METHOD AND APPARATUS FOR ATTENUATING SOUND AND ACOUSTIC NOISE

Abstract: A method and an apparatus for attenuating sound and acoustic noise, wherein there is formed a configuration of acoustic absorbers which prevent unwanted sound from passing the configuration. On the basis of measurements of the sound field which is wanted to be attenuated, and by means of developed computer programs, there is effected a calculation of necessary transfer functions for electro-acoustic transducers of point-absorber type, and thereafter the sound-attenuating configuration of such absorbers (2) is calculated. The electro-acoustic transducers (2) of point-absorber type may, for example, consists of microphones.

Description: Method and apparatus for attenuating sound and acoustic noise The invention relates to a method for attenuating sound and acoustic noise, wherein there is formed a configu¬ ration of acoustic absorbers which prevent unwanted sound from passing the configuration. Further, the invention re¬ lates to an apparatus for such attenuation of sound and acoustic noise. Acoustic noise and high sound level is a substantial environmental problem which is the subject of an increasing interest. Various measures have been used for reducing this problem. A common measure has been to use barriers of diffe¬ rent configurations to prevent propagation of the noise. Such barriers are for example reflecting and/or attenuating sound shields, 2- and 3-layer insulating glass in windows, sound-absorbing coatings and intermediate insulations on or in buildings, machines and traffic devices. Measures of the above-mentioned type are encumbered with different drawbacks: - They constitute a permanent installation and/or structure change. - They also affect the transport of people, animals, organisms, heat, electricity and air through the noise-preventing barrier. - They are badly suited for temporary installation/ removal. It is also known to use so-called counter-sound sys¬ tems for preventing the propagation of sound. Counter-sound systems are characterized in that electrical energy is supplied to electro-acoustic transducers, so that a sound field is set up which interferes with the in-coming sound field and attenuates and amplifies, respectively, this field in different places. Further, there are previously known different types of acoustic absorbers for the attenuation or deadening*-of sound. For example, such absorbers and sound-deadening arrangements are known from US Patent Specifications 2,502,017 and 2,502,019. These known devices are especially intended for indoor assembly, for sound absorption in living rooms or other premises. The individual absorbers according to these patent specifications are hollow bodies the natural frequencies of which are determined by the mechanical di en- sions of the absorbers. Within optics, electromagnetic wave propagation, pro- pagation of waves in elastic materials and gravitation waves, it is further known that it is possible to observe or form so-called point absorbers (resonance absorption of light in atoms, antennas, vibration absorbers, wave power plants) having a so-called absorption cross-section/volume or a so- called absorption length which is greater than the point absorber's own geometrical extent. This only applies when the frequency of the incident waves causes the absorbers to ope¬ rate in a resonant mode. The above principle especially applies when it is the question of common antennas for electromagnetic waves, and in addition in the recent development of wave power plants. Within the acoustics, the principles have, as far as one knows, only been used in so-called acoustic filters. The object -of the invention is to provide a method and an apparatus making it possible to prevent propagation of sound and noise without being encumbered to any appre¬ ciable extent with the above-mentioned drawbacks in con¬ nection with sound-deadening barriers according to the prior art. For achieving the above object there is provided a method of the introductorily stated type which, according to the invention, is characterized in that, on the basis of measurements of the sound field which is wanted to be attenuated, and by means of developed computer programs, there is effected a calculation of necessary transfer func- tions for electro-acoustic transducers of point-absorber type, and thereafter a calculation of the sound-attenuating configuration of such absorbers. The provided apparatus according to the invention comprises a configuration of acoustic absorbers which pre¬ vent unwanted sound from passing the configuration, and is characterized in that the absorbers are constituted by electro-acoustic transducers of point-absorber type having transfer functions which have been calculated on the basis of measurements of the sound field which is wanted to be attenuated, and by means of developed computer programs for calculating said transfer functions. With the expression "electro-acoustic transducers of point-absorber type" it is meant that the characteristic dimensions of the transducers are much smaller than the wave length, i.e. not absorbers of diffraction type. With the expression "transfer functions" is meant the relation between the incoming sound signal and the electri¬ cal signal supplied by the transducer to the electrical- energy consuming element. The electro-acoustic transducers may be of any of the existing types, but is, for example, of electro-dynamic, electro-magnetic, capacitive, piezoelectric or piezoresistive type. As regards the above-mentioned counter-sound systems, it is to be pointed out that .the apparatus according to the present invention is substantially different from the counter-sound systems, since in the present apparatus in principle no energy is supplied to or added to the sound field. The present invention utilizes electro-acoustic tran-s- ducers of point-absorber type in one-, two- and three- dimensional configurations or arrangements. The electro-.- acoustic transducers may be microphones of small and cheap.- construction. As topical microphone -types there may be mentioned electro-dynamic microphones, capacitive micro- phones, coal-dust microphones, piezoelectric or piezoresi¬ stive microphones, etc. The microphones are tuned to one or more desired resonance frequencies and transfer functions. The tuning of the individual microphone is done by means of its mechanical dimensioning and/or electrical means connec¬ ted to the output of the microphone. The electrical means may for example be passive filters or electronic filters of known type. Configurations of several microphones will also be able to be given natural frequencies in addition to the natural frequencies of the individual microphones. These natural frequencies are chosen by means of the pattern of the placing of the microphones in the configuration. Such configurations will prevent the propagation of sound and noise without being encumbered to any appreciable extent with the drawbacks mentioned in the introduction. Further, they will have the advantage that, by means of electronic methods, they may be given a characteristic or a transfer function which can be varied from time to time at will. An apparatus according to the invention will be able to assume an almost unlimited number of different configu¬ rations. In the different configurations, the point absor¬ bers forming part thereof may be arranged in a pattern which is suitable to the purpose and which may be calcula¬ ted by means of sophisticated, mathematical calculations. The invention will " be further described below with reference to the drawing which, in schematic form, shows exemplary embodiments of an apparatus according to the inven- tion, and wherein Figs. 1A and IB show a . longitudinal section and a cross-section, respectively, of a tube containing an array of electro-acoustic transducers; Fig. 2 shows an array corresponding to Fig. 1, but wherein the transducers are connected to passive, electri¬ cal elements; Fig. 3 shows an array corresponding to Fig. 1, but wherein the transducers are connected to active electronic elements; and Fig. 4 shows a tube with an array of a pair of electro- acoustic transducers to which there is connected a device for calculating and generating a signal to the transducers. In the drawing, where like reference numerals desig¬ nate similar elements in the various Figures, there is started from a simple, but illustrating embodiment of the apparatus according to the invention. This is shown in its simplest form in Figs. 1A and IB showing a part of a tube 1 in which there is arranged an array of three electro- acoustic transducers 2 of point absorber type. The trans¬ ducers are arranged aligned after each other and are located in the longitudinal axis of the tube, the transducers being schematically shown to be supported by supports 3. The trans- ducers 2 consist of microphones, and as shown these have no external connections. The sound or noise enters in the direc¬ tion of the shown arrow A, and the transfer functions (resonance frequencies) of the microphones are adapted such that the sound in question is effectively attenuated when p ssing the microphones. The sound is gradually attenuated when passing the microphones and will be substantially eli¬ minated in the frequency range of interest by the use of for example 5 - 7 microphones. The shown embodiment may be adapted for many fields of application, for example for the reduction/elimination of sound from ventilators in venting ducts, etc. In the embodiment of Fig. 2, three microphones 4 are shown to be arranged in a manner corresponding to that of Fig. 1, but each of the microphones is here connected to passive electrical elements 5, e.g. in the form of passive electrical filters, which affect the transfer functions of the microphones in the desired manner. Fig. 3 also shows a configuration corresponding to that of Fig. 1, but the three microphones 6 are here connec- ted to active electronic elements 7 for suitable effect or influence on the transfer functions of the microphones. In Fig. 4, two microphones 8 are shown to be disposed in a manner corresponding to that of the microphones in Figs. 1 - 3. However, i-n this embodiment the microphone out- puts are connected to the input to a device 9 which, from the -supplied signal, i.e. from the energy absorbed by the transducers 8, successively calculates and generates an electrical signal which is supplied to the transducers in order to affect the transfer functions thereof in such- a manner that an attenuation which is as effective as possible is achieved, for example in connection with a varying noise field. The device 9 may essentially consist of a micro¬ processor for the calculation of said signal, and an a pli- fier circuit for supplying the signal with a suitable level to the transducers. As one will understand, apparatuses according to the invention may be adapted for many different fields of appli- cation. Some additional examples will be mentioned below, wherein the utilized electro-acoustic transducers of point absorber type, with possible associated electrical/electro¬ nic connections, for the sake of brevity will be designated only as point absorbers. 1) Attenuation of sound through an open window: One or more point absorbers are suspended in the window opening, in a suitable (calculated) configuration. 2) Attenuation of sound by the placement of point absorbers on window frames and/or window bars: This will permit sound attenuation with opened as well as closed window. 3) Attenuation of sound from machine structures, building sites etc. : Fence structures (permanent or movable) are provided with point absorbers in the crossing points of the fence wires. The fence wire is presupposed to be electrically conducting (steel, aluminium or synthetic fibers with spun-in conductive wire, e.g. made of stain¬ less steel, iron, copper, carbon, etc.) . 4) Sound-attenuating curtains: Mesh nets of conduc¬ tive wire are shaped as curtains provided with a configura¬ tion of point absorbers for sound/noise attenuation in living rooms, around permanent or temporary noise sources in living rooms, dwellings, outdoor areas, etc. 5) Attenuation of sound from road traffic: The partly disfiguring sound shields which are today placed along particularly trafficked roads, may be replaced by fences or rows of light weight posts carrying configurations of point absorbers. It will be easily possible to make these devices self-indicating, i.e. that they are, for example, provided with light signals which at any time indicate the current sound level.

Claims: P a t e n t c l a i m s 1. A method for attenuating sound and acoustic noise, wherein there is formed a configuration of acoustic absor- bers which prevent unwanted sound from passing the configu¬ ration, c h a r a c t e r i z e d i n that, on the basis of measu¬ rements of the sound field which is wanted to be attenuated, and by means of developed computer programs, there is effec- ted a calculation of necessary transfer functions for elec¬ tro-acoustic transducers of point-absorber type, and there¬ after calculation of the sound-attenuating configuration of such absorbers. 2. An apparatus for attenuating sound and acoustic noise, comprising a configuration of acoustic absorbers which prevent unwanted sound from passing the configuration, c h a r a c t e r i z e d i n that the absorbers are con¬ stituted by electro-acoustic transducers (2) of point-absorber type having transfer functions which have been calculated on the basis of measurements of the sound field which is wanted to be attenuated, and by means of developed computer programs for calculating said transfer functions. 3. An apparatus according to claim 2, c h a r a c t e r i z e d i n that the transfer function of at least one transducer (2) essentially is determined only by its electrical and mechanical dimensioning. " - 4. An apparatus according to claim 2 or 3, c h a r a c t e r i z e d i n that at least one transducer (4) is connected to passive electrical elements (5) affecting its transfer function. 5. An apparatus according to claim 2, 3 or 4, c h a r a c t e r i z e d i n that at least one transducer (6) is connected to active electronic elements (7) affecting its transfer function. 6. An apparatus acccording to any of the claims 2 - 5, c h a r a c t e r i z e d i.n that to at least one of the transducers (8) there is connected a device (9) which, from the energy absorbed by the transducer, successively cal- culates and generates an electrical signal which is supplied to the transducer (8) and affects the transfer function there¬ of.

Inventors: BOENKE KNUT, BOENKE, KNUT
Applicants: BOENKE KNUT, BOENKE, KNUT
Application year: 1986
Main classification: G10K11
IPC codes: G10K11