GB2191063A - Active noise suppression - Google Patents

Active noise suppression Download PDF

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Publication number
GB2191063A
GB2191063A GB08710377A GB8710377A GB2191063A GB 2191063 A GB2191063 A GB 2191063A GB 08710377 A GB08710377 A GB 08710377A GB 8710377 A GB8710377 A GB 8710377A GB 2191063 A GB2191063 A GB 2191063A
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United Kingdom
Prior art keywords
disturbance
suppression
compensation signals
adaptive
transducers
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Withdrawn
Application number
GB08710377A
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GB8710377D0 (en
Inventor
Jonathan Neil Smith
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Plessey Co Ltd
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Plessey Co Ltd
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Filing date
Publication date
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Publication of GB8710377D0 publication Critical patent/GB8710377D0/en
Publication of GB2191063A publication Critical patent/GB2191063A/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
    • G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
    • G10K11/1781—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase characterised by the analysis of input or output signals, e.g. frequency range, modes, transfer functions
    • G10K11/17813—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase characterised by the analysis of input or output signals, e.g. frequency range, modes, transfer functions characterised by the analysis of the acoustic paths, e.g. estimating, calibrating or testing of transfer functions or cross-terms
    • G10K11/17817—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase characterised by the analysis of input or output signals, e.g. frequency range, modes, transfer functions characterised by the analysis of the acoustic paths, e.g. estimating, calibrating or testing of transfer functions or cross-terms between the output signals and the error signals, i.e. secondary path
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
    • G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
    • G10K11/1785—Methods, e.g. algorithms; Devices
    • G10K11/17853—Methods, e.g. algorithms; Devices of the filter
    • G10K11/17854—Methods, e.g. algorithms; Devices of the filter the filter being an adaptive filter
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/16—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
    • G10K11/175—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound
    • G10K11/178—Methods or devices for protecting against, or for damping, noise or other acoustic waves in general using interference effects; Masking sound by electro-acoustically regenerating the original acoustic waves in anti-phase
    • G10K11/1787—General system configurations
    • G10K11/17875—General system configurations using an error signal without a reference signal, e.g. pure feedback
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/30—Means
    • G10K2210/301—Computational
    • G10K2210/3011—Single acoustic input
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/30—Means
    • G10K2210/301—Computational
    • G10K2210/3019—Cross-terms between multiple in's and out's
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/30—Means
    • G10K2210/301—Computational
    • G10K2210/3023—Estimation of noise, e.g. on error signals
    • G10K2210/30232—Transfer functions, e.g. impulse response
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/30—Means
    • G10K2210/301—Computational
    • G10K2210/3032—Harmonics or sub-harmonics
    • G—PHYSICS
    • G10—MUSICAL INSTRUMENTS; ACOUSTICS
    • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00—Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/30—Means
    • G10K2210/301—Computational
    • G10K2210/3046—Multiple acoustic inputs, multiple acoustic outputs

Landscapes

  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Multimedia (AREA)
  • Soundproofing, Sound Blocking, And Sound Damping (AREA)

Abstract

A system and a method of adaptive disturbance suppression comprising a plurality of sensors 2 and a plurality of transducers 4 coupled via a signal processor 6. The transducers sequentially produce compensation signals in accordance with an adaptive algorithm in order to provide a disturbance cancellation effect. The algorithm is based upon a matrix of transfer functions which is continually being updated by the signal processor in dependence upon the resultant disturbance measured by the sensors. Hitherto, multiple channel disturbance suppression systems have not re- estimated the compensation signals required whilst contemporaneously maintaining a disturbance cancellation effect. <IMAGE>

Description

SPECIFICATION Adaptive Disturbance Suppression The present invention relates to a method of adaptive disturbance suppression and to a system for the adaptive suppression of disturbance in apparatus such as machinery which causes noise/ vibration in an unenclosed area.
Disturbance suppression is frequently required in order to maintain health and safety standards, or to minimise mechanical interference with sensitive equipment. At most audible frequencies, acoustic screening or passive mountings provide satisfactory reductions, but these lose their effectiveness at low frequencies.
Suppression of vibrations from apparatus can be effected at the mounting points of the apparatus but the contradictory requirements of high compliance and strong support may cause severe design problems. Furthermore, with some apparatus, the acoustic suppression system required to enable satisfactory acoustic silence may be of impractical size.
These types of problem can be approached by artificially superimposing a compensating disturbance, with the same amplitude but opposite phase to the original disturbance, for negating the original disturbance.
Known disturbance suppression systems include single channel systems comprising one sensor and one transducer. Such systems are ineffectual at suppressing vibrations from apparatus with more than one mounting point or in more than one direction, since the energy in the original disturbance merely dissipates through the other mounting points. When such single channel systems are used to suppress acoustic disturbance, cancellation is only effective in those areas of the disturbance where the phase of the compensation signals matches that of the original disturbance.
Consequently the single channel system is very ineffective for negating a disturbance over an area.
It is an object of the present invention to provide an adaptive system and method for effective suppression of a disturbance which method is applicable for effective suppression of a disturbance over an area.
Afurther object of the present invention is to provide an adaptive system and method for effective vibration cancellation in apparatus with multiple mounting points and/or with movement being in more than one direction.
Accordingly, there is provided a system for the adaptive suppression of disturbance in apparatus, the system comprising a plurality of sensors arranged to sense a disturbance in the apparatus, a plurality of transducers for coupling to the sensors and arranged to produce compensation signals for providing a disturbance cancellation effect, wherein each transducer is arranged to provide compensation signals in succession in accordance with an adaptive algorithm, such that the compensation signals are progressively modified in dependence upon compensation signals previously produced by the transducers, whereby effective suppression of the disturbance is automatically optimised.
The present invention also provides a method of adaptive disturbance suppression, the method comprising an iterative algorithm wherein a compensation signal, sequentially produced by a plurality of transducers in accordance with a matrix of transfer functions, is superimposed upon a disturbance, whereby the resultant disturbance is monitored simultaneously by a plurality of sensors, so as to provide a further matrix of transfer functions, whilst comtemporaneously maintaining a disturbance cancellation effect and progressively modifying the compensation signal.
The present invention will now further be described with reference to the accompanying drawing, of which: Figure lisa schematic block diagram according to the present invention; and Figure 2 is an exemplary diagram of a nearly periodic disturbance partially divided into digital samples.
The present invention relates to a system and a method which addresses the problem of effective suppression of a vibrational or acoustic disturbance in an unenclosed area. As shown in Figure 1 the system comprises a plurality of sensors 2 and a plurality of transducers 4 coupled to the sensors 2 via a signal processor 6 for producing compensation signals.
The system is a multiple channel system since it utilises a plurality of sensors 2 and transducers 4.
Since there is a plurality of sources of compensation signals and a plurality of sensors for monitoring the resultant disturbance, it enables the system to effectively suppress a disturbance over an area.
Furthermore an initial assumption is made that the cancelling system and structure are linear, although modest non-linearities may be tolerated.
The system provides a cancelling disturbance in accordance with an algorithm. This algorithm is based upon the 'adaptive transfer function' approach, and is designed to cancel the harmonics of a very nearly periodic disturbance. The algorithm employs a transfer function matrix which is square and so requires an equal number of input data channels and output data channels. Usually, each input data channel corresponds to a respective sensor 2 and each output data channel corresponds to a respective transducer 4. The algorithm is, thus designed to provide output data for the transducers 4 on the basis of the input data obtained from the sensors 2 in order to produce the required cancelling disturbance.
The signal processor 6 may, however, accommodate for an unequal number of sensors 2 and transducers 4 since the additional data channels can be considered as free variables.
The system considers the disturbance over a given time period T, such that the length of the time period T corresponds to one cycle of the fundamental frequency of the disturbance as illustrated by Figure 2. Digital samples are taken at constant intervals phase locked to the disturbance.
The operation of the system may be expressed in algebraic terms as follows, assume there are n digital samples corresponding to n channels of input data from the sensors 2. Consider when a compensating signal Aj(t) is superimposed upon the disturbance by the ith transducer 4. The corresponding resultant disturbance monitored by the jth sensor 2 can be written thus, Bj (t)=lj1 (T) * A, (t), where I jj (T) is a transfer function. For compensation signals produced by all n transducers 4, then
The above matrix equation must be solved for An so as to predict the effect of the compensating signals An upon the system.In order to derive Ant the transfer function matrix requires a deconvolution which is computationally inefficient and therefore difficult to realise in practice.
However, by transforming the transfer function matrix equation to frequency domain this operation is simplified. In the present invention the transformation is carried out by means of an un-windowed fast fourier transform since there is the assumption that the disturbance is very nearly periodic about the time period. Thus
where T,j(f) is the transfer function in frequency domain, and b1(f)=F{B1(t)} a,(f)=F{Ai(t)} for some function F. Hence a(f)=LT(f)i1b(f).
If the apparatus to be quietened has an original disturbance C(t) or correspondingly c(f), then utilising the principle of superposition, to provide the required compensation signals then b(f)=-c(f).
The required compensation signals are, therefore, a(f)=(T(f))-' (-c(f)) in order to negate the original disturbance.
However, it is unlikely that the disturbance will be completely suppressed and some residue c'(f) will remain. Indeed, the original disturbance may vary and if there are frequency changes then the transfer function (T(f))-l for each harmonic will be inaccurate and therefore the disturbance will not be effectively suppressed. The problem may be alleviated by ensuring that the algorithm is adaptive such that the cancelling disturbance can be modified in accordance with variations in the original disturbance. The system monitors the residues c'(f) and re-estimates the transfer function matrix (T(f))-'.
So to cancel the residue c'(f) in the next subsequent pass, the compensation signals are: a(f)=(T(f))-' (-c(f))+(T'(f))-' (-c'(f)) The transfer function is continually being updated so for the rth pass, the compensation signals can be written, thus ar(f)=ar-1(f)+Tr-1(~cr) Hitherto disturbance suppression systems have not re-estimated the compensation signals whilst maintaining a cancelling disturbance.
Initially the transfer function matrix is measured by superimposing an arbitrary signal through each transducer 4 sequentially, and, measuring the resultant disturbance at each sensor 2.
Since this is a multiple channel system there will be cross-coupling terms between the various parts of the apparatus and between the sensors 2 and the transducers 4. Consequently re-estimating the compensation signals is more complicated than for single channel systems. The present invention introduces changes in the compensation signals sequentially through each channel whilst simultaneously monitoring the change in the resultant disturbance through all of the channels, thus enabling the re-estimation of the transfer function matrix to be simplified.
The method of disturbance suppression in an area of free space according to the present invention, is as follows: First, the disturbance c(t) is measured at each sensor; Second, an arbitrary signal is superimposed, through channel one a1 (t). The effect of this signal a1(t) on the disturbance is measured through each channel bn(t), thus generating one column of the transfer function matrix. So
and
The arbitrary signal a1(t) is then removed; Third, step two is repeated for all channels 2 to n, thus generating a complete transfer function matrix, T(t); and Fourth, the transfer function matrix is then inverted and multiplied by minus the disturbance c(t) obtained in step one.
Thus the required compensation signals in frequency domain are If the matrix is singular at any frequency, or the variation introduced by the compensation signals is negligible, then a flag is set by the signal processor 6 preventing the output of any compensation signals at that frequency.
The system has now generated a complete transfer function matrix and the following iterative loop re-estimates the said matrix: Fifth, the transfer function matrix is inverted and a compensation signal a',(t) is superimposed through channel one. The resultant disturbance is measured throughout all channels and this response is compared with the previous response, thus generating a new set of values for column one of the transfer matrix; Sixth, step five is repeated for all channels 2 to n to obtain a new transfer function matrix; and Seventh, the said matrix is then inverted and multiplied by minus the current residue c'(t).
At the end of each pass, the compensation signals obtained from that pass are combined with the compensation signals from the previous pass for each channel. Thus the re-estimated transfer function matrix enables accumulated compensation signals to be superimposed upon the original disturbance.
Steps five to seven of the algorithm are repeated in order to continuously re-estimate the compensation signals required thereby facilitating long term suppression of the disturbance.
If the frequency of the disturbance varies, then the data from the sensors 2 will be the sum of the new disturbance and the old compensation signals. On the following pass, the change at any frequency actually due to both the change in disturbance and the re-estimated compensation signals appear two be only due to the latter. The compensation signals generated at the end of the pass are thus inaccurate.
On the next pass, however, the measured variations in the resultant disturbance after superimposing the compensation signals, are actually due to the latter, so the transfer function estimate is accurate. The accumulated compensation signals are then superimposed on the current residue, to renew the cancellation.
Thus the disturbance suppression system according to the present invention maintains a compensating disturbance while re-estimating the compensation signals using the above single linear interpolation method. The foregoing description is provided by way of example only and modifications may be incorporated into the system and method without departing from the scope of the present invention.

Claims (7)

1. A system for the adaptive suppression of disturbance in apparatus, the system comprising a plurality of sensors arranged to sense a disturbance in the apparatus, a plurality of transducers for coupling to the sensors, and arranged to produce compensation signals for providing a disturbance cancellation effect, wherein each transducer is arranged to provide compensation signals in succession in accordance with an adaptive algorithm, such that the compensation signals are progressively modified in dependence upon compensation signals previously produced by the transducers and whereby effective suppression of the disturbance is automatically optimised.
2. A method of adaptive disturbance suppression comprising an iterative algorithm wherein a compensation signal, sequentially produced by a plurality of transducers in accordance with a matrix of transfer functions, is superimposed upon a disturbance, whereby the resultant disturbance is monitored simultaneously by a plurality of sensors, so as to provide a further matrix of transfer functions, whilst contemporaneously maintaining a disturbance cancellation effect and progressively modifying the compensation signal.
3. A method as claimed in claim 2, wherein the matrix of transfer functions is a square matrix.
4. A method as claimed in claims 2 or 3, wherein the disturbance to be suppressed is substantially periodic.
5. A method as claimed in claim 4, wherein the transducers are arranged to produce a compensation signal at the fundamental and harmonic frequencies of the disturbance to be suppressed.
6. A system for the adaptive suppression of disturbance as substantially hereinbefore described.
7. A method of adaptive disturbance suppression as substantially hereinbefore described.
GB08710377A 1986-05-01 1987-05-01 Active noise suppression Withdrawn GB2191063A (en)

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GB868610744A GB8610744D0 (en) 1986-05-01 1986-05-01 Adaptive disturbance suppression

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GB2191063A true GB2191063A (en) 1987-12-02

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Cited By (17)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3834853A1 (en) * 1988-10-13 1990-04-26 Bayerische Motoren Werke Ag Arrangement for reducing the noise level in the interior of a motor vehicle
WO1991010226A1 (en) * 1989-12-30 1991-07-11 Noise Cancellation Technologies, Inc. Active vibration reducing system
US5060271A (en) * 1990-05-04 1991-10-22 Ford Motor Company Active muffler with dynamic tuning
US5063598A (en) * 1990-04-25 1991-11-05 Ford Motor Company Active noise control system with two stage conditioning
WO1992008223A1 (en) * 1990-10-29 1992-05-14 Noise Cancellation Technologies Inc. Active vibration control system
US5119902A (en) * 1990-04-25 1992-06-09 Ford Motor Company Active muffler transducer arrangement
EP0515518A1 (en) * 1990-02-13 1992-12-02 University of Maryland, College Park Repetitive sound or vibration phenomena cancellation arrangement with multiple sensors and actuators
US5210805A (en) * 1992-04-06 1993-05-11 Ford Motor Company Transducer flux optimization
US5229556A (en) * 1990-04-25 1993-07-20 Ford Motor Company Internal ported band pass enclosure for sound cancellation
US5233540A (en) * 1990-08-30 1993-08-03 The Boeing Company Method and apparatus for actively reducing repetitive vibrations
US5233137A (en) * 1990-04-25 1993-08-03 Ford Motor Company Protective anc loudspeaker membrane
US5245552A (en) * 1990-10-31 1993-09-14 The Boeing Company Method and apparatus for actively reducing multiple-source repetitive vibrations
US5319165A (en) * 1990-04-25 1994-06-07 Ford Motor Company Dual bandpass secondary source
EP0601934A1 (en) * 1992-12-11 1994-06-15 Decaux, Jean-Claude Improvements to methods and devices used to protect from external noises a given volume, preferably located inside a room
US5323466A (en) * 1990-04-25 1994-06-21 Ford Motor Company Tandem transducer magnet structure
WO2004027751A1 (en) * 2002-05-27 2004-04-01 Efremov Vladimir A Method for reducing the noise level of an aviation engine and system for carrying out said method
EP2043383A1 (en) * 2007-09-27 2009-04-01 Harman Becker Automotive Systems GmbH Active noise control using bass management

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0043565A1 (en) * 1980-07-03 1982-01-13 Hitachi, Ltd. Vibration/noise reduction device for electrical apparatus
GB2088951A (en) * 1980-12-05 1982-06-16 Lord Corp Acoustic attenuators with active sound cancelling
GB2097629A (en) * 1981-04-15 1982-11-03 Nat Res Dev Methods and apparatus for active sound control
GB2149614A (en) * 1983-10-31 1985-06-12 Secr Defence Active noise reduction apparatus
GB2154830A (en) * 1984-02-21 1985-09-11 Nat Res Dev Attenuation of sound waves

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0043565A1 (en) * 1980-07-03 1982-01-13 Hitachi, Ltd. Vibration/noise reduction device for electrical apparatus
GB2088951A (en) * 1980-12-05 1982-06-16 Lord Corp Acoustic attenuators with active sound cancelling
GB2097629A (en) * 1981-04-15 1982-11-03 Nat Res Dev Methods and apparatus for active sound control
GB2149614A (en) * 1983-10-31 1985-06-12 Secr Defence Active noise reduction apparatus
GB2154830A (en) * 1984-02-21 1985-09-11 Nat Res Dev Attenuation of sound waves

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Cited By (25)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3834853A1 (en) * 1988-10-13 1990-04-26 Bayerische Motoren Werke Ag Arrangement for reducing the noise level in the interior of a motor vehicle
DE3834853C2 (en) * 1988-10-13 1999-12-02 Bayerische Motoren Werke Ag Arrangement for reducing the noise level in the interior of a motor vehicle
WO1991010226A1 (en) * 1989-12-30 1991-07-11 Noise Cancellation Technologies, Inc. Active vibration reducing system
EP0515518A1 (en) * 1990-02-13 1992-12-02 University of Maryland, College Park Repetitive sound or vibration phenomena cancellation arrangement with multiple sensors and actuators
EP0515518A4 (en) * 1990-02-13 1993-06-30 The University Of Maryland Repetitive phenomena cancellation arrangement with multiple sensors and actuators
US5063598A (en) * 1990-04-25 1991-11-05 Ford Motor Company Active noise control system with two stage conditioning
US5119902A (en) * 1990-04-25 1992-06-09 Ford Motor Company Active muffler transducer arrangement
US5432857A (en) * 1990-04-25 1995-07-11 Ford Motor Company Dual bandpass secondary source
US5323466A (en) * 1990-04-25 1994-06-21 Ford Motor Company Tandem transducer magnet structure
US5229556A (en) * 1990-04-25 1993-07-20 Ford Motor Company Internal ported band pass enclosure for sound cancellation
US5233137A (en) * 1990-04-25 1993-08-03 Ford Motor Company Protective anc loudspeaker membrane
US5319165A (en) * 1990-04-25 1994-06-07 Ford Motor Company Dual bandpass secondary source
US5060271A (en) * 1990-05-04 1991-10-22 Ford Motor Company Active muffler with dynamic tuning
US5233540A (en) * 1990-08-30 1993-08-03 The Boeing Company Method and apparatus for actively reducing repetitive vibrations
AU660423B2 (en) * 1990-10-29 1995-06-29 Noise Cancellation Technologies, Inc. Active vibration control system
WO1992008223A1 (en) * 1990-10-29 1992-05-14 Noise Cancellation Technologies Inc. Active vibration control system
US5245552A (en) * 1990-10-31 1993-09-14 The Boeing Company Method and apparatus for actively reducing multiple-source repetitive vibrations
US5343533A (en) * 1992-04-06 1994-08-30 Ford Motor Company Transducer flux optimization
US5210805A (en) * 1992-04-06 1993-05-11 Ford Motor Company Transducer flux optimization
FR2699205A1 (en) * 1992-12-11 1994-06-17 Decaux Jean Claude Improvements to methods and devices for protecting a given volume from outside noise, preferably located inside a room.
EP0601934A1 (en) * 1992-12-11 1994-06-15 Decaux, Jean-Claude Improvements to methods and devices used to protect from external noises a given volume, preferably located inside a room
US5438624A (en) * 1992-12-11 1995-08-01 Jean-Claude Decaux Processes and devices for protecting a given volume, preferably arranged inside a room, from outside noises
WO2004027751A1 (en) * 2002-05-27 2004-04-01 Efremov Vladimir A Method for reducing the noise level of an aviation engine and system for carrying out said method
EP2043383A1 (en) * 2007-09-27 2009-04-01 Harman Becker Automotive Systems GmbH Active noise control using bass management
US8559648B2 (en) 2007-09-27 2013-10-15 Harman Becker Automotive Systems Gmbh Active noise control using bass management

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Publication number Publication date
GB8610744D0 (en) 1986-06-04
GB8710377D0 (en) 1987-06-03

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