GB2308521A - Adaptive noise cancellation apparatus cancels turbulence-induced noise - Google Patents

Adaptive noise cancellation apparatus cancels turbulence-induced noise Download PDF

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Publication number
GB2308521A
GB2308521A GB9617828A GB9617828A GB2308521A GB 2308521 A GB2308521 A GB 2308521A GB 9617828 A GB9617828 A GB 9617828A GB 9617828 A GB9617828 A GB 9617828A GB 2308521 A GB2308521 A GB 2308521A
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United Kingdom
Prior art keywords
noise
sensors
summator
signal
filter
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GB9617828A
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GB9617828D0 (en
GB2308521B (en
Inventor
Steven David Massie
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Roke Manor Research Ltd
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Roke Manor Research Ltd
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Priority claimed from GBGB9526420.6A external-priority patent/GB9526420D0/en
Application filed by Roke Manor Research Ltd filed Critical Roke Manor Research Ltd
Priority to GB9617828A priority Critical patent/GB2308521B/en
Publication of GB9617828D0 publication Critical patent/GB9617828D0/en
Publication of GB2308521A publication Critical patent/GB2308521A/en
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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H3/00—Measuring characteristics of vibrations by using a detector in a fluid
    • 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
    • G10K2210/10—Applications
    • G10K2210/101—One dimensional
    • 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
    • G10K2210/10—Applications
    • G10K2210/108—Communication systems, e.g. where useful sound is kept and noise is cancelled
    • 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
    • G10K2210/10—Applications
    • G10K2210/112—Ducts
    • 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
    • G10K2210/10—Applications
    • G10K2210/128—Vehicles
    • 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
    • G10K2210/3025—Determination of spectrum characteristics, e.g. FFT
    • 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
    • G10K2210/3026—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
    • G10K2210/3028—Filtering, e.g. Kalman filters or special analogue or digital filters
    • 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
    • G10K2210/50—Miscellaneous
    • G10K2210/505—Echo cancellation, e.g. multipath-, ghost- or reverberation-cancellation

Abstract

Apparatus for cancelling unwanted noise due to turbulent fluid flow past an acoustic sensor is disclosed. A plurality of sensors 8, 10, 12 are connected to an adaptive noise cancellation system. The cancellation system comprises a summator means 18, an input to which is connected to one of the sensors 10, and the remainder of the sensors 10, 12 are connected to inputs to the summator means 18 via controllable filter means 14, 16 which operate to delay and shift the respective monitored signals representative of each of the signals detected by the sensors 10, 12, such that a summation signal provided at an output of the summator means 18 is substantially devoid of noise. A restoring controllable filter 24, connected to the output of the summator means 18, operates to restore the frequency response of the summation signal, thereby providing an output signal devoid of turbulence-induced noise and representative of a wanted signal detected by the sensors 8, 10, 12. This apparatus can be applied to acoustic sensors in pipes or ducts, or mounted on vehicles or aircraft, and sonar sensors underneath ships or towed arrays, or mounted on submarines.

Description

NOISE CANCELLATION APPARATUS The present invention relates to noise cancellation apparatus, and in particular to apparatus which automatically cancels turbulence-induced noise.
The invention will find application in the reduction of turbulence-induced noise in acoustic sensing systems, in which there is constant, or slowly-varying relative motion between the propagation of fluid and the acoustic sensors.
The apparatus is applicable to, but not limited to, sonar sensors underneath ships, towed arrays, or on submarines, acoustic sensors mounted on vehicles, acoustic sensors mounted on aircraft, acoustic sensors mounted out of doors, and subject to wind, and, acoustic sensors in pipes or ducts.
Acoustic sensors, microphones, and sonar transducers are intended to measure the pressure variations associated with a propagating acoustic wave. However, turbulent fluid flow past an acoustic sensor gives rise to unwanted pressure variations that interfere with the wanted signal. The known way tackling this is to place a large porous windshield structure around the sensor to reduce air flow and damp out the turbulence. This approach can be effective, but does considerably increase the size of the sensor and therefore makes the sensor more conspicuous, and results in increased disturbance of the fluid flow pattern in the vicinity of the sensor.
An aim of the present invention is the provide noise cancellation apparatus which automatically cancels turbulence induced noise and which avoids the use of the above mentioned windshield structure.
According to the present invention there is provided noise cancellation apparatus comprising a plurality of acoustic sensors which are arranged in the flow of a fluid medium and are connected to an adaptive cancellation system which is arranged to remove noise generated through turbulence in the fluid medium.
According to an aspect of the present invention there is provided a method of cancelling noise from a wanted signal, wherein the noise and the wanted signal are detected by a plurality of sensors, the said wanted signal being detected substantially contemporaneously by each sensor, and the said noise being substantially correlated at each sensor after a timelag, the said method comprising the steps of: delaying at least one sensor response signal by means of at least one controllable filter means such that the detected noise signals of at least two sensors are correlated, subtracting at least one of the response signals and at least one of the delayed response signals from each other by means of a summator means, and restoring a frequency response of the signal at an output of the summator means by means of a further controllable filter means.
An embodiment of the present invention will now be described with reference to the following drawings, wherein, FIGURE 1 shows a turbulent flow passing a number of acoustic sensors; FIGURE 2 shows noise cancellation apparatus in accordance with the present invention; FIGURE 3 shows five signal wave form diagrams representative of signals at various points in the apparatus shown in Figure 2, and, FIGURE 4 shows three diagrams representing the frequency response of signals at three points of the apparatus, shown in Figure 2.
Referring to Figure 1, a fluid flow is shown in a direction of arrow 2. The turbulent fluid flow consists of a number of circulating eddies 4, that are convected along with the general drift of the fluid. The eddies 4 are associated with variations of pressure. Although turbulent flows evolve constantly, eddies may persist for considerable periods of time.
If omni-directional acoustic sensors 6 (pressure transducers) are arranged along a streamline in a turbulent fluid flow, the turbulence-induced signals picked up by adjacent sensors may be strongly correlated at a time-lag T corresponding to the time taken for the fluid to travel from one sensor to the other. The time-lag T may be determined by the rate of fluid flow v and the separation of the sensors x, in accordance with equation (1).
v (1) x Although turbulence induced noise will be highly correlated at each of the sensors 6, after a time-lag T, a wanted acoustic signal 7, generated from a distant source, will be detected by each of the acoustic sensors 6, substantially contemporaneously.
Referring to Figure 2, there is shown a block diagram of noise cancellation apparatus which operates to substantially reduce the correlated turbulence noise. In Figure 2, two or more acoustic sensors, which may be microphones 8, 10, 12 are arranged along, or close to, a streamline of the fluid flow. The microphones feed into an adaptive cancellation system comprising two controllable filters 14, 16, a summator 18 and a filter controller 20. The microphone 8 is connected to an input of the controllable filter 14, the microphone 10 is connected to an input of the summator 18, and the microphone 12 is connected to an input of the controllable filter 16. The controllable filters 14, 16 each have an output which is connected to a further input of the summator 18, respectively.The controllable filters 14, 16 each have a further output which is connected to a respective input of an inverse filter controller 22. The adaptive cancellation system includes a feedback filter controller 20, which receives an output from the summator 18, and provides two output signals which are applied to a further input of each controllable filter 14, 16. The output of the summator 18 is also connected to an input of a third controllable filter 24, which is controlled by the inverse filter controller 22, and may be an inverse or equaliser filter, from which is generated the output signal.
The adaptive cancellation system may use any of the standard or specialist techniques for interference cancellation, for example, a Least Mean Squares (LMS) algorithm, the function of which is well known in the art and is used to control the filters 14, 16, so that, at the output of the summator 18 turbulence-induced signals picked up by adjacent sensors which are strongly correlated at the time-lag T, corresponding to the time taken for the fluid to travel from one sensor to the other, are filtered out.
Turbulence noise cancellation is achieved by the apparatus shown in Figure 2, in accordance with the following general operating principles. Signals at the output of the summator 18, are representative of a sum of the output signals from the filters 14, 16 and the straight through channel from the microphone 10.
The output signal from the summator 18, may be a discrete time signal sampled in accordance with a sampling rate of the apparatus. As will be appreciated the output signals may also be continuous time signals.
The feedback filter controller 20, operates to adjust each of a plurality of coefficients of the controllable filters 14, 16, such that the respective monitored signals representative of the received acoustic signals by each of the microphones are delayed and shifted in accordance with the time-lag T. An effect of delaying and shifting each of the monitored signals is that the turbulence-induced noise is thereafter highly correlated, and by further arranging for the coefficients of the filters 14, 16, to be adapted to scale and invert the monitored signal samples, the signal generated at the output of the summator 18, will have the turbulence-induced noise substantially subtracted from the monitored signal provided by the straight through microphone 10.
This arrangement of delaying and adding the monitored signals is illustrated by the signal diagrams given in Figure 3. Monitored signals representative of the acoustic signal detected by microphone 8, is shown in Figure 3(a), whereas the monitored signal detected by the straight through microphone 10, is shown in Figure 3(b), and the monitored signal detected by the microphone 12, is shown in Figure 3(c). After the monitored signals from the outer microphones 8, 12, have been filtered by the controllable filters 14, 16, and added to the monitored signal from the straight through microphone 10, the signal produced at the output of the summator 18, is shown in Figure 3(d). In Figure 3(d), a signal diagram representative of the signal at the output of the summator 18, is shown to have had the turbulence noise cancelled.Furthermore, wanted signal peaks 30, 32, have been correspondingly delayed and inverted, and are therefore present in the output signal from the summator 30a, 30b, 32a, 32b, along with representations of the wanted signal 30, 32, as present in the monitored signal from the straight through microphone 10.
The signal at the output of the summator 18, therefore comprises a composition of the wanted signal 30, 32, and delayed and inverted version of the wanted signal 30a, 30b, 32a, 32b. In order to utilise all energy corresponding to the wanted signal, the inverse filter controller 22, operates to adjust the impulse response coefficients of the third controllable filter 24, such that the delayed and inverted versions of the wanted signals 30a, 30b, 32a, 32b, are advanced and again inverted, such that the wanted signal peaks 30, 32, in a final output signal from the controllable filter represented as the diagram shown in Figure 3(e), comprises substantially all of the energy appertaining to the wanted signal detected by the microphones 8, 10, 12.
The operation of the third controllable filter 24, may be seen in the frequency domain as re-building the frequency response appertaining to the monitored signal from the microphone 10.
This is because the frequency response of signals at the output of the summator 18, will exhibit substantial frequency notches in accordance with the delay and inversion operation of the upper and lower controllable filters 14, 16. This is illustrated in Figure 4(a), where two frequency notches 40, 42, introduced into the frequency response of the composite signal at the output of the summator 18, are illustrated. Therefore, in order to restore the frequency response of the composite signal, the third controllable filter 24, operates to introduce frequency peaks 44, 46, in accordance with the frequency notches 40, 42, shown in Figure 4(b), thereby substantially cancelling the frequency notches 40, 42. The effect on the frequency response at the output of the third controllable filter 24, is represented by the frequency response diagram in Figure 4(c).In the time domain, this is equivalent to combining the versions of the wanted signal so as to combine substantially all of the energy appertaining to the wanted signal detected by the microphones 8, 10, 12.
In a situation where the frequency response of signals at the output of the summator 18, comprises deep frequency notches, it may not be appropriate to re-build the frequency response by introducing corresponding high frequency peaks. This is because an attempt by the restoring filter to introduce a frequency peak to cancel a deep frequency notch, would have an effect of greatly increasing noise in the correspondingly restored signal. In this situation therefore, the inverse filter controller operates to establish which parts of the frequency band of the wanted signal can be re-built through an introduction of frequency peaks, and which parts of the frequency band may only be re-built to a limited extent by introducing frequency peaks, which have reduced amplitude so as to suppress noise which would otherwise be amplified, thereby maintaining a low signal to noise ratio.As with any digital signal processor, perfect results are only possible with an infinite latency.
Although the example embodiment of the invention has been described with reference to apparatus with three microphones, it will be appreciated that the principles of operation would be extended to any number, and more particularly resolution of the wanted signals from the turbulenceinduced noise will increase with more acoustic sensors or microphones.
To calculate appropriate values for the coefficients of the third controllable filter 24, the inverse filter controller 22 receives the current set of impulse response coefficients from each of the two controllable filters 14, 16, and performs a calculation on those coefficients to produce an output signal for controllable filter 24.
There are many ways in which the coefficients in the filter 24 could be calculated, and one example would be to use the assumption that the acoustic signal arrives at all microphones 8, 10, 12 simultaneously. The summator 18 adds together the output signals from the filters 14,16, and the straight through channel from the microphone 10 at the sampling rate. The output signal from summator 18, is subjected to a calculation for generating the inverse filter. The inverse filter provides the same output frequency response as that obtained from microphone 10 in the absence of turbulence.
The invention as described above reduces the level of turbulence-induced noise in an acoustic sensor system without the need for a bulky windshield assembly. This will enable the acoustic sensor system to achieve greater sensitivity and range.
In addition, the reduction in size, or complete absence, of windshield assembly minimise any disturbance to the airflow caused by the acoustic sensor system, as well as rendering the presence of the acoustic sensor less conspicuous.

Claims (12)

1. A noise cancellation apparatus comprising a plurality of sensors which are arranged in the flow of a fluid medium and are connected to an adaptive cancellation system which is arranged to remove noise generated through turbulence in the fluid medium.
2. A noise cancellation apparatus as claimed in Claim 1, wherein the adaptive cancellation system comprises a summator means, an input of which summator means is connected to one of the plurality of sensors, and a remainder of the plurality of sensors are connected to the summator means via controllable filter means, which controllable filter means operate to scale and adjust a plurality of monitored signals representative of signals detected by the said sensors, such that a summation signal provided at an output of the summator means is substantially devoid of noise, and a restoring controllable filter connected to the output of the summator means, which operates to restore the frequency response of the summation signal, thereby providing an output signal devoid of turbulence-induced noise and representative of a wanted signal detected by the said sensors.
3. A noise cancellation apparatus as claimed in Claim 2, wherein the adaptive cancellation system further comprises an inverse filter controller, which operates to generate filter coefficients of the restoring filter in accordance with impulse responses of the said controllable filter means in communication therewith.
4. A noise cancellation apparatus as claimed in Claims 2 or 3, wherein the adaptive cancellation system further comprises a feedback filter controller being connected to the output of the summator means for recursively adapting the impulse responses of the said controllable filter means, in accordance with any noise present in the summation signal.
5. A noise cancellation apparatus as claimed in Claim 4, wherein the feedback filter operates in accordance with a Least Means Squares algorithm to perform the noise cancellation.
6. A noise cancellation apparatus as claimed in any preceding Claim, wherein the sensors are arranged in an array extending substantially in a direction of propagation of a noise signal.
7. A noise cancellation apparatus as claimed in any preceding Claim, wherein the sensors are acoustic sensors.
8. A noise cancellation apparatus according to Claim 5, wherein the acoustic sensors are microphones.
9. A noise cancellation apparatus according to Claim 6, wherein the microphones are three microphones.
10. Apparatus as claimed in Claim 9, wherein the said controllable filter means are two controllable filters, a first of which controllable filter has an input connected to a first of the three microphones, the summator has an input connected to a second of the three microphones, a second of the two controllable filter has an input connected to a third of the three microphones, and wherein said first and second controllable filters each have an output connected to a further input of said summator.
11. A noise cancellation apparatus as claimed in any preceding Claim, wherein the restoring controllable filter is an equalising filter.
12. A method of cancelling noise from a wanted signal, wherein the noise and the wanted signal are detected by a plurality of sensors, the said wanted signal being detected substantially contemporaneously by each sensor, and the said noise being substantially correlated at each sensor after a time-lag, the said method comprising the steps of: delaying at least one sensor response signal by means of at least one controllable filter means such that the detected noise signals of at least two sensors are correlated, subtracting at least one of the response signals and at least one of the delayed response signals from each other by means of a summator means, and restoring a frequency response of the signal on output of the summator means by means of a further controllable filter means. 12. Apparatus substantially as hereinbefore described with reference to Figures 2, 3 and 4 of the accompanying drawings.
GB9617828A 1995-12-22 1996-08-27 Noise cancellation apparatus Expired - Lifetime GB2308521B (en)

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Application Number Priority Date Filing Date Title
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Application Number Priority Date Filing Date Title
GBGB9526420.6A GB9526420D0 (en) 1995-12-22 1995-12-22 Noise cancellation apparatus
GB9617828A GB2308521B (en) 1995-12-22 1996-08-27 Noise cancellation apparatus

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GB2308521A true GB2308521A (en) 1997-06-25
GB2308521B GB2308521B (en) 1999-10-13

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Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4195360A (en) * 1973-10-16 1980-03-25 Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of National Defence Signal processing circuit
US4283767A (en) * 1979-05-03 1981-08-11 Raytheon Company Multiple correlator reference processor
US4388711A (en) * 1981-07-28 1983-06-14 The United States Of America As Represented By The Secretary Of The Navy Optimum flow noise cancelling hydrophone module
EP0560559A2 (en) * 1992-03-09 1993-09-15 Geco A.S. Method of cancelling turbulence noise in a fluid-filled seismic streamer
US5359663A (en) * 1993-09-02 1994-10-25 The United States Of America As Represented By The Secretary Of The Navy Method and system for suppressing noise induced in a fluid medium by a body moving therethrough
EP0681193A1 (en) * 1994-05-05 1995-11-08 Western Atlas International, Inc. Method of attenuating coherent noise in marine seismic data
WO1996010247A1 (en) * 1994-09-29 1996-04-04 The Boeing Company Active noise control in a duct with highly turbulent airflow

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4195360A (en) * 1973-10-16 1980-03-25 Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of National Defence Signal processing circuit
US4283767A (en) * 1979-05-03 1981-08-11 Raytheon Company Multiple correlator reference processor
US4388711A (en) * 1981-07-28 1983-06-14 The United States Of America As Represented By The Secretary Of The Navy Optimum flow noise cancelling hydrophone module
EP0560559A2 (en) * 1992-03-09 1993-09-15 Geco A.S. Method of cancelling turbulence noise in a fluid-filled seismic streamer
US5359663A (en) * 1993-09-02 1994-10-25 The United States Of America As Represented By The Secretary Of The Navy Method and system for suppressing noise induced in a fluid medium by a body moving therethrough
EP0681193A1 (en) * 1994-05-05 1995-11-08 Western Atlas International, Inc. Method of attenuating coherent noise in marine seismic data
WO1996010247A1 (en) * 1994-09-29 1996-04-04 The Boeing Company Active noise control in a duct with highly turbulent airflow

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GB9617828D0 (en) 1996-10-09
GB2308521B (en) 1999-10-13

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Expiry date: 20160826