ACTIVE ACOUSTIC ATTENUATION SYSTEM FOR HIGHER ORDER MODE NON-UNIFORM SOUND FIELD IN A DUCT

Abstract: A system is provided for increasing the frequency range of an active acoustic attenuation system in a duct without increasing cut-off frequency f c of the duct or otherwise splitting or partitioning the duct into separate ducts or chambers. The frequency range is increased above f c to include higher order modes. A plurality of cancelling model sets are provided. Each transverse portion of the acoustic pressure wave has its own set of an adaptive filter model, cancelling speaker, and error microphone. A single input microphone may service all sets.

Description: The invention relates to active acoustic attenuation systems, and more particularly provides a system suitable for cancelling undesirable output sound in a duct for higher order mode non-uniform sound fields. The invention arose during continuing development efforts relating to the subject matter shown and described in U.S. Patents 4,677,677, 4,677,676 and 4,665,549, and allowed U.S. application S.N. 992,282, filed October 23, 1986, all assigned to the assignee of the present invention and incorporated herein by reference. A sound wave propagating axially through a rectangular duct has a cut-off frequency f c = c 2L where c is the speed of sound in the duct and L is the longer of the transverse dimensions of the duct. Acoustic frequencies below the cut-off frequency f c provide plane and uniform pressure acoustic waves extending transversely across the duct at a given instant in time. Acoustic frequencies above f c allow non-uniform pressure acoustic waves in the duct due to higher order modes. For example, an air conditioning duct may have transverse dimensions of 0.61m by 1.83m. The longer transverse dimension is 1.85m. The speed of sound in air is 344m per second. Substituting these quantities into the above equation yields a cut-off frequency f c of 94 Hertz. In circular ducts similar considerations apply when the duct diameter is approximately equal to one-half of the wavelength. Exact equations may be found in L. J. Eriksson, Journal of Acoustic Society of America, 68(2), Aug. 1980, pp. 545-550. Active attenuation involves injecting a cancelling acoustic wave to destructively interfere with and cancel an input acoustic wave. In the given example, the acoustic wave can be presumed as a plane uniform pressure wave extending transversely across the duct at a given instant in time only at frequencies less than 94 Hertz. At frequencies less than 94 Hertz, there is less than a half wavelength across the longer transverse dimension of the duct. At frequencies above 94 Hertz, the wavelength becomes shorter and there is more than a half wavelength across the duct, i.e. a higher order mode with a non-uniform sound field may propagate through the duct. In an active acoustic attenuation system, the output acoustic wave is sensed with an error microphone which supplies an error signal to a control model which in turn supplies a correction signal to a cancelling loudspeaker which injects an acoustic wave to destructively interfere with the input acoustic wave and cancel same such that the output sound at the error microphone is zero. A known method and apparatus for the attenuation of sound is shown in UK-A-2088951. In this document, an upstream microphone array disposed in a duct, senses sound in the duct and converts it to an electrical signal. A modified adaptive filter generates a cancelling sound to be emitted by a speaker in the duct and a further downstream error microphone detects the sound resulting from the combination of the source sound and the cancelling sound. This microphone output is employed as an error signal by the adaptive filter which adjusts the signal driving the speaker so that the cancelling signal more nearly approximates the mirror image of the source sound. If the sound wave travelling through the duct is a plane wave having a uniform pressure across the duct, then it does not matter where the cancelling speaker and error microphone are placed along the cross section of the duct. In the above example for a 0.61m by 1.83m duct, if a plane wave with uniform pressure is desired, the acoustic frequency must be below 94 Hertz. if it is desired to attenuate higher frequencies using plane uniform pressure waves, then the duct must be split into separate ducts of smaller cross section or the duct must be partitioned into separate chambers to reduce the longer transverse dimension L to less than c / 2f at the frequency f that is to be attenuated. In the above example, splitting the duct into two separate ducts with a central partition would yield a pair of ducts each having transverse dimensions of 0.61m by 0.91m. Each duct would have a cut-off frequency f c of 188 Hertz. The above noted approach to increasing the cut-off frequency f c is not economically practicable because active acoustic attenuation systems are often retrofitted to existing ductwork, and it is not economically feasible to replace an entire duct with separate smaller ducts or to insert partitions extending through the duct to provide separate ducts or chambers. According to one aspect of the present invention there is provided an active attenuation system for attenuating in an acoustic system an undersired elastic wave propagating in an elastic medium, said elastic wave having non-uniform pressure distribution in said medium at a given instant in time along a direction transverse to the direction of propagation such that said wave has a plurality of portions along the transverse direction including at least one positive pressure portion and at least one negative pressure portion, the system comprising:- a plurality of output transducers, one for each of said positive and negative pressure portions of said undesired elastic wave, said output transducers introducing a plurality of cancelling elastic waves into said medium; a plurality of error transducers, one for each of said positive and negative pressure portions of said undesired elastic wave, said error transducers sensing the combined said undesired elastic wave and said cancelling elastic waves and providing a plurality of error signals; and a plurality of adaptive filter models modelling said acoustic system, one for each of said positive and negative pressure portions of said undesired elastic wave, each said model having an error input from a respective said error transducer and outputting a correction signal to a respective said output transducer to introduce a respective said cancelling elastic wave, such that each said portion of said undesired elastic wave has its own set of an adaptive filter model, output transducer, and error transducer. According to another aspect of the present invention there is provided a method for attenuating in an acoustic system an undesired elastic wave propagating in an elastic medium, said elastic wave having non-uniform pressure distribution in said medium at a given instant in time along a direction transverse to the direction of propagation such that said wave has a plurality of portions along the transverse direction including at least one positive pressure portion and at least one negative pressure portion, the method comprising the steps of:- introducing a plurality of cancelling elastic waves into said medium from a plurality of output transducers, one for each of said positive and negative pressure portions of said undesired elastic wave, for attenuating said undesired elastic wave; sensing the combined said undesired elastic wave and said cancelling elastic waves with a plurality of error transducers, one for each of said positive and negative pressure portions of said undesired elastic wave, and providing a plurality of error signals; and modelling said acoustic system with a plurality of adaptive filter models, one for each of said positive and negative pressure portions of said undesired elastic wave, each said model having an error input from a respective said error transducer and outputting a correction signal to a respective said output transducer to introduce a respective said cancelling elastic wave, such that each said portion of said undesired elastic wave has its own set of an adaptive filter model, output transducer, and error transducer. Preferably the attenuation is for an acoustic system including an axially extending duct having an input for said undesired elastic wave which propagates therethrough to an output and wherein said undesired elastic wave has N portions extending transversely across the duct, N being ≧ 2, and includes said at lease one positive pressure portion and said at lease one negative pressure portion. The invention eliminates the need to reduce the longer transverse dimension L of the duct to less than c / 2f . Instead, the invention increases the frequency range above f c to include higher order modes. Consequently, the present invention provides a method for increasing the frequency range of an active acoustic attenuation system in a duct without increasing cut-off frequency f c of the duct or otherwise splitting the duct into separate ducts or partitioning the duct into separate chambers. This solves the above noted problem in a particularly simple and cost effective manner. A plurality N of cancelling model sets can be provided. Each set has its own adaptive filter model, cancelling speaker, and error microphone. A single input microphone may service all sets. The duct has a transverse dimension greater than a half wavelength, and there is non-uniform acoustic pressure transversely across the duct at a given instant in time. Thus the invention can also be used with modes that have non-uniform pressure distribution in both transverse dimensions of a rectangular or other shape duct. The invention may also be used with modes that have non-uniform pressure distribution in both the radial and circumferential dimensions of a circular duct. Examples of the present invention will now be described with reference to the accompanying drawings, in which:- FIG. 1 is a schematic illustration of acoustic system modeling in accordance with above noted incorporated U.S. patents 4,677,676 and 4,677,677. FIG. 1 shows the acoustic pressure distribution of the plane wave mode. FIG. 2 is a sectional view of the acoustic pressure distribution taken along line 2-2 of the duct of FIG. 1. FIG. 3 is a schematic illustration showing the duct of FIG. 1 and the acoustic pressure distribution of the first higher order mode. FIG. 4 is a sectional view of the acoustic pressure distribution taken along line 4-4 of FIG. 3. FIG. 5 is a schematic illustration showing the duct of FIG. 1 and the acoustic pressure distribution of the second higher order mode. FIG. 6 is a sectional view of the acoustic pressure distribution taken along line 6-6 of FIG. 5. FIG. 7 is a schematic illustration of an active acoustic attentuation system embodying the invention. FIG. 1 shows a modeling system in accordance with incorporated U.S. Patent 4,677,677, FIG. 5, and like reference numerals are used from said patent where appropriate to facilitate clarity. The acoustic system 2 includes an axially extending duct 4 having an input 6 for receiving input noise and an output 8 for radiating or outputting output noise. The acoustic wave providing the noise propagates axially left to right through the duct. The acoustic system is modeled with an adaptive filter model 40 having a model input 42 from input microphone or transducer 10 and an error input 44 from error microphone or transducer 16, and outputting a correction signal at 46 to omnidirectional output speaker or transducer 14 to introduce cancelling sound waves such that the error signal at 44 approaches a given value such as zero. The cancelling acoustic wave from output transducer 14 is introduced into duct 4 for attenuating the output acoustic wave. Error transducer 16 senses the combined output acoustic wave and cancelling acoustic wave and provides an error signal at 44. The acoustic system is modeling with an adaptive filter model 40, as in the noted incorporated patents. The input acoustic wave is sensed with input transducer 10, or alternatively an input signal is provided at 42 from a tachometer or the like which gives the frequency of a periodic input acoustic wave, such as from an engine or the like, without actually measuring or sensing such noise. FIG. 2 shows a cross sectional view of duct 4 at a given instant in time for the above noted example, where the duct has transverse dimensions of 0.61m by 1.83m. The cut-off frequency f c of the acoustic wave travelling axially in the duct (out of the page in FIG. 2) is given by f c = c 2L , where f c is the cut-off frequency, c is the speed of sound in the duct, and L is the longer of the transverse dimensions of the duct, namely 1.83m. Thus in the example given, f c = 94 Hertz. Acoustic frequencies below 94 Hertz provide plane and uniform pressure acoustic waves in the duct. This is shown at wave 402 in FIG. 1 having positive pressure across the entire transverse dimension of the duct at a given instant in time as shown at the plus sign 402 in FIG. 2. At acoustic frequencies greater than f c , there may be a non-uniform acoustic pressure wave at a given instant in time across the duct due to higher order modes. This is because the transverse dimension of the duct is greater than one-half the wavelength of the acoustic wave. FIG. 3 shows the first higher order mode wherein the acoustic frequency is greater than f c . In the example shown, for a 0.61m by 1.83m duct, the acoustic frequency is greater than 94 Hertz. The acoustic wave at a given instant in time has a positive pressure portion 404, as shown in FIG. 3 and at the plus sign in FIG. 4. At the same given instant in time, the acoustic wave also has a negative pressure portion 406, as shown in FIG. 3 and at the minus sign in FIG. 4. This first higher order mode has a node 408 between wave portions 404 and 406. FIGs. 5 and 6 show the second higher order mode with a portion 410 of positive pressure, a portion 412 of negative pressure, and a portion 414 of positive pressure, separated by respective nodes 416 and 418 at a given instant in time. The acoustic frequency is greater than 2f c , i.e. greater than 188 hertz. In the second higher order mode, there are two pressure nodes 416 and 418, each separating a portion of the acoustic wave of positive and negative pressure. Further higher order modes continue in like manner. For example, the third higher order mode associated with the transverse dimension L has four Portions separated by three pressure nodes at a given instant in time. One manner of insuring plane uniform pressure acoustic waves across the transverse dimension of the duct at a given instant in time is to increase the cut-off frequency f c . This may be accomplished by splitting the duct into separate ducts or partitioning the duct into separate chambers to reduce the longer transverse dimension L to less than c / 2f . For example, in FIG. 6, partitions may be provided axially longitudinally to split or partition the duct into three separate ducts or chambers each having transverse dimensions of 0.61m by 0.91m, such that only a half wavelength at 282 hertz can fit within each duct chamber. This raises the overall cut-off frequency to 282 hertz, without higher order modes in any of the separate chambers. This enables active acoustic attenuation of plane uniform pressure acoustic waves of frequencies up to 282 hertz. Most active acoustic attenuation systems are retrofitted to existing ductwork, and hence the above noted approach of partitioning the duct into separate ducts or chambers is usually not economically feasible because of the substantial installation and retrofit cost of installing such partitions in existing ductwork. Without the partitions, only frequencies below 94 hertz, in the above example, will have a plane uniform pressure acoustic wave across the duct free of higher order modes. The present invention provides a system for increasing the frequency range of an active acoustic attenuation system without increasing cut-off frequency f c or otherwise splitting the duct into separate ducts or partitioning the duct into separate chambers to reduce the longer transverse dimension L to less than c / 2f . FIG. 7 shows a system in accordance with the invention, and uses like reference numerals from FIG. 1 and the above noted incorporated patents where appropriate to facilitate clarity. A plurality of cancelling acoustic waves are output into the duct from a plurality of output transducers or speakers 14, 214, 314, one for each negative or positive pressure portion of the acoustic wave, for attenuating the output acoustic wave providing the output noise. The combined output acoustic wave and the cancelling acoustic waves are sensed by a plurality of error transducers or microphones 16, 216, 316, one for each portion of the acoustic wave, respectively, which error microphones provide error signals at 44, 244, 344, respectively. The acoustic system is modeled with a plurality of adaptive filter models 40, 240, 340, one for each portion of the acoustic wave, respectively. Each adaptive filter model has an error input 44, 244, 344, from a respective one of the error microphones and outputs a correction signal at 46, 246, 346, to a respective one of the output speakers 14, 214, 314, to introduce the respective auxiliary cancelling acoustic wave. The sound from speaker 14 travels back along a feedback path to the input transducer provided by input microphone 10. Likewise, sound from speakers 214 and 314 travel back along feedback paths to input microphone 10. The feedback path from speaker 14 to input microphone 10 is modeled with the same model 40 such that model 40 adaptively models both the acoustic system 4 and the feedback path. Likewise, the feedback path from speaker 214 to input microphone 10 is modeled with the same model 240 such that model 240 adaptively models both acoustic system 4 and the noted feedback path. Likewise, the feedback path from speaker 314 to input microphone 10 is modeled with the same model 340 such that model 340 adaptively models both duct 4 and the noted feedback path. None of the models 40, 240 or 340 uses separate on-line modeling of duct 4 and off-line modeling of the respective feedback path. Off-line modeling of the respective feedback paths using broadband noise to pre-train a separate dedicated feedback filter is not necessary. The feedback path is part of the model used for adaptively modeling the entire system. Each model is an adaptive recursive filter model having a transfer function with both poles and zeros, as in the noted incorporated patents. The use of poles to model the feedback path is significant. Individual finite impulse response (FIR) filters are not adequate to truly adaptively cancel direct and feedback noise. Instead, a single infinite impulse response (IIR) filter is needed to provide truly adaptive cancellation of the direct noise and acoustic feedback. Thus, each of models 40, 240 and 340 adaptively recursively models the acoustic system and the feedback path on-line. Since each model is recursive, it provides the IIR characteristic present in the acoustic feedback loop wherein an impulse will continually feed upon itself in feedback manner to provide an infinite response. The feedback path from speaker 14 to input microphone 10 is modeled by using the error signal at 44. The feedback paths from speakers 214 and 314 to input microphone 10 are modeled by using the respective error signals at 244 and 344 from respective error microphones 216 and 316. The feedback path from speaker 14 to input microphone 10 is modeled by using the error signal at 44 as one input to model 40 and the correction signal at 46 as another input to model 40, FIG. 7 of incorporated U.S. Patent 4,677,676. Likewise, each of the feedback paths from speakers 214 and 314 to input microphone 10 are modeled by using the respective error signals at 244 and 344 from the respective error microphones 216 and 316 as one input to the respective models 240 and 340 and the respective correction signals 246 and 346 to the respective speakers 214 and 314 as another input to the respective model 240 and 340 as in FIG. 7 of incorporated U.S. Patent 4,677,676. The system of FIG. 7 increases the frequency range of the active acoustic attenuation system above f c . N acoustic waves are output into the duct from N output transducer speakers 14, 214, 314, for attenuating the output acoustic wave providing the output noise at 8. The combined output acoustic wave and the N acoustic waves from the N speakers are sensed with N error transducers 16, 216, 316, providing N error signals 44, 244, 344. The acoustic system is modeled with N adaptive filter models 40, 240, 340, having error inputs from respective error microphones 16, 216, 316, and outputting N correction signals 46, 246, 346, respectively, to the N speakers 14, 214, 314, such that the N error signals approach respective given values. In FIG. 7, N = 3. N equals the number of portions of negative and positive pressure present in the acoustic wave extending transversely across the duct at a given instant in time. For example, in a first higher order mode system, N = 2. In a second higher order mode system, N = 3, as in FIG. 7. One or more input signals representing the input acoustic wave providing the input noise at 6 are provided to the adaptive filter models 40, 240, 340. Only a single input signal need be provided, and the same such input signal may be input to each of the adaptive filter models, at 42. In FIG. 7, an input microphone 10 provides a single input transducer sensing the input acoustic wave and supplying such input signal. Alternatively, the input signal may be provided by a transducer such as a tachometer which provides the frequency of a periodic input acoustic wave such as from an engine or the like. Further alternatively, the input signal may be provided by one or more error signals, in the case of a periodic noise source, J.C. Burgess, Journal of Acoustic Society of America, 70(3), Sep. 1981, pp. 715-726. Further alternatively, a plurality of input transducers such as microphones 10, 210, 310, may be provided, each sensing the input noise and providing a separate input signal respectively to models 40, 240, 340. It has been found that multiple input microphones are not needed. It is believed that this is because the acoustic pressure at position 10 is related to the acoustic pressure at the other positions such as 210 and 310 by appropriate transfer functions which are adaptively modeled and compensated in the respective models by the coefficients in the numerators and denominators of the IIR pole-zero filter models, particularly if a high number of coefficients are used. In FIG. 7, N random noise sources 140, 241, 341, introduce noise into each of the N models 40, 240, 340, respectively, such that each of the N error microphones 16,216,316, respectively, also senses the auxiliary noise from the auxiliary noise sources and additionally models each respective output transducer speaker 14, 214, 314, and each respective error path from each respective speaker to each respective error microphone 16, 216, 316, respectively, all on-line without separate modeling and without dedicated pretraining, as in FIGs. 19 and 20 of incorporated U.S. Patent 4,677,676. The noise from each auxiliary noise source is random and uncorrelated to the input acoustic wave providing the input noise at 6, and is provided by a Galois sequence, M.P. Schroeder, "Number Theory in Science and Communications", Berlin: Springer-Verlag, 1984, page 252-261. The Galois sequence is a psuedorandom sequence that repeats after 2 M - 1 , where M is the number of stages in a shaft register. The Galois sequence is preferred because it it easy to calculate and can easily have a period much longer than the response time of the system. The auxiliary noise sources 140, 241, 341, enable additional adaptive modeling of the characteristics of each of the speakers 14, 214, 314, and the error paths from such speakers to the output microphones, 16, 216, 316, on an on-line basis. In one embodiment, local baffles 4a, 4b, are provided in duct 4 between the speakers 14, 214, 314, to minimize interaction between the speakers. The baffles are local and extend only adjacent the speakers, and do not extend along the length of the duct nor between the output microphones 16, 216, 316. Local baffles are easy to install during installation of the speakers 14, 214, 314, and do not involve substantial additional retrofit cost as compared to splitting or otherwise partitioning the duct into separate ducts or chambers along the entire or substantially the entire axial length thereof. Each model 40, 240, 340, comprises a recursive least mean square filter including a first algorithm 12, FIG. 7 of incorporated U.S. Patent 4,677,676, having a first input 42 from the input microphone, a second input 49 from its respective error signal 44 from its respective error microphone, and an output, and a second algorithm 22 having a first input from its respective correction signal 46 to its respective output speaker, a second input 47 from its respective error signal 44 from its respective error microphone, and an output, and a summing junction 48 having inputs from the outputs of the first and second algorithms, and an output providing the respective correction signal 46 to the respective one of the N output speakers. In another embodiment, FIGs. 8 and 9 of incorporated U.S. Patent 4,677,676, each of the N models 40, 240, 340, includes a first algorithm 12 having a first input 42 from the input microphone, a second input 49 from the respective error signal 44 from its respective one of the N error microphones, and an output, a first summing junction 52 having a first input from the respective error signal 44 from the respective one of the N error microphones, a second input from the respective correction signal 46 to the respective one of the N speakers, and an output 54, second algorithm means 22 having a first input from the output 54 of the first summing junction 52, a second input 47 from the respective error signal 44 from the respective one of the N error microphones and an output, and a second summing junction 48 having inputs from the outputs of the first and second algorithms 12 and 22, and an output providing the respective correction signal 46 to the respective one of the N output speakers. The system of FIG. 7 may be extended for use in both transverse dimensions of the duct for applications where both transverse dimensions are greater than a half wavelength resulting in higher order modes that have non-uniform sound fields in both transverse directions at a given instant in time. The system of FIG. 7 may be extended for use in circular ducts containing higher order modes that have non-uniform sound fields in both radial and circumferential directions at a given instant in time. In general, the active attenuation system of FIG. 7 may be used for attenuation of an undesired elastic wave in an elastic medium. The elastic wave has non-uniform pressure distribution in the medium at a given instant in time along a direction transverse to the direction of propagation such that the wave has a plurality of portions along the transverse direction at the given instant in time including at least one positive pressure portion and at least one negative pressure portion. It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.

Claims: An active attenuation system for attenuating in an acoustic system an undesired elastic wave propagating in an elastic medium, said elastic wave having non-uniform pressure distribution in said medium at a given instant in time along a direction transverse to the direction of propagation such that said wave has a plurality of portions along the transverse direction including at least one positive pressure portion (404) and at least one negative pressure portion (406), characterized in that it comprises : a plurality of output transducers (14, 214, 314), one for each of said positive and negative pressure portions of said undesired elastic wave, said output transducers introducing a plurality of cancelling elastic waves into said medium; a plurality of error transducers (16, 216, 316), one for each of said positive and negative pressure portions of said undesired elastic wave, said error transducers sensing the combined said undesired elastic wave and said cancelling elastic waves and providing a plurality of error signals (44, 244, 344); and a plurality of adaptive filter models (40, 240, 340) modelling said acoustic system, one for each of said positive and negative pressure portions of said undesired elastic wave, each said model having an error input (44, 244, 344) from a respective said error transducer and outputting a correction signal (46, 246, 346) to a respective said output transducer to introduce a respective said cancelling elastic wave, such that each said portion of said undesired elastic wave has its own set of an adaptive filter model, output transducer, and error transducer. An active attenuation system according to claim 1 for an acoustic system including an axially extending duct (4) having an input (6) for said undesired elastic wave which propagates therethrough to an output (8) and wherein said undesired elastic wave has N portions extending transversely across the duct, N being ≧ 2, and includes said at least one positive pressure portion and said at least one negative pressure portion, the system including:- N said adaptive filter models adaptively modelling said acoustic system, each model having a respective said error input (44, 244, 344) from a respective one of N said error transducers and outputting a said correction signal (46, 246, 346) to a respective one of N said output transducers (14, 214, 314) to introduce a respective one of N said cancelling waves such that each of said N error signals approaches a given value. An active attenuation system according to claim 2 comprising input transducer means (10, 210, 310) providing one or more input signals (42) representing said undesired elastic wave at said input, and wherein each of said N filter models adaptively models said acoustic system on-line without dedicated off-line pre-training and also adaptively models the feedback path from the respective one of said N output transducers to said input transducer means on-line for both broad band and narrow band acoustic waves without dedicated off-line pre-training, and outputs its respective said correction signal to its respective one of said N output transducers to introduce its respective one of said N cancelling waves. An active attenuation system according to claim 3 wherein each of said N models comprises means adaptively modelling its respective said feedback path as part of said respective model itself without a separate model dedicated solely to said respective feedback path and pre-trained thereto. An active attenuation system according to claim 3 or 4 wherein each said model comprises an adaptive recursive filter. An active attenuation system according to claim 5 wherein each said filter has a transfer function with both poles and zeros. An active attenuation system according to claim 6 wherein each said model comprises a recursive least mean square filter. An active attenuation system according to claim 3 or 4 wherein each of said N models comprises:- first algorithm means having a first input from said input transducer means, a second input of its respective error signal from its respective one of said N error transducers, and an output; second algorithm means having a first input of its respective said correction signal from its respective one of said N output transducers, a second input of its respective said error signal from its respective one of said N error transducers, and an output; a summing junction having inputs from said outputs of said first and second algorithm means, and an output providing the respective said correction signal to the respective one of said N output transducers. An active attenuation system according to claim 3 or 4 wherein each of said N models comprises:- first algorithm means having a first input from said input transducer means, a second input of its respective said error signal from its respective one of said N error transducers, and an output; second algorithm means having a first input of said output cancelling wave, a second input of its respective said error signal from its respective one of said N error transducers, and an output; and a summing junction having inputs from said outputs of said first and second algorithm means, and an output providing the respective said correction signal to the respective one of said N output transducers. An active attenuation system according to claim 3 or 4 wherein each of said N models comprises:- first algorithm means having a first input from said input transducer means, a second input of the respective said error signal from its respective one of said N error transducers, and an output; a first summing junction having a first input of its respective said error signal from its respective one of said N error transducers, a second input of its respective said correction signal to the respective one of said N output transducers, and an output; second algorithm means having a first input from said output of said first summing junction, a second input of the respective said error signal from the respective one of said N error transducers, and an output; and a second summing junction having inputs from said outputs of said first and second algorithm means, and an output providing the respective said correction signal to the respective one of said N output transducers. An active attenuation system according to any one of claims 3 to 10 wherein each of said N output transducers is a microphone, said input transducer means is one or more microphones, and each of said N output transducers is a speaker. An active attenuation system according to any one of claims 2 to 11 comprising:- auxiliary noise source means (140, 241, 341) introducing auxiliary noise into each of said N adaptive filter models which is random and uncorrelated with said undesired elastic wave at said input; and a second set of N adaptive filter models each having a model input from said auxiliary noise source means and an error input from a respective one of said N error transducers. An active attenuation system according to claim 12 comprising summer means summing auxiliary noise from said auxiliary noise source means with the outputs of each of said first mentioned N filter models and supplying the result as the respective said correction signal to the respective one of said N output transducers. An active attenuation system according to claim 13 wherein each of said adaptive filter models in said second set of N models comprises algorithm means, and the system further comprising second summer means summing the outputs of the respective one of said N error transducers and N algorithm means, and multiplier means multiplying the output of said second summer means with auxiliary noise from said auxiliary noise source means and supplying the result as a weight update signal to said algorithm means. An active attenuation system according to claim 12 wherein each of said N adaptive filter models adaptively models said acoustic system on-line without dedicated off-line pre-training, and also models the feedback path from the respective one of said N output transducers to said input transducer means on-line without dedicated off-line pre-training, each of said N models having a model input from said input transducer means and an error input from the respective one of said N error transducers and outputting a correction signal to the respective one of said N output transducers to introduce the respective one of said N cancelling waves such that the respective one of said N error signals approaches a given value, and wherein said second set of N adaptive filter models, each adaptively modelling both a respective said error path and a respective one of said N output transducers on-line without dedicated off-line pre-training; and a copy of each of said models in said second set of N adaptive filter models, each copy being in a respective one of said first mentioned N adaptive filter models to compensate for both the respective said error path and the respective one of said N output transducers adaptively on-line. An active attenuation system according to any one of claims 2 to 15 comprising local baffle means in said duct between said N output transducers to minimize interaction therebetween, said baffle means being local to said output transducers and not extending between said N error transducers. A method for attenuating in an acoustic system an undesired elastic wave propagating in an elastic medium, said elastic wave having non-uniform pressure distribution in said medium at a given instant in time along a direction transverse to the direction of propagation such that said wave has a plurality of portions along the transverse direction including at least one positive pressure portion (404) and at least one negative pressure portion (406), characterized by the steps of:- introducing a plurality of cancelling elastic waves into said medium from a plurality of output transducers (14, 214, 314), one for each of said positive and negative pressure portions of said undesired elastic wave, for attenuating said undesired elastic wave; sensing the combined said undesired elastic wave and said cancelling elastic waves with a plurality of error transducers (16, 216, 316), one for each of said positive and negative pressure portions of said undesired elastic wave, and providing a plurality of error signals (44, 244, 344); and modelling said acoustic system with a plurality of adaptive filter models (40, 240, 340), one for each of said positive and negative pressure portions of said undesired elastic wave, each said model having an error input (44, 244, 344) from a respective said error transducer and outputting a correction signal (46, 246, 346) to a respective said output transducer to introduce a respective said cancelling elastic wave, such that each said portion of said undesired elastic wave has its own set of an adaptive filter model, output transducer, and error transducer. A method according to claim 17 for an acoustic system including an axially extending duct (4) having an input (6) for said undesired elastic wave which propagates therethrough to an output (8) and wherein said undesired elastic wave has N portions extending transversely across the duct, N being ≧ 2, and includes said at least one positive pressure portion and said at least one negative pressure portion, the method including:- modelling said acoustic system with N said adaptive filter models each having respective error inputs (44, 244, 344) from a respective one of N said error transducers and outputting N said correction signals (46, 246, 346) to N respective said output transducers (14, 214, 314) to introduce N respective said cancelling waves such that said N error signals approach respective given values. A method according to claim 17 or 18 comprising modelling said acoustic system with adaptive recursive filter models each having a transfer function with both poles and zeros. A method according to claim 17 or 18 comprising modelling said acoustic system with adaptive recursive least mean square filter models. A method according to any one of claims 17 to 20 comprising:- sensing said undesired elastic wave at said input with input transducer means (10, 210, 310); modelling each of the feedback paths from said output transducers to said input transducer means with the same respective adaptive filter model, without a separate model pre-trained solely to the respective feedback path, by modelling each said feedback path as part of said respective adaptive filter model such that each said adaptive filter model adaptively models both said acoustic system and said respective feedback path, without separate modelling of said acoustic system and said respective feedback path and without dedicated pre-training of said respective adaptive filter model with a broad band acoustic signal. A method according to claim 21 comprising modelling each of said feedback paths by using the respective said error signal from the respective said error transducer. A method according to claim 21 comprising modelling each of said feedback paths by using the respective said error signal from the respective said error transducer as one input to the respective said model and the respective said correction signal to the respective said output transducer as another input to the respective said model. A method according to claim 18 comprising providing one or more input signals representing said undesired elastic wave at said input, and modelling said acoustic system with said adaptive filter models having inputs from said one or more input signals. A method according to claim 24 comprising providing a single said input signal representing said undesired elastic wave at said input, and inputting the same said input signal to each of said adaptive filter models. A method according to claim 25 comprising providing a single input transducer sensing said undesired elastic wave at said input and supplying said input signal. A method according to claim 24 comprising providing a plurality of said input signals, one for each of said adaptive filter models, respectively. A method according to claim 27 comprising providing a plurality of input transducers sensing said undesired elastic wave at said input and supplying said input signals, respectively, A method according to any one of claims 17 to 29 comprising providing auxiliary noise source means (140, 241, 341) and introducing noise therefrom into each of said models, such that each of said error transducers also senses the auxiliary noise from said auxiliary noise source means. A method according to claim 29 wherein said models additionally model each respective output transducer and each respective error path from each respective output transducer to each respective said error transducer, all on-line without separate modelling and without dedicated pre-training. A method according to claim 29 or 30 comprising introducing noise from said auxiliary noise source means which is random and uncorrelated with said undesired elastic wave at said input. A method according to claim 31 wherein said auxiliary noise source means comprises N auxiliary noise sources, and comprises introducing noise from each of said N noise source into a respective one of said models such that each of said error transducers also senses the auxiliary noise from its respective one of said N auxiliary noise sources. A method according to any one of claims 17 to 32 comprising providing local baffle means in said duct between said output transducers to minimize interaction therebetween.

Inventors: ALLIE, MARK C., ERIKSSON, LARRY J., HOOPS, RICHARD H.
Applicants: NELSON INDUSTRIES, INC.
Application year: 1989
Main classification: G10K11
IPC codes: H04R3, G10K11