The Active Control of Sound

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The active control of soundActive control of sound results from destructiveinterference between the sound field of an originalacoustic source and that from a controllable array ofsecondary acoustic sources. For this destructiveinterference to occur over an appreciable region of spacethe sound field of the secondary sources must match thatfrom the primary sourc e in both time a nd spa ce. Thespatial matching requirement leads to an upperfrequency of applicability of active control. Active controlcomplements conventional passive methods of soundcontrol, which do not work well at low frequencies.Practical feedforward controllers, using a multichannelgeneralisation of the well known L M S adaptivealgorithm, have been developed, using as many as 16loudspeakers and 32 microphones, and applied withconsiderable success in the control of low-frequencypropeller noise inside aircraft and low-frequency enginenoise inside cars.

by S . Elliott and P. A. Nelson

IntroductionThe pressurc fluctuations that weperceive as sound arc gcncrallgvery small modula tions of a muchlarger steady, ambient pressure.For example, in air at normalatmospheric pressurc, a very loudsound, with a sound pre ssure levelof 100 dB (me asur ed with respe ctto a refcrcnce level of 20 pPa) is amodulation of only about 2 Pa(2 N/m2) on top of an ambientpressure of about 105 Pa. Soundpropagate s as a longitudinal wavemoti on, which involvcs aninteraction between thccompressibility of the air and itsinertia, at a wave velocity in air ofabout 340 ms-1, which is a factorof O6 or so slower than thc speedof propagation of electromagneticradiation. This means that thewavelength of a sound wave is verymuch shorter than that of an

electromagnetic wave of acomparabl e frequency. Thefrequency range of audible soundis from about 20 Hz to 20 kHz,which gives a wavelength in air ofbetween 17 m and 17 mm. Thisrange of wavelengths corresponds

to the VHF to microwave region ofelectromagnetic waves, whichobviously have ve q much higherfrequencies.

An important property of soundwaves at normal amplitudcs is thatthcy propagate linearly, so that thenet effect at any one point of twoseparate s ound waves will be thesuperposition of the effects of thesound waves acting individually.This linear property of soundmea ns that, if an artificiallygenerated sound wave can beengineered to be exactly out ofphase with that from someannoying acoustic s ource, the twowaves will destructively interfereand thc rcsult will be silence. Thisis the basis of active soundcontrol. To turn this laboratorycuriosity into a useful noisecontrol strategy requires anunderstanding of both the

acoustics and electrical controltechnology appropriate to theparticular problem underconsideration. In this brief reviewwe will consider these two asp ectsof the problem separately, andthen look at some successful

practical applications of t h etechnique. But first we outline thehistorical development of activesound control and the reasonswhy it has rapidly come of ageover the past few years. Theinterested rea der is also referredto some of the o ther reviewarticles which have been publishedover the last few years,-4 and tothe recent textbook on the subjectwritten by the present authors.5

In a pioneering piecc of work,first published in 1934, thcGerman physicist Paul Luegoutlined the basic philosophy ofactive sound control in ducts, andin free spa ce. Fig. 1 is reproducedfrom this original patent andclearly shows, in diagram 1, thedetection of an offendingsinusoidal sound wave S,propagating in a duct usingmicrophone M , whose electricaloutput is passed through someelectronic controller I to drive anacoustic secondary source L . Thissource generates the antiphaseacoustic wave S, that cancels theinitial wave. Diagram 3 illustratesthe cancellation of nonsinusoidalacoustic waveforms, and diagrams2 and 4 illustrate, in a ratheridealised way, the generation of an

actively generated acousticshadow behind a s econdarysource L when excited by a three-dimensional , freely propagatingsound field generated by source A .It should be remembered that thispatent was published nearly 50years ago , only a few years afte rdevices for thc transduction ofsound into electrical signals andvice versa had first become widelyavailable (Guicking).

Notable dcvelopments weremade in the United States in the1950s by Har ry OlsonR 1953), whoidentified the possible applicationof the technology to controllingthe noise in cars and aircraft, andWilliam ConoverU 1956), who was

working on controlling the soundradiated by large transformers.Fig. 2 is reproduced fromConovers 1956 paper a nd show s afeedforward contr ol strategy inwhich a sinusoidal referencesignal, at a harmonic of the line

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H 4 diagram 1 4f

AJ-

V

IA I diagram 3diagram 2-_-- -_-------

A I

-------- --- -

diagram L

Diagrams from Paul Lueg s 1934 paten16

-equency, is manuallv adjust ed inniplitude and phase before being:d to thc se co nda n loudspeaker .he amplitude and phase arearied to minimise the pressure atome distant microphone locationnd the hope was that inancelling the sou nd at theiicr ophone position a null woulde generated in the soundadiation pattern of the-ansformc r. Conover discussedi e possibility of having anutomatic control system tod o r m he ampli tude and phasedjustment, but felt that, since the:vel could c han ge by 6 dB in anour, such a control system wasseyond the state of the art at thatme.

ilks about using multipleecondary loudspeaker s to obtain

eductions in radiated sound overmattern. Such multichanncl:edforward active sound controlystems ar e now starting to findheir wav into production, not soiuc h for the control of radiated

In the s ame paper, Conover also

larger angle in the directivity

5ound, but m ore for the activecontrol of enclosed sou nd fields iapplications where the alternatingpressure waveforms are almostperiodic. Examples are the low-frequency boom inside cars due tthe engi ne firing frequency, andthe low-frequency drone in thepassenger cabin of aircraft du e tcthe propellers.

The physical reasons why thescapplications a re confined torelatively low audio frequencieswill be discussed in the nextsection, and after that theadvances in control strategy whicallow rapid adaptation of thecontroller (compared to thatenvisaged by Conover) will bedescribed. This rapid adaptatio n ineeded for the automotiveapplication in particular since thcfrequency of excitation is

constantly varying with enginespeed and the level of excitation ivery dependent o n the engine loa,Adaptation times of about 0.1 shave been achieved for amultichannel active control systeiin such a n application.

Acoustic principles of active

Destructive interference at apoint between two wavefield5 thathave the sam e frequency isfamiliar n a number of differenttypes of wave ph enomena. Thedassic demonstrations ofinterference were performed in

optics by Thomas Young at thebeginning of the nineteenthcentury. Fig. 3 , for example,illustatcs Young's famouscxperiment in which two radiallyexpanding waveforms weregenerated by illuminating a pieceof paper with two closely spacedthin slits cut into it. Although thetwo waves interfere destructively atcertain points in suchdemonstrations, there are otherpoints at which the two wavesinterfere constructively and soincrease the amplit ude of thewave. It was Young who firstclearly understood how theprinciple of superposition appliesto interference between waves.

When sound is controlled bydestructive interference betweenthe original sound wave and thatgenerated by a controllablesecondary sourc e, we are generallynot just seeking to cancel at apoint but to extend the area overwhich this destructive cancellationoccurs over as wide a region ofspace as possible. Thc mainacoustic objective of designing anactive sound control system is thusto match, as u d l as we can, thespatial variations of the soundfields generated by the original(primary) acoustic sour ce and thecontrollable (secondary ) acousticsource. This geometricrequirement is in addition to thetemporal requirement: that theacoustic waveform produce d bythe se con day source must exactlymirror that of the primary source.The spatial matching of soundfields is only possible in a rathersmall number of physicalsituations, and the se define thegeometries in which active soundcontrol will work best.

which the spatial variation of theprimary and secondary soundfields can be visualised is the one-dimensional case of plane so undwaves propagating in a duct, asdiscussed by Lucg.b The spatialdistribution of a sinusoidal soundwave propagating from left to rightin a duct is illustrated at oneinstant in time in Fig. 4a Thedistribution of a sound wavegenerated by the secondary sourceis also shown in this figure inwhich the waves on eithe r side of

sound control

Perhaps the simplest situation in

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2 Manually adaptive, feedfo ward system for the active control of transformer noise , proposed in 1956 by William ConoverY

. .transmission line by an open-circuit termination. Such an

the secondary source ar epropagating away from it. If thetwo waves are added together atevery point in space the net

pressure distribution is asillustrated in Fig. 4b. Clearly thetwo soun d fields destructivelyinterfere to the right of thesecondary source, so the originalsound wave has been silenced inthis region. The two soun d fieldsconstructively interfere to the leftof the secondary so urce , however,and a standing wave is formed.

It should bc remembered thatthis net pressure distribution onlyexists at one instant in time; atsome later time, when the wavehas progrcssed, the pressurewaveform due to the primarysource will be at a maximum nextto the secondary source. Thewaveform generated by the

secondary sou rce will then be at aminimum and at this instant thetwo waves not only cancel eachother out to the right of thesccondary sou rce, but also thewhole of the standin g wave to theleft (which is all in-phase) is alsozero. The diagram shown by Luegand reproduced in Fig. 1 is thusphysically correct at one veryparticular instant in time. but doesnot really illustrate the overallbehaviour of the active controlsystem.

The physic al effect of thesecondary source is to drive thepressure immediately in front of itto zero at all times. The originalsound wave propagating in the

duct thus sees a large impedancediscontinuity and is reflected backalong the duct, exactly as wouldha me n in an electrical

3 Thomas Young s interfe renceexperiment n optics

impedance discontinuity couldalso be engineered, for example,by using a passive tuned sidebranch in the duct. This

arrangement wouId only work atcertain discrete frequencieshowever, and t he ad vantage of anactive system is that the soundwaves can be reflected back alongthe duct over a broad range offrequencies. Active so und controldoes not introduce any physicalobstruction into the duct, whichmay impede thc air flow, and thisis in contrast with conventionalnoise control methods, whichgenerally require splitters orbaffled expansion chambers.

The seco nd examplc ofmatching thc spatial distributionsof sccondary and primary soundfields that we will consider is thatof two sinusoidal monopole

acoustic sources radiating intofree space. Such an arrangem entis illustrated in Fig. 5 , in which thepositions of the wavefronts due tothe two sources are drawn indifferent colours.

In Fig. Sa the frequency of thetwo sources is low, so that theacoustic wavelength is large andthe wavefronts are well separatedcompared with the distancebetween the sources. In this casethe so und waves generated by thetwo sources a re reasonably wellmatched some distance from thesources; that is, the distancebetween the two sets of wavefronts(which corresponds to the phasedifference between the waveforms)is small in comparison with thedistancc between successive wavefronts from either source (whichcorresponds to a complete cycle of

the waveform). If o ne so urce isarranged to be out of phase withthe ot her, then clearly the twowavefields will destructivelyinterfere, to a large extent, in thisfar-field region. Close to thesour ces the pres sure levels willstill bc significant and the spatialdistribution of the sound field willbe rather complicated, but thecancellation of the far-fieldpressure will still result in a greatlydiminished net acoustic poweroirtprrt from the two sources. Theactive control of radiated soundcan clearly be very successfulwhen the original source iscompact and the secondary sourcecan be positioned a small distance

away, compared with the acousticwavelength.In Fig. Sa, a monopole type

acoustic sour ce has beenconverted into a dipole type sourcewith a subsequent decrease in

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l

secondary sourcea

secondary sourceb

4 Active control of a plane sinusoidal sound wave propagating from right to leftin a duct: (a) spatial distributions of pressure at one instant due to primary wave(blue line) and secondary source (red line): b ) nett pressure field showingdestructive interference to the right of the second ary source, and a standing wave

to the left

a

b

V5into free space: (a) at a frequency at which the wavelength is large compared tothe spacing of the sources: b) at a frequency at which the wavelength is smallcompared to the spacing of the sources

Wavefronts from a primary source (*) and a secondary source 0 ) ropagating

radiation efficiency. In Fig. 5b, thewavefronts are much closertogether, indicating that t hefrequency of the acoustic sourcesis higher, and the wavelength inthis case is not large compared tothe separation of the sources. Thissituation is more analogous to theYoung's slit experiment illustratedin Fig. 3, in which the interference

between the wavefields is at somepoints destructive, but at othe rpoints constructive, even far fromthe sources. In this case the totalacoustic powcr radiated by the twosources will be greater than thatgenerated by the primary sourceon its own. This undersirablefeature can be suppressed bygradually reducing the strength ofthe secondary source as thefrequency incrcascs.1 but alth oughenhancement of the net poweroutput can be prevented, it is stillnot possible to achieve significantreductions in the power outputonce the separation between thesources becomes comparable withhalf the acoustic wavelength.

Fig. 5 illustrates a veryimportant feature of active soundcontrol systems: generally they willonly work well if the ac oust icwavelength is long co rnpared withthe separation bctwccn theprimary and seco ndar j sourcc. Itis unusual in practice to be able toposition a secondary sourcc muchcloser than a metre or so from theprimary source, even assumingthat the primary source isrelatively compact compared withthis distance and does behave as amonopole acoustic source. Thisgeometric constraint imposes anupper frequency limitation on theeffective operation of such activesound control systems of a fewhundred hem . It is verysignificant, however, that activesound control will generally workprogressively better t he lower thcfrequency becomes (i.e. the longerthe wavelength), whereasconventional, passive methods ofnoise cont rol generally getprogressively worse at lowerfrequencies. This observation leadsone to the conclusion that theactive control of sound will notreplace conventional, passive,noise control solutions in themajority of applications wherehigh-frequency noise is importa nt.There are some ar eas, however, inwhich low-frequency sound

predominates and conventionalsolutions would be t oo bulky orweighty to be practical, in theinterior of cars and aircraft forexample. It is here that activesound control comes into its own.

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An enclosed sound field differsfrom a freely propagating one inthat, at certain frequencies,resonances can be set up withinthe enclosure that will cause anincrease in the acoustic responseof the enclosure at thesefrequencies. The characteristicpressure distribution in arectangular enclosure for o ne ofthese acoustic resonances (orroom modes) is illustrated inFig. 6, in which the sinusoidalpressure waveform varies inamplitude (being zero along thenodal planes) an d is either in-phase (blue) or out-of-phase (red)with the acoustic source drivingthe enclosure (which is assumedto be that in the bot tom left-handcorner marked P ) . A secondaryacoustic source in the oppositecorner, marked S, will coupl e intothis room mode in the oppositephase to the original source, andso Fig. 6 could equally wellillustrate the pr essure distributionin the enclosu re when driven bysource S with the regions colouredin red being in-phase with sourccS and th ose in blue being out-of-phase with S . Since the spatialdistributions of the pressure in theenclosure due to the two sourcesare identical, but their waveformsare exactly out-of-phase, thencomplete cancellation of this roommode ca n bc achieved by drivingthe se condary sourc e to preciselyt h e same extent as the primarysource.

In practice, however, theseroom modes a re fairly heavilydamped (with a typical Q factor ofabout five) and even it anenclosure is driven by a singlesinusoidal source, then manyroom modes will be excited tosome degree. Althoughcancellation under suchcircumsta nces will not b e perfect,

secondary source S A

~~~ ~

6resonance (room mode

Pressure distribution in a rectangular enclosure due to one acoustic

the suppression of a dominantacolfstic resonance is clearlypossible, and by using multiplesecondary sources some controlover a number of room modes canbe achieved, giving reductions inoverall pressure level over a widerfrequency range. It is anunfortunate fact that the numb erof acoustic modes in a room withnatural frequency below theexcitation frequency, rises inproportion to thc cube of the

excitation frequency. This meansthat, even if a large number ofsecondary sources ar e used, thererapidly comes a point, as the

excitation lrequenc y is increased,when the number of room modessignificantly excited in theenclosure is very much larger thanthe number of secondary sources,and overall, global control of th esound field is not possible usingan active control systcm. This verydefinite upper frequency limit inenclosures is due to much thesame reasons as the upperfrequency limit in the free field,namely the requirement for good

spatial matching of the soundfields from primary and secondarysources breaking down as thewavelength gets smaller.

a b

7 a ) Physical block diagram and b ) equivalent electrical block diagram of a feedback active sound control system

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1201

7

31.5 6 125 250 500 l 2 k 4 k 8kfrequency, HZ

8 1/3 octave sound pressure level spectrum in the cockpit of a jet aircraft(upper curve) and at the pilot s ear when wearing a conventional headset (middlecurve). The lowest curve is the spectrum at the pilot s ear using a headset withactive noise reduction ANR) eveloped by Racal Acoustics

3 Electrical controller strategies

controllers in active sound controlsystems can be broadly divided

into those operating using afeedforward principle (such as thatof ConoveP) and those operatingusing a feedback principle (suchas that of OlsonR). he feedback

in active sound controlThe design of electrical

control approach requires n oknowledge of the waveform of thfprimary source, and is most oftenimplemented as the simplenegative feedback loop illustratedin Fig. 7 a , in which a loudspeakeris driven by the signal from aclosely-spaced microphone afterpassing through a high-gaininverting amplifier. The equivalen

L I

a

b

9 Block diagrams of an adaptive digital filter used for (a) the adaptivecancellation of electrical noise, and b) he active control of sound. c) Anotherversion of b) assuming the filter is slowly time-varying

block diagram f or thisarrangement is illustrated in Fig.7b, nwhich -A is the gain of theinverting amplifier and C(s) is thepurely electrical transfer functionbetween the loudspeaker input a ndmicrophone output, whichcontains the electroacousticresponse of both thesetransducers, together with theresponse of the acoustic couplingbetween t hem. p is the acousticpressure at the Gicrophone due tothe primary source alone and p isthe acoustic pressure at themicrophone with the active controlsystem operating. The transferfunction of the complete feedbackcontrol system clearly has theform

P S ) 1

P J S ) 1+AC s) 1)~

Provided the phase shift round theelectroacoustic loop C(s) is not toogreat, the amplifier gain A can bemade large, and significantreductions in the pressure at themicrophone can be achieved.

The problem obviously comeswhen the phase shift round theexternal loop C s) approaches180 (as it will do at higherfrequencies, since C(s) containssome element of delay), in whichcasc the system can becomeunstable if the amplifier gain is toohigh. Although compcnsators canbe used in series with theamplifier to extend the usablefrequency range, this tendency toinstability at higher frequenciesdetermines the practical limits ofoperation of such a fcedba\cksystem. Thc stability problem isfurther complicated by thechanges in the response of theelectroacoustic loop C(s) becauseof the variability in the res ponse ofthe transducers a nd the acousticenvironment in which theyoperate.

Despite these practicaldifficulties, a num ber ofcompanies have developedproduction versions of feedback

active control systems forcontrolling the noise inside theearmuffs of headsets. Fig. 8, forexample, shows a 1/3 octavesound pressure spectrumrepresentative of that in the cabinof a military jet aircraft (uppercurve) and that at the ear of thepilot wearing a conventionalheadset (middle curve). The noiselevel at the ear is still clearlyexcessive at low frequencies andcan interfere with the perceptionof speech, whose range offrequencies and levels are also

ri

f

U - M - I132 E L E C T R O N I C S C O M M U N I C AT I O N E N G I N E E R I N GJOURNAL AUGUST 1990

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shown as the grey area in thecentre of the figure. The lowercurve represents the noise at thepilot's e ar when wearing a headsetfitted with an active noisereduction (ANR) system developedat Racal Acoustics.

The other co ntrol strategy whichcan be adopted (feedfonvardcontol) is applicable whe n areference signal can be generatedwhich is related to the soundradiated by the primary source. Anexample of this approach hasalready been illustrated in Fig. 2 ,as used by Conover in the activecontrol of transformer noise.Because the sound field generatedby the primary source is generallynonstationary, such a feedfonvardcontroller must be adaptive totrack these changes in the primaryfield. Conover illustrates manua ladaptation to compensate forchanges that occur in the soundpropagating from his transformer,due partly to atmospheric changesover periods of several hours. In

controlling the engine noise insidecars, however, changes in theamplitude, phase a nd frequency ofthe primary pressure field occurmuch morc quickly than this, on atimescale of a sec ond, and a wayof rapidly adapt ing such afeedfonvard controller must bcfound.

One very successful area ofsignal processing that has

developed rapidly over the pastfew decades is that of adaptivedigital filtering. In Fig. 9 wecompare block diagrams in whichan adaptive digital filter is used inan electrical noise cancellationproblem (Fig. 9a and in which anadaptive digital filter is used a s thefeedforward controllcr in an active

sound control system (Fig. Yb) ,such as that illustrated in Fig. 2. I nFigs. 9a and 9b, x n) is thesampled reference signal. d n ) isthe 'desired signal' in the electricalcancellation problem of Fig. Yaand the signal due to the primaryfield in the active sound cont rolproblem of Fig. 9b. e n ) is theresidual electrical e rro r signal inFig. 9a and the residual acoustical

? primanlM secondary

signal, due to both primary andsecondary so urces, in Fig. 9b. Themajor difference between thesetwo diagrams is the presence ofthe electroacoustic path C betwcenthe input to the loudspeaker andoutput from the microphone inFig. 9b. The presence of this 'errorpath' means that the usual

algorithms used to update suchadaptive digital filtcrs, such as th eL M S algorithm, will not generallyconverge in su ch applications. TheL M S algorithm for the electricalnoise cancellation problem (Fig.9 a may be written as

w(n+ l ) = w n ) + a e n ) x n ) 2)

where w(n) is the vector of FIRfilter coefficients at the nth sample

matrix of M X Kadaptive FIR filters error paths

matrix of L X M

10 Block diagram of a multichannel feedfow ard active sound control system

11inside a car

Schematic diagram of a six-loudspeaker, eight-microphone active sound control system for reducing the engine boom

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time, x n ) is the vector of previousreference signals, e n ) is theinstantaneous error signal and a isa convergence coefficient.

The failure of this algorithm inFig. 9b is due to t he presence ofthe err or path operating on e n )but not on x n ) ,which biases theestimate of the crosscorrelationbetween these signals, which is thebasis for the update quantitye n ) x n ) sed in eqn. 2. A solutionto this problem, put forwardindependently by Widrow et a1.12and Burgess in 1981, was thatthe order of the transfer functionsof the filter and err or path can benotionally reversed (Fig. 9c , inwhich case the adaptive filter isoperating directly on the errorsignal, as in Fig. 9a, but is nowbeing driven by x n ) after havingbeen filtered by a representation ofthe erro r path C to produce thefiltered reference signal ~ n ) .hiscom mut ing of the element s of theblock diagram is not strictly validif the adaptive filter is timevarying, but it does suggest amodification to the LMS algorithmwhich has been very successfulused in practical applications.

w n + l ) = w n ) - a e n ) r n ) 3)where r n ) s now th e vector ofprevious reference signals filteredby a representation of the errorpath C . Eqn. 3 is known a s thefiltered x LMS algorithm.Although in practice the filteredreference signal now has to begenerated (by passing X M ) throughsome electrical model of the errorpath) to perform the update on theadaptive filter, so that someknowledge of the system undercontrol is required, theconvergence of the algorithm ha sbeen found to be very robust toerrors in this error path model.

To actively control the soundthroughout an enclosure it isgenerally necessary to use anumber of secondary acousticsources to minimise the sum ofthe squared pres sures at a numberof e rro r microphones. The generalblock diagram of such amultichannel adaptive activecontrol system is shown in Fig. 10,in which it has been additionally

assumed that multiple referencesignals are being used to drive amatrix of adaptive control filters.The multichannel generalisation of

the filtered x LMS algorithm,which minimises the sum of thesquar ed error si nals by adjustingeach of the coefkcients of an arrayof di gital filters, can be written asI4

w n + l ) = w n ) - a R n ) e n ) 4)

where w n ) s a vector containingthe coeff icients of all the adaptivefilters, e n ) s a vector containingall the error signals and R n ) is amatrix containing each of thedelayed reference signals passedthrough every error path fromeach loudspeaker to eachmicrophone. The calculation of theexact least squares solution forw n ) nvolves the inverse of thematrix RT n)R n) . nd by carefullyordering the various filtercoefficients in the vector w n ) hismatrix can be arranged to beblock Toeplitz,I5 which can resultin more efficient numericalsolution. The multichannel LMSalgorithm described by eqn. 4 hasbeen implemente d, for example, in

a practical system for the controlof the sound at the first threeharmonics of the blade-passingfrequency inside a propeller

12 A-weighted sound pressure level due to the engine firing frequency at head height in the four seat positions of the smallhatchback car illustrated when accelerated hard in second gear

~ standard car; ----- with active sound control system

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aircraft u.sing 16 loudspeakers and32 microphones. It is acousticallydesirable to minimise the sum ofthe squares of the pressures at agreater number of microphonesthan there are secondary sources,and recent work has suggestedthat under such conditions thestability of the multichannel L M Salgorithm is even more robust toerrors in the estimates of the errorpath th an the single-channelfiltered x algorithm.

In practical situations, then,where som e prior knowledge ofthe primary sound field isavailable, feedfonvard controlmethods using adaptive digitalfilters can provide stable contro lthat can still tracknonstationarities. The algorith msgencrally used to adapt suchdigital filters can also begeneralised to cope with the caSeof multiple reference signals,secondary sources and err ormicrophones.

4 Some examples of thepractical application of activesound control inside cars andaircraft

As remarked earlier, the soundinside cars and propeller aircrafttcnds to be dominated by the low-frequency periodic noise fro m thecar engine, or from the propellersweeping past the aircraft fuselage.In both these cases, referencesignals of the appropriatefrequency are readily generatedfrom once-per-revolutiontachometer signals obtained fromthe car or aircraft engines.Adaptive feedfonvard controlsystems have been built and uscdin practice for both these

applications a nd it is informativeto briefly discuss the commonfeatures and the differencesbetween the two contr ol systems.

Although the sire of passengercabin in a 50-seat propelleraircraft, as used for the flight testsof a practical c ontrol system, isobviously somewhat larger tha nthe size of a typical car interior,the upper frequency of operation(about 200-300 HI) of bothsystems is quite similar. This isbecause the acoustic limit onglobal control is due to the rise inthe acoustic modal density withfrequency, which is similar in thetwo cases. Because of thedifference in size, however, thenumber of secondaryloudspeakcrs (16) and errormicrophones (32) required tomaintain reas onable active controlin the aircraft cabin isconsiderably higher th an that

10

a

b

13cabin of a British Aerospace 748 propeller aircraft at the blade passing frequency:(a) in the standard aircraft; b) with active noise control

Spatial distribution of the normalised sound pressure level in the passenger

necessary to control the noise atany one speed in the car , which istypically two loudspcakcrs andfour m icrophones. The frequencyof excitation in a ca r can vary by afactor of u p to ten to one as theengine is taken from idle tomaximum speed, whereas therotational spced of the propellersis normally kept within a muchnarrower range. This means that atwo-loudspeaker, four-microphonesystem is not usually sufficient tocouple into the variety of acousticmodes that a re excited over thespecd range experienced in a car,and a four- o r six-loudspeaker,eight-microphone system is morctypically used. The controlle r forthe car, then, has to manage asmallcr number 01 channels than

that for the aircraft, but must beable to adapt more quickly to copewith the rapid changes in enginespccd and engine load.16 Aschematic diagram of a practicalcontrol system for a car is shownin Fig. 1 1, The positioning of theloudspeakers and microphones,together with the fine tuning of theadaptation coefficient and variousother aigorithm parameters mustbe optimised from one mode l ofvehicle to anoth er f or the bestresults. Fig. 12 shows the soundpressure level at the engine firingfrequency only, passed through anA-weighting frequency-selectivefilter, at various positions in asmall hatchback car as it is beingaccelerated hard along a test trackin sccond gear . The solid lines ar e

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the levels in the normalproduction ca r and the dashedlines ar e with the benefit of a f o u rloudspeaker, eight-microphoneactive control system. Largereductions of 10- 15 dB a reachieved in the front seats by theactive control system above anengine speed of about 3000 RPM(which corresponds to a n enginefiring frequenc y of 100 Hz), and atsomewhat lower engine speeds inthe rear seats. The engine firingfrequency is not the onlycomponent of the sound in a carbut, as cars become morepowerful and lighter, this low-frequency 'boo m' increasinglydominates the overall sound level,and is very difficult to controlusing conventional, passivetechniques to th e extent shown inFig. 12 without significantlyincreasing the overall weight of t hevehicle.

A similar dilemma faces thedesign er of a propeller aircraft:reductions in the dominant low-

frequency sound in the passengercompartment can only beachieved, using conventionalmethods, with a significant weightpenalty. Flight trials of an activesound control system in a BAe748, however, have shown that u pto 14 dB can be taken off the su mof the squared outputs of the 32control microphones at the bladepassin g frequ ency of 88 Hz using16 internal loudspeakers assecondary sources.17 Fig. 13 is anisomet ric plot of the magn itud e ofthe pressure at 88 Hz measured atthe 32 error microphonesuniformly distributed at seatedhead height in the passenger cabinof the aircraft used f or the in-flight

experiments. The normalisedsound pre ssure levels in decibelsare plotted as dashed points at thefour microphone positions acrossthe cabin (from port to starboard)and at ten seat row positions(from forward bulkhead to row10) going along the cabin. Fig. 13ashows the sound field withoutactive control and Fig. 13b showsthe pressure field, measured in thesame positions, after the activecontrol system was switched on.The overall reduction in level anda genera l flattening of the spatialvariation in the soun d field can beseen clearly.

5 ConclusionsAlthough the basic principles of

active sound control have beenknown for over 50 years, it is onlyrecently that advances in digitalsignal-processing technology haveenabled practical multichannel

active control systems to berealised. The possibilities openedup by these advances have alsostimulated a re-examination of thebasic physical principles of activesound control.

the sou nd field from a n arra y ofcontrolled secondary sources todestructively interfere with tha tfrom some original primarysource of sound over a usefulvolume of space if

a) the waveform of thesecondary source is the mirrorimage of the primary source (atemporal constraint) and

b) he soundfield distributionfrom the secondary source nearlymatches that from the primarysource (a spatial constraint).In practice, the latter conditioncan often only be met bypositioning the secondary sourcewithin a fraction of an acousticwavelength of the primary source.This leads to a fundamental upperfrequency range of operation forglobal active sou nd contro l of afew hundred hertz. Active noisecontrol systems working in a morerestricted volume, such as anearmuff, can work to somewhathigher frequencies, but physicallimitations always limit the upp erfrequency range of operation.

The fact that active controlworks better at lower frequenciescomplements more conventionalnoise control methods, usingabsorptive materials for example,which tend to work better athigher audio frequencies. Inapplications where strong low-frequency c omponents are aproblem, and in which the

additional weight associated withpassive shielding or absorptioncannot be tolerated, active soundcontrol offers an attractivealternative. Current applicationsbeing developed includecontrolling the low-frequencyengine noise in cars and the low-frequency propeller noise in thepassenger cabins of aircraft. Inboth these applications verysignificant reductions in soundpressure level have beenexperimentally demonstrated.

In general, it is only possible tor

AcknowledgmentsFundamental work on active

control at the Institute of Soundand Vibration Research atSoutha mpton University has beensupported by the DTI and UKSERC. The practical applicationsin aircraft and cars have been theresult of co-operation with BritishAerospace and Lotus Engineering,

and we a re grateful to Lotus forsupplying Figs. 11 and 12 in thispaper a nd to Racal Acoustics forproviding Fig. 8.

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0 IEE: 1990First received 2 st May and in revisedform 22nd June 1990

The authors ar c with the Institute ofSound an d Vibration Research, T h eUniversity, Southampton SOY 5 UK.

136 ELECTRONICS COMMUNICATION ENGINE ERING JOURNAL AUGUST 1990

The Active Control of Sound

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