambers, where said passage comprises at least one diffuser for leading the gas flow mto a second one of said at least two acoustic chambers, the diffuser of said passage bemg of a different type than a radial diffuser. 4. A device according to any of the preceding claims, wherem sound absorptive material is provided withm at least one of the at least one acoustic chamber. 5. A device accordmg to any of the preceding claims, wherem sound absorptive material is provided with each of the at least one acoustic chamber. 6. A device according to any of the preceding claims, wherem the local natural frequency, f e , of at least one system comprising the gas of two consecutive acoustic chambers AC : and AC-, + i and the gas of the passage interconnecting said two acoustic chambers, approximated by the following expression: V-, and V-, + 1 being the volumes of the chambers AC-, and AC ] + ] respectively (the volume V ] + ] being set to infinite when the chamber AC-, is connected to an exterior environment or an exterior chamber in a downstream direction) , a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L being the length of the passage, and c being the local sound velocity, is at the most 0.75 times a characteristic frequency of the flow system. 7. A device according to any of the preceding claims, further comprising at least one diffuser for diffusing at least part of said gas flow through at least one of said at least one passage. 8. A device according to claim 6, wherein said local natural frequency is at the most 0.5 times the characteristic frequency of the flow system. 9. A device according to claim 8, wherem said local natural frequency is at the most 0.4 times the characteristic frequency of the flow system. 10. A device according to claim 9, wherem said local natural frequency is at the most 0.3 times the characteristic frequency of the flow system. 11. A device accordmg to claim 10, wherem said local natural frequency is at the most 0.25 times the characteristic frequency of the flow system. 12. A device accordmg to claim 11, wherem said local natural frequency is at the most 0.2 times the characteristic frequency of the flow system. 13. A device according to claim 12, wherein said local natural frequency is at the most 0.15 times the characteristic frequency of the flow system. 14. A device according to claim 13, wherein said local natural frequency is at the most 0.1 times the characteristic frequency of the flow system. 15. A device according to any of the preceding claims, wherein ζ-, ' of each acoustic chamber is at the most 1.0. 16. A device according to any of the preceding claims, wherein ζ-, ' of at least one acoustic chamber is at the most 0.75. 17. A device according to claim 16, wherein ζ-, ' of at least one acoustic chamber is at the most 0.5. 18. A device according to claim 17, wherein , of at least one acoustic chamber is at the most 0.25. 19. A device according to claim 18, wherein ζ D ' of at least one acoustic chamber is at the most 0. 20. A device according to claim 19, wherein ζ 3 ' is of at least one acoustic chamber less than 0. 21. A device according to any of the preceding claims, wherein the average sound attenuation AdB conferred by the transitions of cross-sectional area, approximated by the expression given under (i) in claim 1 is at least 3.7 dB when the device comprises no more than two acoustic chambers, 2.9 dB when the device comprises three acoustic chambers, 1.9 dB when the device comprises 4 or more acoustic chambers . 22. A device according to claim 21, wherem the average sound attenuation AdB conferred by the transitions of cross-sectional area, approximated by the expression given under (I) m claim 1 is at least 5.6 dB when the device comprises no more than two acoustic chambers, - 4.8 dB when the device comprises three acoustic chambers, 3.7 dB when the device comprises 4 or more acoustic chambers . 23. A device according to any of the preceding claims, wherem curvatures, preventing flow separation, are applied to at least part of the contour of at least one of the at least one passage and/or the let pipe. 24. A device accordmg to any of the preceding claims, comprising one or more radial diffusers and/or one or more axial diffusers and/or one or more circular conical diffusers and/or one or more annular diffusers and/or a plurality of conical diffusers arranged on a substantially cylindrical sur- face and/or one or more diffusers for reversing the direction of flow and/or one or more double diversion diffusers. 25. A device according to claim 24, wherem each of the at least one acoustic chamber is substantially cylindrical, and wherein one or more outlets from said at least one diffuser are located substantially at the axial centre of the chamber associated with said diffuser. 26. A device according to any of the preceding claims, wherem each of the at least one acoustic chamber is substantially cylindrical, said chamber thereby defining a cylinder axis, and wherem one or more outlets from said least one diffuser are located at a distance from the cylinder axis of approximately two thirds of the radius of the acoustic chamber. 27. A device according to any of claims 6-26, comprising m acoustic chambers, m being at least 2, where the following applies to each of at least m-1 of the acoustic chambers: the mlet passage leading gas to the chamber is provided with a diffuser or a diffuser element, a curvature or curvatures, preventing flow separation, are applied to at least part of the contour of all passages leading gas out of the chamber, the pressure loss across the chamber expressed as ζ-, ' is less than or equal to 1.5, the approximated average sound attenuation AdB conferred by each transition of cross-sectional area is at least: 2.9 dB for m=2, 1.8 dB for m=3, 1.1 dB for m>3, and wherem the following applies to each of at least m-2 of the passages interconnecting two consecutive acoustic chambers or interconnecting an acoustic chamber and the exterior environment or chamber: - the approximated local natural frequency, f P/ is at the most 0.75 times a characteristic frequency of the flow system. 28. A device accordmg to 27 where the following applies to each of at least m-1 of the acoustic chambers: the pressure loss across the chamber expressed as ζ j ' is less than or equal to 1.0, the approximated average sound attenuation AdB conferred by each transition of cross-sectional area is at least: 3.7 dB for m=2, 2.9 dB for m=3, 1.8 dB for m>3. 29. A device according to 28 where the following applies to each of at least m-1 of the acoustic chambers: the pressure loss across the chamber expressed as ζ, ' is less than or equal to 1.0, the approximated average sound attenuation AdB conferred by each transition of cross-sectional area is at least: 5.6 dB for m=2, - 4.8 dB for m=3, 3.7 dB for m>3, and wherem the following applies to each of at least m-2 of the passages interconnecting two consecutive acoustic chambers or interconnecting an acoustic chamber and the exterior environment or chamber: the approximated local natural frequency, f e , is at the most 0.5 times a characteristic frequency of the flow system. 30. A device according to any of claims 27-29 wherem the criteria of any of claims 27-29 applies to all acoustic chambers and to all passages. 31. A device according to any of the preceding claims, where the sound level of self-generated noise of each one of said at least one acoustic chamber at maximum gas flow rate is less than 5 dB (A) higher than the self-generated noise of a circular cylindrical reference chamber through-flowed at said gas flow rate, the cross-sectional area of the mlet passage leading gas mto said acoustic chamber being ai, the cross-sectional area of the passage leading gas from said acoustic chamber being a 2 , said reference chamber: being of the same volume as each of said at least one acoustic chamber, having a length equal to its diameter, having flat end caps, being provided with centrally positioned holes in its flat end caps, having a first end cap which is connected to a cylindrical mlet pipe of a cross-sectional area which is approximately equal to ai, the terminating surface of said cylindrical mlet pipe being aligned with said first end cap, having a second end cap which is connected to a cylindrical outlet pipe of a cross-sectional area which is approximately equal to a 2 , said cylindrical outlet pipe having a rounded inner edge at its interconnection with said second end cap and being aligned with said second end cap . 32. A device according to any of the preceding claims, where the sound level of self-generated noise of each one of said at least one acoustic chamber at maximum gas flow rate is less than 5 dB (A) higher than the self-generated noise of a circular cylindrical reference chamber through-flowed at said gas flow rate, the cross-sectional area of the let passage leading gas mto said acoustic chamber bemg a l the cross-sectional area of the passage leading gas from said acoustic chamber being a , said reference chamber: being of the same volume as each of said at least one acoustic chamber, - having a length equal to its diameter, having flat end caps, bemg provided with centrally positioned holes in its flat end caps, having a first end cap which is connected to a cylindrical mlet pipe of a cross-sectional area which is approximately equal to a , the terminating surface of said cylindrical let pipe being aligned with said first end cap, having a second end cap which is connected to a cylindrical outlet pipe of a cross-sectional area which is approximately equal to a 2 , said cylindrical outlet pipe having a rounded inner edge at its interconnection with said second end cap and being aligned with said second end cap. 33. A device according to any of the preceding claims, wherem curvatures, preventing flow separation, are applied to at least part of the contour of the outlet and/or mlet of a pipe (4,6) or passage (12) of the device, said passage being the mlet pipe (6) or its extension mto the device and/or the outlet pipe (4) or its extension mto the device and/or a passage (12) connecting two chambers (1,2) . 34. A device according to any of the preceding claims, wherein at least one passage is annular, constituted by an inner cylinder (42, 43) and by an outer cylinder (44, 45). 35. A device according to claim 34, wherem at least one of the at least one passage which is annular is a passage connecting two chambers (1,2) . 36. A device according to claim 34 or 35, wherem the at least pipe one passage being annular constitutes a flow cross sectional area increase m the flow direction. 37. A device according to any of claims 34-36 m which said annular passage (12) comprises a constant flow area part (12b) and an outlet diffuser part (12c) . 38. A device according to any of claims 34-37 m which said inner cylinder extends mto said first chamber (1) by a cylinder (41) of substantially the same diameter as said inner cylinder, and said outer cylinder is connected to a flow- guidmg body (21) with a curvature (20) . 39. A device according to any of claims 34-38 ir which sound absorptive material is contained withm said cylinder (42, 43) and/or withm a continuation cylinder (41) extending into said first chamber (1) and/or withm a continuation cylinder (43) extending into said second chamber (2). 40. A device accordmg to claim 39 wherem at least part of the continuation cylinder (41) is perforated. 41. A device according to any of claims 34-40 m which the outflow from said connecting passage (12) passes mto an annular passage (30) inside said second chamber (2), said annular passage (30) comprising at least a perforated portion of an inner cylinder (43) and an outer, perforated cylinder (46), both said cylinders (43, 46) separating sound absorptive materials (Bd, Be) from gas flow withm said second chamber. 42. A device according to claim 41 in which the outflow from said connecting passage (12) passes directly mto an annular passage ( 30 ) . 43. A device accordmg to any of claims 34-42 m which the distance (Dl) between the mlet (10a) to said first chamber (1) and the mlet (12a) to said annular passage (12) is so large that substantially no unstable flow occurs m the chamber (1). 44. A device accordmg to any of claims 41-43 wherem the inner cylinder (43) has a substantially non-perforated portion which constitutes at least part of the inner cylinder constituting an inner wall of the outlet diffusor part (12) . 45. A device according to any of claims 41-44 wherem the substantially non-perforated portion of the inner cylinder (43) constitutes at least part of an inner wall of the chamber (2) . 46. A device according to any of the preceding claims wherem at least one of the at least one passage is provided with a diffuser or a diffuser element at its outlet. 47. A device according to any of the preceding claims, where the generatrix of at least part of at least one curved passage selected from said at least one passage is wound in a peripheral direction, at least part of the curved passage having a plane spiral form. 48. A device accordmg to any of claims 1-46, wherem the generatrix of at least a part of at least one curved passage selected from said at least one passage is wound in a peripheral direction, said part of said curved passage extending m a longitudinal direction, so as to form a screwlike helical form. 49. A device according to claim 47 or 48, wherem the curved part of the curved passage (12) extends radially over an angle between 0° and 90°. 50. A device according to claim 47 or 48, where the curved part of the curved passage (12) extends radially over an angle between 90° and 180°. 51. A device according to claim 47 or 48, wherem the curved part of the curved passage (12) extends radially over an angle between 180° and 270°. 52. A device according to claim 47 or 48, wherem the curved part of the curved passage (12) extends radially over an angle between 270° and 360°. 53. A device according to claim 47 or 48, wherem the curved part of the curved passage (12) extends radially over an angle between 360° and 720°. 54. A device according to claim 47 or 48, wherem the curved part of the curved passage (12) extends radially over an angle of 720° or more. 55. A device according to any of the preceding claims, comprising at least two acoustic chambers, wherem the curved passage (12) interconnects two chambers, a first of which surrounds a second one. 56. A device according to any of the preceding claims, where the generatrix of the curved part of the passage (12) extends along a surface of revolution. 57. A device according to claim 56, wherem at least part of the surface of revolution is conical. 58. A device according to any of the preceding claims, where a first part of the curved passage (12) extends m a first longitudinal direction and a second part of the curved passage (12) extends in a second longitudinal direction which is opposite to said first longitudinal direction. 59. A device according to any of the preceding claims, wherem the at least one curved passage consitutes a tail pipe interconnecting one of said at least one acoustic chamber and an exterior environment or an exterior chamber, and wherem at least part of the curved passage s wound around said casing on an outer surface of the casing. 60. A device according to any of the preceding claims, and comprising at least one monolithic body positioned upstream or downstream of an let passage and/or an outlet passage of the one or more chambers . 61. A device according to claim 60, wherein said at least one monolithic body is positioned substantially immediately upstream or substantially immediately downstream of said mlet passage and/or said outlet passage of the one or more chambers. 62. A device accordmg to claim 60 or 61 wherem the at least one monolithic body if of an annular form. 63. A device according to any of claims 60-62 wherem the gas flow leaves the passage m a tangential direction at least two locations along the passage. 64. A device according to any of the preceding claims, further comprising at least one resonance chamber which is not through- flowed with or without a therein enclosed sound absorptive material and communicating with one of said at least one acoustic chamber through at least one communication passage, the volume of the at least one resonance chamber and the geometric configuration of the at least one resonance chamber and the communication passage or passages being designed to have a selected center attenuation frequency so as to supplement the sound attenuation achieved with said acoustic chamber or chambers at frequencies of said flow system around said center attenuation frequency. 65. A αevice according to 64, wherem the volume of the at least one : esonance chamber is adjustable for instance by means of displaceable adjustment means so as to allow for a variable center attenuation frequency. 66. A device according to claim 65 or 66 wherem one or more resonant chambers (60) are contained m said casing with the aim of providing added sound attenuation, said resonance chambers being designed as resonators with selected center frequencies . 67. A device according to any of the preceding claims, wherein the following applies to at least one selected chamuer selected from said at least one acoustic chamber: the mean cross-sectional area A-, of each of said selected chamber is at least four times the largest of: - the sum of all cross-sectional areas of passages leading gas to the selected chamber, ai, and the sum of all cross-sectional areas of passages leading gas from the selected chamber, a 2 , the mean cross-sectional area, A-,, being defined as the mean value of all cross-sectional areas along a mean trajectory for sound waves travelling from across the selected chamber, - the volume of the at least one selected chamber, V-, , is at least 8^(a, + a 2 )l 2j , a, and a 2 being defined as stated above, the cross-sectional area, A, and the volume, V-, , mcluding any elements belonging to sound absorptive material side the selected chamber and any other parts being in acoustic communication with the selected chamber. 68. A device accordmg to claim 67, where the volume of the at least one selected chamber, V , is at least \ 6 J(a + a )/ 2) . 69. A device according to claim 67 or 68, wherein at least one of said at least one selected chamber comprises deflecting means for deflecting the flow mside the chamber. 70. A device according to claim 69, wherem the deflecting means are formed so as to deflect the flow mside said at least one selected chamber at least 90 degrees. 71. A device according to any of the preceding claims, said at least one acoustic chamber havmg an let and one outlet, the distance between said mlet and said outlet, D, being so large that substantially no unstable flow occurs mside said selected chamber . 72. A device accordmg to claim 72 in which, with the aim of preventing unstable flow in said at least one acoustic chamber, said distance D is 10-50o larger than the distance at which substantially unstable flow would occur. 73. A device according to any of the preceding claims, where chambers with swirl-provokmg passage mlet and/or passage outlet flow directions are formed in such a way and/or provided with such internal guides, e.g., guide plates, that essentially no major swirl occurs in the selected chamber. 74. A device according to claim 73, wherem said guides are designed and dimensioned so that flow resistance caused by the guides is minimised. 75. A device according to any of claims 67-73, comprising at least one selected chamber from which at least one passage leads gas from the selected chamber to the exterior environment or chamber, said passage (s) each having an acoustically effective length L, which is at least 1.5 times the direct and straight distance m space between passage mlet and passage outlet . 76. A device according to any of claims 67-75, comprising at least two selected chambers fulfilling the criteria of any of claims 67-75 and being interconnected by one or more passages each having an acoustically effective length L which is at least 1.5 times the direct and straight distance in space between passage mlet and passage outlet. 77. A device according to any of claims 67-76, comprising at least one selected chamber from which at least one passage leads gas from the chamber to the exterior environment or chamber and said passage or passages all have an acoustically effective length L, which is at least equal to the length of said at least one selected chamber. 78. A device according to any of claims 67-77, the device comprising at least two selected chambers fulfilling the criteria of any of claims 67-77 and being connected by one or more passages each having an acoustically effective length L which is at least equal to the sum of the lengths of said two selected chambers, Lj and L 2 . 79. A device according to any of the preceding claims, each of said passages having forms causing a flow trajectory with the passage or passages to deflect, the passage form(s) being such that the radius of curvature of the trajectory at most points is greater than or equal to the transverse straight-line distance across the passage when measured perpendicular to the trajectory tangent at said points. 80. A flow system comprising at least one device according to any of the preceding claims. 81. A flow system accordmg to claim 80, and having a characteristic frequency, the local natural frequency, f„, of at least one system of the device comprising the gas of two consecutive acoustic chambers AC-, and AC 3 + ι and the gas of the passage interconnecting said two acoustic chambers, approximated by the following expression: V-, and V : + 1 being the volumes of the chambers AC-, and AC J + ι respectively (the volume V : + 1 being set to infinite when the chamber AC D is connected to an exterior environment or an exterior chamber a downstream direction) , a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L bemg the length of the passage, and c being the local sound velocity, is at the most 0.75 times said characteristic frequency. 82. A flow system according to claim 81, wherem said local natural frequency, f e , is at the most 0.5 times said characteristic frequency. 83. A flow system according to claim 82, wherem said local natural frequency, f e , is at the most 0.4 times said characteristic frequency. 84. A flow system according to claim 83, wherem said local natural frequency, f e , is at the most 0.3 times said characteristic frequency. 85. A flow system according to claim 84, wherem said local natural frequency, f e , is at the most 0.25 times said characteristic frequency . 86. A flow system according to claim 85, wherem said local natural frequency, f e , is at the most 0.2 times the said characteristic frequency. 87. A flow system according to claim 86, wherem said local natural frequency, f e , is at the most 0.15 times said characteristic frequency. 88. A flow system according to claim 87, wherem said local natural frequency, f e , is at the most 0.1 times said characteristic frequency. 89. A flow system according to any of claims 81-88, and comprising an internal combustion engine, the characteristic frequency being the firing frequency of the engine. 90. A flow system according to any of claims 81-88, and comprising a gas turbine and/or a compressor. 91. A flow system according to claim 90 the characteristic frequency of the gas turbine or the compressor being a predominant low-frequent peak sound level frequency, determmable by measurement or from known design features, and/or a mid-octave band frequency which is lower than the measure peak sound level octave band and below which frequency the sound level generally falls of with further lowered frequency. 92. A vehicle comprising a combustion engine, such as a diesel engine, a petrol engine or a gas engine, and/or a gas turbine and/or a compressor and at least one device according to any of claims 1-79., the at least one device being comprised m the exhaust system of the vehicle. 93. A vehicle according to claim 92, the vehicle being a ship or boat. 94. A stationary plant comprising a combustion engine, such as a diesel engine, a petrol engine or a gas engine, and/or a gas turbine and/or a compressor and at least one device according to any of claims 1-79. 95. A method for dimensioning a device for silencing a gas flow directed therethrough, said device being adapted for installation in a flow system and comprising: a casing, - at least one acoustic chamber contained m the casing, said chamber being through-flowed by gas, at least one let pipe for leading gas mto one of said at least one acoustic chamber, at least one passage of a length L and of a representative cross-sectional area a for leading gas from each one of the at least one acoustic chamber to another of the at least one acoustic chamber or to an exterior environment or an exterior chamber, optionally one or more monolithic bodies comprised in each of one or more of said at least one acoustic chamber, said device showing at least one transition of cross-sectional area for the flow of the gas between a relatively lower cross- sectional area ai and a relatively higher cross-sectional area A the method comprising comprising approximating: (I) the local natural frequency, f e , of one or more mass systems comprising the gas mass of two consecutive acoustic chambers, AC-, and AC 3 + ι, by applying the following expression: V D and V-,+1 being the volumes of the chambers AC-, and AC D + ι respectively (the volume V : + ι being set to infinite when the chamber AC-, is connected to an exterior environment or an exterior chamber a downstream direction) , a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L being the length of the passage, and c being the local sound velocity, (II) the sound attenuation AdB, conferred by one or more of the at least one transition of cross-sectional area by applying the following expression: AdB t = Λ:log, a A x being the relatively higher cross -sectional area at the i ' th transition of cross -sectional area of the gas flow, a x being the relatively lower cross-sectional area at the 1 ' th transition of cross-sectional area of the gas flow, and k is an empirically determined constant for adapting two or more of the following parameters relative to each other: (a) the length L and the representative cross-sectional area a of one or more of the at least one passage, (b) the number of changes, n, of transitions of cross-sectional area for the flow of the gas between a relatively lower cross-sectional area a, and a relatively higher cross- sectional area A] , (c) the number of acoustic chambers, m, comprised in the casing, (d) the relatively lower cross-sectional area a λ (ι=l..n) and the relatively higher cross-sectional area A (ι=l..n) of each one of the n changes of cross-sectional area, (e) the volume or volumes, V (j=l..m), of each one of the m acoustic chambers comprised in the casing, (f) the natural frequency, f„, of one or more domains of the device comprising two consecutive acoustic chambers AC η and AC-, + i , or (g) the total sound attenuation conferred by the device. 96. A method according to claim 95, said method comprising adapting : (a) the length L and the representative cross-sectional area a of each one of the at least one passage, (b) the number of changes, n, of cross-sectional area for the flow of the gas between the relatively lower cross-sectional area a, and the relatively higher cross-sectional area A l f (c) the number of acoustic chambers, m, comprised in the casing, (d) the relatively lower cross-sectional area a; (ι=l..n) and the relatively higher cross-sectional area A, (ι=l..n) of each one of the n changes of cross-sectional area, (e) the volume or volumes, V (j=l..m), of each one of the m acoustic chambers comprised the casing, (f) the natural frequency, f„, of any domain of the device comprising two consecutive acoustic chambers AC 3 and AC ] + 1 , (g) the total sound attenuation AdB conferred by the device, relative to each other by performing the steps of: (l) restricting the values of one or more of the parameters (a) -(g) by assigning a predetermined value or a predetermined limit to each one of said one or more parameters, In) assigning values to the parameters a, L, a L and A x if predetermined values have not been assigned to those parameters at step (1), (m) approximating the local natural frequency, f e , of any domain of the device comprising two consecutive acoustic chambers AC-, and AC-, + 1 by applying the following expression: V, and V-, + 1 being the volumes of the chambers AC-, and AC ] + i respectively (the volume V? being set to infinite when the chamber Ac η is connected to an exterior environment or an exterior chamber in a downstream direction) , a being a representative cross-sectional area of the passage interconnecting the two consecutive acoustic chambers, L being the length of the passage, and c being the local sound velocity, (iv) if a value or a limit for the local natrural frequency has been prescribed at step (1), comparing said value or limit to f„ as determined at step (m), (v) approximating the total sound attenuation ] AdB conferred by the device by applying the following expression : Y AdB = k∑log^- ft α, n being the total number of changes of cross-sectional area of the gas flow, A ± being the relatively higher cross - sectional area at the i ' th change of cross-sectional area of the gas flow, a x being the relatively lower cross-sectional area at the 1 ' th change of cross-sectional area of the gas flow, and k is an empirically determmed constant, (vi) if a value or a limit for the total sound attenuation of the device has been prescribed at step (1) , comparing said value or limit to AdB as determined at step v (vn) if the comparisons performed at steps (iv) and (vi) reveal that the limitations prescribed at step (I) are not fulfilled, updating the values of the parameters set at step (n) and repeating steps (m)-(vιι) until the limitations prescribed at step (l) are achieved. 97. A method according to claim 95 or 96, further comprising applying expressions known per se for approximating the increase or decrease of static pressure at one or more of said at least one transition of cross-sectional area. 98. A method according to claim 96 and 97, comprising assigning a predetermined limit or value to the pressure loss over the device at step (l) of claim 96, and repeating steps (m)-(vιι) of claim 96 until said predetermined limit or value for the pressure loss over the device is achieved. 99. A method according to any of claims 96-98, further comprising repeatmg steps (m)-(vιι) of claim 96 until said parameters are mutually optimized. 100. A method according to any of claims 95-99 wherem the value of k is in the range 5-7. 101. A method accordmg to claim 100 wherem the value of k is m the range 6-6.5. 102. A method according to claim 101 wherein the value of k is 6.25. 103. A method according to any of claims 95-102, the device further comprising at least one resonance chamber with or without a therein enclosed sound absorptive material and communicating with one of said at least one acoustic chamber through at least one communication passage, the method comprising adjusting the volume of the at least one resonance chamber and the geometric configuration of the at least one resonance chamber and the communication passage or passages being so as to have a selected center attenuation frequency so as to supplement the sound attenuation achieved with said acoustic chamber or chambers at frequencies of said flow system around said center attenuation frequency. 104. A method according to claim 103, the volume of the at least one resonance chamber being adjustable for instance by means of displaceable adjustment means, the method comprising adjusting the range of the volume of the at least one resonance chamber so as to allow for a variable center attenuation frequency withm a given range. 105. A method according to claim 103 or 104, the device comprising one or more resonant chambers (60) contained in said casing with the aim of providing added sound attenuation, the method comprising designing the volumes of said resonance chambers as resonators with selected center frequencies. 106. A method of silencing a gas flow directed therethrough, said method comprising leading gas flow through at least one device according to any of claims 1-79. 107. A method according to claim 106, wherem the gas flows m a flow system according to any of claims 80-91.

Inventors: FREDERIKSEN EYVIND, FREDERIKSEN SVEND, FREDERIKSEN, EYVIND, FREDERIKSEN, SVEND
Applicants: FREDERIKSEN EYVIND, FREDERIKSEN SVEND, SILENTOR AS, FREDERIKSEN, EYVIND, FREDERIKSEN, SVEND, SILENTOR A/S
Application year: 1997
Main classification: F01N1
IPC codes: F01N1, F01N3