SOUND ATTENUATOR

Abstract: A ventilating system for transport of air in environments with requirements concerning sound comprising a plurality of channel sections and a sound-attenuating device comprising both active and passive sound attenuation. The passive sound attenuation comprises a reactive part and a resistive part, and the active, the reactive and the resistive attenuation are adapted to attend to attenuation within separate frequency ranges.

Description: Sound attenuator TECHNICAL FIELD The present invention relates to a sound attenuator in a plant for transport of gases. More particularly, it relates to a combination attenuator comprising one passive and one active sound attenuation. In particular, the invention relates to a ventilation plant comprising a system of channels in which sound attenuation occurs by means of a combined active and passive technique. The invention also relates to a control system for changing the passive and the active sound attenuation in real time . BACKGROUND ART A ventilation plant comprises a plurality of channels, sound attenuators and apparatus for conditioning of air. Such apparatus may, for example, be air humidifiers, heaters and coolers. In a simple design, such a plant comprises an exhaust-air system. In this system, air is sucked from a., plurality of rooms within a building and is sluiced out into ■the open. In another simple design, such a plant comprises a supply-air system. In this system, air is sucked from the open and is sluiced into a large number of rooms within a building. However, often a ventilation plant is a combination of a supply-air system and an exhaust-air system. The plant attends to ventilation in rooms within, for example, an office building or a dwelling house. At one end of the system there is a fan installation, usually comprising one fan for supply air and one fan for exhaust air. At the other end of the installation, channels open out into rooms within the building. These rooms are in many cases associated with requirements with respect to sound. The sound which is thus generated by the fans must therefore, on its way from such a room, undergo sound attenuation such that the sound level in the room does not exceed the requirement. It is known to arrange sound attenuation in a channel system for transport of gas, not only near the fan but also near the orifice. It may be a question of both a flue-gas cleaning plant and an ordinary ventilation plant. In the first case, the channel opens out into the open air and in the second case the channel usually opens out into a room. In both cases, the sound coming out through the orifice is subjected to sound requirements . Most of the sound is usually generated by the fans and it is known to arrange the main sound attenuation close to the fans. However, along the channels, sound is formed by objects projecting into the flowing gas. Sound may also be formed by turbulence in the flowing gas. To prevent sound, generated in any of these ways, from reaching the orifice of the channel, local sound attenuators are arranged at the orifice. One common way of arranging sound attenuators is to insert resistive sound attenuators in the channel. By resistive sound attenuation is to be understood here the fact that the sound energy is absorbed and transformed into heat. To absorb sound with a low frequency, that is, large wavelengths, a relatively thick absorbent is required. This leads to the disadvantage that such a sound attenuator becomes extensive in size. Another disadvantage is that it gives rise to a considerable pressure drop in the channel system. A thicker sound attenuation also becomes heavier and more expensive to manufacture. A sound attenuator with such a thick absorbent still provides quite a moderate sound attenuation at the very lowest frequencies. For high frequencies, on the other hand, the sound attenuation is very good. This results in the problem that the attenuation curve becomes ill adapted to the noise of the fan. A fan provides great sound energy at low frequencies and moderate energies at higher frequencies. The sound attenuator provides poor attenuation at low frequencies and very good attenuation at high frequencies. The sound leaving the sound attenuator is thus a low-frequency, rumbling sound. The sound penetrates the channel wall as well as opens out into spaces and causes noise problem. One known way of eliminating the above problem is to introduce so-called active sound attenuation. By active sound attenuation is to be understood that a sound source is adapted to create an active sound which is in opposition to the sound progressing in the channel system, whereby this sound is extinguished. An active sound attenuation system usually comprises an upstream microphone, a loudspeaker, a downstream microphone, and a control unit. The upstream microphone provides information about the sound to be attenuated whereas the downstream microphone provides information which is used to optimize the system. Experiments have shown that active sound attenuation functions best in systems where sound propagates as a plane wave. This takes place where the channel dimension is small in relation to the wavelength of the sound, which normally only comprises low frequencies. At higher frequencies, transverse modes of the sound arise in the channel, which modes mask the fundamental mode such that the microphones can no longer measure correctly, whereby the active technique is deteriorated. From the patent document US 4,665,549, a hybrid sound attenuator, which comprises active sound attenuation, is previously known. The task to be solved by the known active sound attenuator is to eliminate sound radiating backwards from the active loudspeaker. This sound would otherwise generate unwanted sound at the outside of the channel and could also influence the microphones which control the process. The known sound attenuator solves this problem by placing the active loudspeaker inside the channel. The rear of the loudspeaker is surrounded by absorbents and are preferably placed in a baffle in the channel. However, the arrangement is bulky and encroaches upon the channel dimension as well as on the external dimensions . This results in an increase in the pressure increase in the channel so that more powerful fans have to be installed. Combining resistive sound attenuation with active sound attenuation gives an increased possibility of dimensioning the attenuation so that it coincides better with the -require- ent. It also means that the resistive attenuator may be made thinner, so that the attenuator can be made more slender. Still, at high frequencies, the attenuation becomes greater than the requirement. The active system gives a good attenuation at low frequencies. The resistive attenuation gives a good attenuation at high frequencies . In the frequency range therebetween, the sound attenuation becomes less good. In this context, it is known to increase the thickness of the resistive absorbent such that it provides sufficient attenuation of sound in this intermediate-frequency range. However, the increase in thickness leads to the sound attenuator becoming bulky and to the absorbing properties at high frequencies becoming oversized. As pointed out above, it will also be heavier and more costly to manufacture. SUMMARY OF THE INVENTION The object of the present invention is to suggest ways and means of manufacturing a sound attenuator in a transport system for gas which may be adapted to the predominant sound source and which is not bulky. The sound attenuator shall be inexpensive in manufacture and shall not involve any significant pressure drop. From a second aspect of the invention, a ventilation system for a building or a ship, which incorporates a plurality of sound attenuators of the described type, is referred to. The invention also relates to a method for efficiently attenuating sound in a ventilation system, without significantly increasing the pressure increase. From a third aspect of the invention, a sound attenuator with variable attenuating properties, which may be controlled without intervening in the sound attenuator, is referred to. These objects are achieved according to the invention by a sound attenuator according to the characteristic features described in the characterizing portion of the independent claim 1, by a ventilation system comprising a sound attenua- tor according to the characteristic features described in the characterizing portion of the independent claim 8, by a method according to the characteristic features described in the characterizing portion of the independent claim 10, and by a computer program according to the characteristic features described in the characterizing portion of the independent claim 15. Advantageous embodiments are described in the characterizing portions of the dependent claims. According to the invention, the present object is achieved by a sound attenuator comprising a combination of active sound attenuation, reactive sound attenuation and resistive sound attenuation. Hence, an active system is brought to attenuate sound within substantially a first frequency range, the low- frequency range. A reactive system is adapted to attenuate sound within substantially a second frequency range, the intermediate-frequency range. Finally, a resistive system is brought to attenuate sound within substantially a third frequency range, the high-frequency range. Experiments have verified that the attenuation in a combination attenuator according to the invention substantially follows a serial pattern. The sound is, for example, first subjected to the reactive attenuation whereupon the remaining sound is influenced by the resistive system. Finally, the sound now remaining is attenuated by the active system. • H Φ β β ( ti A υ LΠ o IT) o ID o Lfl rH H CM CM m CO To function correctly, a resonance attenuator should be placed at a point where the relative pressure of the sound has a maximum. Since the resonance attenuator itself influences the sound, more correctly expressed, it should be placed at a point where the sound would have had a maximum. Such a location in a channel system is a quarter of a wavelength in front of a point in the channel where the channel undergoes an abrupt increase in cross section. At such a point, part of the sound is reflected. In an ordinary case, this is achieved by a resistive attenuator. The resistive attenuator has an abrupt change of the channel dimension, in which the absorbent is placed. At high frequencies, the attenuator is perceived to have the dimension as the channel. At low frequencies, the resistive attenuation of the absor- bent is low and is therefore perceived as a change in volume. In the following text, this type of sound attenuator is referred to as a reflection attenuator. In an advantageous embodiment, the reactive attenautor is arranged with a device which allows adjustment of attenuation and frequency. In one embodiment, this is achieved with an actuator-adjustable length of a quarter-wave attenuator. In another embodiment, the effect is achieved by arranging a quarter-wave attenuator with a bottom which is brought into oscillation by means of an actuator and hence offers a stiffness which varies with the sound wave and which is determined by the actuator. Both solutions are suitably brought to exert an influence based on an actuator which is controllable by means of a control unit and which attends to the change. The resistive system comprises a discontinuous recess in the channel in which an absorbent is arranged. In one embodiment it is a soft porous absorbent that fills up the recess. In another embodiment it is a thin absorbent that is brought to constitute a continuation of the internal surface of the channel. In another advantageous embodiment, the resistive attenuator is arranged with an adjustable absorption only. In this case, a controllable actuator brings the absorbent to change its properties. One way of achieving this is to allow the actuator to compress the absorbent, in which case the density and the thickness are influenced. Another way is to place a surface in the vicinity of the absorbent, which surface is brought to vibrate by the actuator. In this case, the resistance, which is experienced by the acoustic wave upon passage of the absorbent, is influenced. In this ' way, an absorbent is influenced to change its absorbent in a controllable manner. The designation actuator is to be understood in a broad sense and may thus comprise both an electromechanical device and a device which, within the porous absorbent, via a signal, may attend to a change of properties in the absorbent. In the third aspect of the invention, the sound attenuator comprises control equipment and means for adjusting both the reactive system and the resistive system. To this end, the control equipment comprises a processor with a program memory. A computer program is thus brought to be executed, whereby instructions are given to the processor to attend to an adjustment of the active, the reactive and- the resistive part of the sound attenuation. In an advantageous embodiment, the control equipment is adapted, on the basis of a given strategy, to automatically adjust the sound attenuator to offer the attenuation which is most suitable for the sound source. In an advantageous embodiment, such a program is supplied to the control equipment via a network, such as, for example, the Internet or via a computer-readable medium, such as a CD-ROM. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in greater detail by description of an embodiment with reference to the accompanying drawing, wherein Figure 1 shows a diagram of the attenuation versus the frequency of a sound attenuator according to the invention, Figure 2 shows a sound attenuation system comprising an active part, a reactive part, and a resistive part, Figure 3 shows a variable resonance attenuator of a quarter- wave attenuator type in a plurality of advantageous embodiments, and Figure 4 shows a variable reflection attenuator in a plurality of advantageous embodiments . DESCRIPTION OF THE PREFERRED EMBODIMENT The sound attenuator system according to the invention comprises an active, a reactive and a resistive part. The diffe- rent parts influence their own frequency ranges. Figure 1 shows the effect of the different parts of the system. Curve A represents the effect of the active attenuation. In a typical case, this part of the attenuation lies in the low- frequency range 50-250 Hz, designated L in the figure. Curve B represents the effect of the reactive attenuation. This typically lies in the intermediate-frequency range 250-630 Hz, designated M in the figure. Curve C indicates the effect of the resistive attenuation. This lies within a wide frequency range but substantially within the high-frequency range 630-5000 Hz, designate H in the figure. Finally, curve D indicates the attenuation resulting from the three coopera- ting attenuations. The resultant curve shows that a considerable attenuation is obtained even at low frequencies and that the attenuation at high frequencies is less than in known systems. This provides a possibility of being able to adapt the attenuation in a better way to the predominant sound picture from the fan. As previously mentioned, the effect of the three sound attenuation systems is substantially serial. The sound is first attenuated by the reactive system, whereupon the remaining sound is attenuated by the resistive system. Finally, the sound now remaining is attenuated by the active system. The sound attenuator system according to the invention is shown in more detail in Figure 2, where an active system comprises a first loudspeaker la, a second loudspeaker lb; a pair of upstream microphones 2a, 2b and a pair of downstream microphones 3a, 3b, as well as a control box 4. The control box comprises processors and electronic equipment for control of the system and communication with the world around. In the example shown it is placed in or near the channel system but may advantageously also be placed together with other control equipment for the ventilation system. The sound attenuator system also comprises a plurality of quarter-wave attenuators 5a, 5b which are placed on both sides of the continuous channel 6. In the example shown, the quarter-wave attenuators are arranged in packages 7a, 7b and transverse to the channel 6. The channel accommodates a pair of reflection attenuators 8a, 8b which comprise a resistive attenuation. In the figure shown, the resistive attenuation is brought about by a thick absorbent 9a, 9b which usually consists of mineral wool. Advantageously, the dynamic flow resistance should be between 0.5 and 2. Between the attenuator packages 7 and the reflection attenuator 8, a plane channel segment 10 is arranged. The object of this channel segment is that the orifice of the packages 7a, 7b should end up a quarter of a wavelength away from the channel of the reflection attenuator. In the example shown, the sound is assumed to enter the channel from the left. At the front edge of the reflection attenuator, a reflection wave is formed which causes the orifice of the quarter-wave attenuator to end up where an acoustic wave with the tuned frequency would have had a relative pressure maximum. The sound around the tuned frequency is hence subjected to a maladjustment and is prevented from propagating. The remaining sound passes the reflection attenuator which comprises an absorbent. In this portion, a large part of the high-frequency part of the remaining sound dissipates, or is transformed into heat. In the same portion of the attenuator, the remaining sound is subjected to an active sound generated by the loudspeakers, which sound is brought into opposition to the propagating sound. To determine the active sound, the two upstream microphones measure the sound some distance upstream of the loudspeakers. The measured sound must be corrected for the active sound which also propagates upstream. Finally, the two downstream microphones measure the remaining sound and pass information to the control box about fine adjustment of the system. Figure 3 shows a plurality of embodiments of a variable quarter-wave attenuator 11. Figure 3a shows an attenuator where a loudspeaker 12 at the bottom of the attenuator causes a controllable impedance. Figure 3b shows an attenuator 11 at the bottom of which a piston 13 is arranged, which is controllable by means of an actuator 14. A variant of this is shown in Figure 3c where a folded portion 15 of the attenuator 11 enables a lower portion 16 of the attenuator to be controlled by means of an actuator 14. Finally, Figure 3d shows an attenuator 11 directed along the channel. A piston 17 at the rear end is arranged to be controllably varied by means of an actuator 14. One advantage of this embodiment is that a plurality of attenuators arranged around the channel may be arranged to cooperate with one or more actuators. A couple of embodiments of a variable resistive attenuator are shown in Figure 4. Figure 4a shows an absorbent 20 which is lowered into a recess 21 in the channel. The bottom 22 of the recess is arranged flexible and connected to a controllable actuator. The actuator is brought to vibrate such that the sound penetrating through the absorbent experiences a greater or smaller impedance. Another way of arranging a variable impedance of a sound-absorbing wool is shown in Figure 4b. Here, the sound-absorbing wool 27 is wound on an inner tube 24 which consists of a perforated sheet. An outer tube 25 of perforated sheet is arranged outside the wool. The outer tube is provided with folded-up flaps 26a, 26b between which a gap is formed. An actuator 23 is arranged to variably bring the flaps away from and closer to each other, whereby the circumference of the outer tube is changed. In this way, the wool located between the inner fixed tube and the outer variable tube may be variably compressed and hence the impedance be changed. Although advantageous, the attenuation system according to the invention is not limited to the embodiments shown. Thus, the various attenuators may be placed in a row, that is, in series with each other without losing the effect. As previously pointed out, the reactive part may also comprise other types of resonance attenuators, such as Helmholtz resonators and membrane attenuators. The attenuation system may also optionally be placed in the channel system. In this case, it is advantageous, for example, to place an attenuation system at the orifice in a channel section which operates a room with strict requirements concerning sound. In case of large channel dimensions, it is advantageous to place a plurality of the sound attenuator according to the inven- tion in parallel. The resistive system may also comprise baffles in the channel comprising a porous absorbent. The active system may also comprise only one microphone. The most common practice is to include at least one downstream microphone .

Claims: CLAIMS 1. A sound attenuator in a system for transport of gas comprising active sound attenuation (A) and passive sound attenuation, characterized in that the passive sound attenuation comprises a reactive part (B) and a resistive part (C) , and that the active, the reactive and the resistive attenuation are each adapted to attend to attenuation within substantially their own frequency range. 2. A sound attenuator according to claim 1, characterized in that the active attenuation is adapted to influence the low- frequency range (L) , that the reactive attenuation is adapted to influence the intermediate-frequency range (M) , and that the resistive attenuation is adapted to influence the high- frequency range (H) . 3. A sound attenuator according to claim 1 or 2, characterized in that the reactive attenuation comprises a plurality of quarter-wave attenuators (5) arranged in packages (7 ) . 4. A sound attenuator according to any of the preceding claims, characterized in that the resistive attenuation is brought about by a reflection attenuator (6) comprising a porous absorbent ( 9) . 5. A sound attenuator according to any of the preceding claims, characterized in that the active attenuation com- prises a sound source (1), an upstream microphone (2), a downstream microphone (3) and a control device (4) comprising a processor for receiving and processing the microphone signals and controlling the sound source. 6. A sound attenuator according to any of the preceding claims, characterized in that the reactive attenuation com- prises a controllable actuator (14) by means of which the reactive impedance in the attenuator is variable, that the resistive attenuation comprises a controllable actuator (23) by means of which the resistive impedance in the attenuator is variable, and that the sound attenuator comprises a control member comprising a processor for controlling the reactor, the resistive and the active attenuation. 7. A sound attenuator according to claim 3, characterized in that the packages (7) are placed upstream of a reflection attenuator (6) . 8. A ventilation system for transport of air in environments involving sound requirements comprising a plurality of channel sections and at least one sound-attenuating device comprising active sound attenuation (A) and passive sound attenuation, characterized in that the passive sound attenuation comprises a reactive part (B) and a resistive part (C) , that the active attenuation is adapted to influence the low-frequency range (L) , that the reactive attenuation is adapted to influence the intermediate-frequency range (M) , and that the resistive attenuation is adapted to influence the high-frequency range (H) . 9. A ventilation system according to claim 8, characterized in that the active attenuation comprises a sound source, two microphones and a control device, that the reactive attenuation comprises a plurality of resonance attenuators, and that the resistive attenuation comprises a reflection attenuator comprising a porous absorbent. 10. A method for sound attenuation in a system for transport of gas comprising active sound attenuation (A) and passive sound attenuation, characterized in that the passive sound attenuation is adapted to comprise a reactive part and a resistive part, that the active sound attenuation is adapted to influence the low-frequency range (L) , that the reactive attenuation is adapted to influence the intermediate- frequency range (M) , and that the resistive attenuation is adapted to influence the high-frequency range (H) . 11. A method according to claim 10, characterized in that the reactive attenuation is adapted to comprise a plurality of quarter-wave attenuators (5) which are arranged in packages (7) . 12. A method according to claims 10-11, characterized in that the resistive attenuation is adapted to comprise a reflection attenuator which is brought to comprise a porous absorbent. 13. A method according to claims 10-12, characterized in that the active attenuation is adapted to comprise a sound source (1), an upstream microphone (2), a downstream microphone (3), and a control device (4) comprising a processor which is adapted to receive and process the microphone signals and to control the sound source. 14. A method according to claims 10-13, characterized in that the .reactive attenuation is brought to- comprise a controllable actuator (14) ' , by means of which the reactive impedance in the attenuator is varied, that the resistive attenuation is brought to comprise a controllable actuator (23) by means of which the resistive impedance in the attenuator is varied, and that the sound attenuator is brought to comprise a control member comprising a processor which, on the basis of a computer program, is adapted, according to desire, to control the reactive, the resistive and the active sound attenuation. 15. A computer program comprising instructions for influen- cing a processor, in a sound attenuator comprising a variable reactive attenuation, a variable resistive attenuation and a variable active attenuation, to control, according to desire, the reactive, the resistive and the active sound attenuation. 16. A computer program according to claim 15 provided via a network, such as the Internet. 17. A computer-readable medium, characterized in that the medium comprises a computer program according to claim 15.

Inventors: AABOM MATS, JOHANSSON CLAES-GOERAN, LARSSON MATS, NYGREN TOMAS, AABOM, MATS, JOHANSSON, CLAES-GOERAN, LARSSON, MATS, NYGREN, TOMAS
Applicants: AABOM MATS, FLAEKT AB, JOHANSSON CLAES GOERAN, LARSSON MATS, NYGREN TOMAS, AABOM, MATS, ABB FLAEKT AB, JOHANSSON, CLAES-GOERAN, LARSSON, MATS, NYGREN, TOMAS
Application year: 2001
Main classification: E04F17
IPC codes: F24F13, G10K11