Impulse noise: some definitions, physical acoustics and other considerations.

Impulse noise: Some definitions, physical acoustics and other considerations RogerP. Hamernikand KengD. Hsueh AuditoryResearchLaboratories, State University ofNew York•Plattsburgh, 107BeaumontHall, Plattsburgh,New York 12901

(Received1 September1990;revised1January1991;accepted18February 1991)

An overviewof the impulsenoise(blast wave) stimulusis presentedwith an emphasison examiningthoseparametersthat havebeentraditionallyusedto quantifythe stimulusfor the purposeof understanding its effectson hearing. PACS numbers:43.66.Ed, 43.50.Pn, 43.50.Qp [WAY]

INTRODUCTION

The primary purposeof this introductory paper is to definethe impulsestimulusthat will be the focusof the next severalpapers,and to discusssomeof the characteristics that distinguishit from "ordinary" acousticstimuli. In addition, this paper will presentsomeof the variousterms that have beenusedto quantifythe impulsefor the purposeof understandingits effectson the auditory system.This paper presentsno new information, but rather relies heavily on the informationpresentedin the referencedpapers.It is hoped that perhapsa slightlydifferentperspectiveon the impulse stimulusmight help to clarify some of the terminology which hasevolvedoverthe pastcoupleof decades.

I. IMPULSE

AND IMPACT

NOISE

DEFINED

The terms impulsenoiseand impact noisehave often beenusedinterchangeably whenan adjectivewasrequiredto describehigh-peaksound-pressure level ($PL) noisetransients.This casualuseof terminologyhas, at times,led to confusion. The following phenomenologicaldefinitions probablyrepresentthe mostfrequentlyusedor implied convention.

(1) Impulse noise:a noisetransientthat arisesas the resultof a suddenreleaseof energy(most oftenelectricalor chemical)into the atmosphere.Typicalsourcesof suchtransientsareexplosivedischarges suchasgunfire,circuitbreakers, etc. The physicalcharacteristicsof theseimpulsesare largelydependentuponthe geometryand scaleof the source, i.e., the shapeof the explosivecharge,sizeof the discharge, etc. The receivedwaveformis further dependentupon the environmentin whichit propagates. Peakoverpressures can often exceedone atmosphere( 194-dBpeak $PL). (2) Impact noise:a noise transient that arisesas the resultof the impactbetweentwo objects,e.g.,hammerstriking a metalplate,punchpress,etc.The impactsare essentially the resultof a rapid releaseof energythrough primarily mechanicalmechanisms.The physicalcharacteristics of impactsare largelydependentuponthe mechanicalproperties of the impactingobjectsaswell ason the transmission path. A comprehensive reviewof impactnoisesourcesalongwith more indepth classificationand discussionof the mechanismsof productionis presentedby Akay (1978). 189

J. Acoust.Soc.Am.90 (1), July1991

II. PHYSICAL BASIS FOR THE DISTINCTION IMPACT AND IMPULSE NOISE

BETWEEN

Thesetwo classesof transientswhile generallyhaving differentoriginsare basicallythe samekind of an acoustic/ macroacousticeventwhosephysicalunderstandingcan be derivedfrom a commonset of principles.Thus the above classification of transientsasimpactor impulseis somewhat artificialsinceboth fall alonga continuumof signals,whose

temporalsignatures, whiletheycanbebothrelativelysimple or quitecomplex,areprobablybestcategorizedby their peak overpressures. It is the magnitudeof the overpressures that determinethe specificrulesor equationsthat governthe behaviorof thesetransients.The peakoverpressure will determine,for example,whetherthe linearizedwaveequationor some form of a nonlinear equation can best describethe propagationof thetransients.Also,for thepurposeof understandingtheeffectsthat thesetransientshaveonhearing,the peak overpressure is the parameterwhich indicates,for example,that the auditorysystemmay be operatingin its nonlinear rangeor that the mechanismof cochleardamageis primarily mechanicalor metabolicin origin. In practice,mostof theimpulsesthat will bediscussed at thismeetingwill havepeakoverpressures in excessof 140dB SPL and the impulsewill consistof a singleshockfront or several shock fronts in the case of a reverberant

enclosure.

The most commonlyencounteredindustrial impacts will generallyhave peak SPLs under 140 dB. Thus historically for reasonssuchasthese,140-dBpeakSPL hasbeenconsidered a fenceseparatingthe impulsefrom the impact stimulus.One hundredandforty dB peakSPL is alsoa convenient number on the basisof physicalacousticsconsiderations. Webster and Blackstock (1977) have shown that at around

the 140-dB level for midfrequencypure tones, nonlinear wavebehaviorbeginsto significantlyaffectthe profileof the wave. At a peak SPL below about 140 dB the weak shock front that characterizesan impulsecannotbe sustainedfor very long. Thus, at levelsbelowapproximately140 dB, the impulseand the impact for all practicalpurposesbehave similarly and no distinction between the two transients should be made. III. SOME SHOCK

WAVE

PHYSICS

The pressuretransients(impulses)which producethe soundsensationsand auditory systemtrauma that are the

0001-4966/91/070189-08500.80

¸ 1991 AcousticalSocietyof America

189

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 84.72.8.38 On: Sat, 17 May 2014 06:40:56

focusof this portionof the meetingare not acousticwaves, i.e., within the domain of linear acoustics,but rather fall

within the domainof macroacoustics and shockwavephysics. The analytical relationsthat predict the behavior of acousticwavesareobtainedfrom a linearizationof the equations of fluid motion. This linearizationis basedupon the assumptionof infinitesimalamplitudesand gradientsof the field variablesand the assumptionof an isentropicprocess. Thushigher-ordertermsin the fieldvariablesanddissipative effectsareneglectedandthe second-order, linear,partialdifferential equation(acousticwave equation),which results from theseassumptions, predictsa wavedisturbance,which propagateswithout distortion.Nonlinearconvectiveeffects in contrastwith dissipativeeffects(viscosity,thermal conductivity) can be measured at sound-pressurelevels of around 140 dB for a 1000-Hz tone. For a pure tone, such nonlinear (convective)effectscan easilybe measuredin the following type of experimentconductedby Webster and

a sound wave. For additional details Morse and Ingard (1968) and Shapiro( 1953) shouldbe consulted. The thickness of the shock front is related to the rise

time of the ideally measuredpressure"jump" acrossthe shock (as distinguishedfrom instrumentationresponse time). Acrossa shockfront, thereare veryhighgradientsof propertychange,viscousstresses becomelarge,and nonadiabaticeffectsmust alsobe considered.The dissipativeeffectstend to erodediscontinuitiesin velocityand temperature and thuslimit how rapidly changesare allowedto take placeacrossthefront. For weakshocks,for example,having overpressures on the orderof 2 K Pa ( • 160-dBpeakSPL), the shockthickness, in standardair, is on the orderof 10- 5 m, whichequatesto a "risetime" of approximately 0.01/•s. At 190-dBpeakSPL (63 K Pa overpressure), thesevalues

areontheorderof 10-6 m and0.001/•s,respectively.

Blackstock, (1977): Given a sound source and receiver,

IV. THE IDEAL IMPULSE FOURIER TRANSFORM

graduallyincreasethe SPL of the sourceand measurethe signaltransmittedto the receiver.At thelowerrangeof SPL

The instantaneous releaseof energyfrom a pointsource in a free field, i.e., a field without any reflectingsurfaces,

there is a linear relation between the source and the received

SPL [ Fig. 1(a) ]. As the SPL of the pure toneincreasesthe source-receiver functiondeviatesfrom linearity and gradually flattensout sothat the receivedSPL of the fundamental toneno longerincreases asthe sourcelevelis increased.The wavedistortsandthe additionalenergyof the sourceappears in the higherharmonicsof the fundamental.In the absence of viscosityand heat conduction,the wave distortion is a resultof a wavespeedthat changesfrom onepointto another (convectiveeffect).Differentportionsof the wave,traveling at differentspeeds,causedistortion.Changesin the local speedof soundasa resultof localtemperaturechangesalso contributeto the distortion.The originalhigh-levelsinusoid graduallydistortsinto a "sawtooth"-like wave [Fig. 1(b) ] referredto asa shockedsoundwave,or a repeatedseriesof shockwaves.Acrossa shockfront, the properties(i.e., velocity,density,pressure,entropy,etc.) of the systemchange discontinuously. Thesepropertiescanberelatedacrossa stable planeshockfront (i.e., a shockfront in which a balance hasbeenattainedbetweenthe competingeffectsof the convectivenonlinearityand dissipation),by usingthe RankineHugeniotrelationsand a suitableequationof state.For example,the speedof the shockfront canbeshownto be given

NOISE WAVEFORM

AND ITS

/'lE'xtra

(a)

Attenua

! Saturation

Line. ar I

_Approac. hto

Plateau

Region

Region II Saturation Source

SPL - dB

(b) 130 dB

by,

U = Co{ 1 + [ (•' + 1)/2•']P}'/2,

( 1)

where ?'= Cv/C• is the ratio of specificheatsat constant pressureand volume,respectively;Co is the speedof sound in the undisturbedmedium, and P = (p--Po )/Po is the shockstrength,i.e., the changein pressureacrossthe shock front relative to that in the undisturbedmediumPo. The pressurechange(p- Po) can be obtainedwith a conventionalmicrophoneor otherpressuretransducer.For a shock travelingthroughair at standardconditions?'= 1.4,and in the limit as P becomesvery small (i.e., a very weak shock wave), Eq. ( 1) showsthat the shockspeed,U approaches the localspeedof soundCo. Thus the amountby which the shockspeedis greaterthan the local speedof soundis one

FIG. 1. (a) The amplitude response functionfor thefundamental of the source toneshowing thedevelopment of saturation. (b) Schematic of the received signalat a fixeddistance fromthesource forvarious source levels

measure of the extent to which the shock wave deviates from

(from Webster and Blackstock, 1977).

190

J. Acoust.Soc. Am., Vol. 90, No. 1, July 1991

142 dB

160 dB

R.P. Hamernikand K. D. Hsueh:Impulsenoisedefinitions

190


I

I

I

I

I

t_

I

I I I IIII

i

i

i i iiiii

i

i

i

i iiiii

i

i

i

i

40-

(at) FriedlanderWave PF(t) = (1- t/c)e't/c m

•

20-

pmntof transitionto

• i/

lO-

g:

•

•B/•ctslope

oI

I

t•_

cq •

I

T = 6c I

-' I

Time

I

I

I

(a)

-lO

I

'I

I Frequency I I I IIIII I•I I I IIIII ' ' ';p'{•'•,,c

! I I!

I

!

•

I

I

i

I

i iiiii

I

.

I

i

i

i iiiii

I

i

i

I iiiii

I

i

i

Arg[P F(o))]=tan-•[(1- o•2c2)/(2o)c)]

(b) FriedlanderWave withrisetime

I

p(t) =ps(t) +PF(t)

ps(t)= t/b, b>0

•

/i•

I

p•(t) =(1-r/c)e-,'/c, t'=t-b

.

-2

I

I ,

m

I

I

I'

Time

, 'I

III

I

I

I I I IIII

I

I

I I I IIII

!

I

I

I I•

f'•,='•/2•C Frequency •

I

T=b+6c I

i

. (b)

i

• I

I

I

I

I

!

IIII

I

i

I

I

I

I

I

I !11

i

i

i

i

i

i

I

8

FIG, 2. (a) The idealimpulseor Friedlanderwavewith zerorisetime. (b) The idealimpulsewith a finiterisetime. i

-4.

produces a pressure-time, p (t), signatureof theapproximate formshownin Fig. 2 (a). An impulsesignatureof thistypeis

i

(c)

-8i

i

i

i

often referred to asa Friedlander wave, named after the British mathematician

i

i

!

iiI

i ii

1/2•tc

Frequency --•

Frederick G. Friedlander who used such

waveformsin the studyof diffraction (Friedlander, 1946).

Theimpulse isidealinthesense thatitsp(t) profile can bemodeledby Eq. (2). This relationdescribes a wavewhose rise time is zero and has no reflections:

pF(t) ={0, --OO t 1, an approximateexpression for the amplitudespectrumcanbeshownto be

IP½o)I = (xf•/co2b) [ 1- cos(cob) ],/2.

(9)

p(t)dt(Ns) m2 .

(10)

The "impulse"ofp(t) is a quantityoftenusedin structural mechanics whenimpulsiveloadsmustbetakeninto account, but althoughmuch of the peripheralauditiorysystemis a mechanicalsystem,the "impulse" has not beenappliedto problemsassociatedwith noisetrauma; (b) the time-integratedsquaredsoundpressure(Young, 1970) alsoreferred to as soundexposure,i.e.,

rp2(t)dt [(3)s],

(11)

and (c) the energyflux (energyper unit area), i.e.

•o rp2( t)dt (j/m2). pc

(12)

This calculationis valid only for the specialcaseof a free progressivewave (plane wave). If indeedthis relation applies to a given situation,then atmosphericpressureand temperatureshouldbe stated,and the appropriatevalueof pc used.However,if oneis generallymorecomfortable with energylikeunits,Eq. (12) can still be usedeventhoughthe planewaveassumption may not bevalid. If thisis done,then it mustberecognizedthat the resultis not the actualvalueof the energyflux. It is also important to specifythe precise valueofpcthatwasusedin thecalculations, in orderthatthe

quantityforp2(t)dtcanbe recovered sothat comparisons amongdata from differentlaboratoriescan be made. The abovethreerelationscanbe usedto computethree different quantities,in addition to peak and duration parameters, whichcanbeusedto quantifyan impulsein thetimedomain. In orderto furtherquantifyrealistic(nonideal) impulse noisestimuli in the time domain, it has alsobeencustomary

to definean "effective"duration of an impulsein several differentways.Someof thesesuggested temporaldefinitions havetheir originsin a desireto obtaina temporalindexthat couldbe relatedto the energyof an impulseresponsible for causingtrauma. Referringto Fig. 5, someof thesedefined durations (Lemche, 1987) are:

From Eq. (9), one can showthat, for frequencies f• 1/2rrc (or coc>>1), the envelopof the amplitudespectrumdeclines at -- 12 dB/oct.

It can be shown that this is true for all

frequenciesabovef= 1/3b. For frequencies below 1/3b, the spectralenvelopis similar to that of the ideal zero risetime impulse.The transitionpoint betweenthe --6- and -- 12dB/oct slopesmovestoward the lower frequenciesas b increases.Theserelationsareshownschematicallyin Fig. 4. A relatedpresentationcanbe found in Kryter (1970). 192


(a) Aduration --(t• --to ) (b) B duration = (t 1 -- to ) q- (t 3 -- t2 ) (Coles et al., 1968)

(c) C duration = (t• --to ) + (t3 --t2 ) q- (t5 --t4 ) + ... (Pfander et al., 1980) R.P. Hamernikand K. D. Hsueh:Impulsenoisedefinitions

(13) 192


(d) D duration = (t, -- to) (Smoorenburg, 1982) •kPmax (e) peak SPL = 20 log • APref •

(a)

•

•

whereApref-- 2 X 10- 5 Pa.

(c)

When p(t) is relativelywell behaved,mostof the above parameterscaneasilybe obtained.In the frequencydomain

Pe= peakpressure

Pe---

Pe---

c:: 20 dB

10 dB

a number of different terms have often been used in the liter-

O

ature to identify the ordinateaxis of the Fourier pressure spectrum.For impulseswhere the Fourier pressurespectrum, IP(a•)I, is a continuous functionof frequency,theordinate is a quantity that representsa spectraldensity, i.e., pressureper unit frequency.An applicationof the Fourier transformto p (t) yields

to/' Time

•

(b)

(d)

FIG. 5. Schematic drawingsof severalsimpleimpulsewaveforms that illustrate the varioustemporalpointswhich are usedto determinethe (a) ,4 duration,(b) B duration,(c) C durationand(d) D durationofanimpulse.

I

I

I

(14)

I

i

i

i

i

i

i

iiii

i

I

I

i

i

iiii

i

i

i

i

i

i

_

PeakSPL= 155dB re:2Xl0-Spa

• (a)

12

(a)IP(co)l(•/Hz), the Fourier pressure spectrum

(d)

pc = 406 mks rayIs

=

v

•

o.8

•f=122Hz = 4O96

= 500

_

kHz

-

-

70.4

-

_

i

i

i

i

2

4

6

8

Time I

i

i

i i i iili

i

.........

I

I I I IIII

;•)

......

Frequency (kHz) i

i

i I I I

O-

I

o

--

-20

]........

(msec)

x

-

I

I

I

I

I IIlI

I

(e) A-Weighted

I

IIIII

I

I

I

I

II

Af _= 1 22 HZ

'•

N = 4096

e•F:16

-60-

I

pc = 406 mks rayIs

N

-40 -

I

24

500kHz

X

-80 •

re:6XIO-4j/m2/Hz rs=500 kHz, N=4096

(b) pc= 406 mksrayls &f_-122Hz

-1oo

•--

I o

i

i

I I I IIII

i

i

I I I IIII

I

I

I I I IIIi

I

1

I I I I i111

10

I

I

I I I I

Frequency (kHz) i

i

I •

_

.•

i

L•

Frequency (kHz)

A-Weighted

0.10

ß Unweighted [] A-Weighted pc = 406 inks rayIs

0.08 0.06

-40

-60 -80

-IOO

re:6XIO-4j/m2/Hz fs=500 kHz, N=4096

(C) pC= 406 mksrayls &f; 122Hz

i i i i i i illI i i I I i illl i i i i iI IO Frequency (kHz)

o.oo 0.125

0.25

0.5

1.0

2.0

4.0

8.0

16.0

Frequency (kHz)

FIG. 6. (a) A typicalimpulse waveform generated bya shocktube.(b) Therelativesoundexposure spectrum levelfortheimpulseshownin ( a). (c) TheAweighted sound exposure spectrum levelfortheimpulse shown in (a). (d) Theenergy fluxspectral density fortheimpulse shown in (a). (e) TheA-weighted energyfluxspectraldensityfor theimpulseshownin (a). (f) An unweighted andan A-weightedoctavebandanalysis of thewaveformshownin (a). 193

J. Acoust.Soc.Am.,Vol. 90, No. 1, July1991


193


or

(b) IP(co)12

s/Hz

,

the Fourier pressuresquaredspectrum (the sound exposurespectrum); (15) or

an energyterminologypersistsin the noisestandardsliterature, and this is the primary reasonfor someof the above discussion relatingto the unnecessary contortioninvolving pc. It is probablyadvisableto avoidaltogetherthe useof an energyterminologyandadheresolelyto terminologythat is basedupon the quantity measured,i.e., pressureand the quantitiesderivedfrom the pressurerepresented by Eqs. (11) and (15).

pc

an energy (flux) spectrum in the case of a plane wave. (16)

As in the time domainandwith the samecaveats,if energylike termsare soughtregardless of the validity of the plane

waveassumption, IP(o)I • cansimplybedivided bya clearly specified pc to producea quantitywith unitsof energy(flux) spectraldensity.In both the time and frequencydomains,if a "level" is desiredany of the abovethreequantitiescanbe referencedto someappropriatequantitywith matchingdimensions to producea relative(dB) scale.Figure6 (a) illustratesa typicalimpulsewaveformproducedby a shocktube and measuredin anechoicsurroundings.Figure 6(b) illustratesthe relativeenergy(flux) spectraldensity(soundexposure spectrum level) of this waveform. The reference

VI. MEASUREMENT

AND ANALYSIS

CONSIDERATIONS

Becauseof the extremelyhighlevelsandshortdurations of many impulses,most conventionalsound-pressure level instrumentationmay not be suitablefor the measurementof many typesof impulsenoiseand piezoelectricor piezoresistive typesof transducersmay be required.Similarly, since muchcontemporaryinstrumentationin thisfieldreliesupon digital technologyin someelementof the measurementand analysissequence, caremustbe exercisedin the selectionof suitableequipment.Thereareseveralgoodpublicationsthat addressinstrumentationrequirementsin greatdetail,for example, Garinther and Moreland (1965), Crockerand Sutherland ( 1968), Patterson et al. (1980), and Lemche ( 1987).

In digital systemsthe minimum samplingfrequency(digitization rate) of 160K samples/sthat hasbeenrecommended

quantity forthedBscale was choosen as6X10-4j?m2?Hz,(Lemche, 1987) iseasily achieved incontemporary PC-

where pc= 406mks rayls. Withthisreference information o based data acquisition systems, and low-cost 1-MHz systems

an absolute ordinatescalerepresenting IP(o)12canbe re-

covered.For purposesof comparison,this samespectrumis shownin Fig. 6(d) plottedin termsof the absoluteenergy flux densityalong the ordinate. While relative scalesare at timesconvenientto use,theycanbe difficultto obtainquantitative information from unlessreferencequantitiesare clearlyspecifieda practicenot alwaysobservedin the literature. Sincethe time- and frequency-domainanalysisof an impulseare equivalent,analyticalproceedures involvingthe Fourier transformcaneasilybe checkedby applyingParseval's theorem:

p2(t)dt=1

trum, IP(o)I, is symetricrelativeto the ordinateaxis,the positiveand negativefrequencycomponentsare identical,

IP(co)l 2do.

(17)

Becauseof the differentialspectralsensitivityof the ear, it iscommonto applyweightingfunctions to thesoundexposure spectra[Eq. (15)]. The most commonlyusedin the noisetraumaliteratureistheA-weighting.The precisevalue of the A-weightingfunctionacrossthe rangeof audiblefrequenciesto beappliedcanbefoundin a numberof references (Beranek,1988and ANSI, S1.42, 1986). The A-weighted spectracorresponding to the spectrashownin Fig. 6 (b) and (d) are shownin Fig. 6 (c) and (e), respectively. The effect of A-weightingis clearlyseenwhenFig. 6(d) and (e) are compared.The sameinformationis presentedin an octave bandformatandplottedasa bar graphin Fig. 6(f), a form that is often convenientwhencomparisons to audiometric dataneedto bemade.In additionto the abovetemporaland frequency-domain parameters, thetotalnumberof impulses and somemeasureof interstimulusintervalmay be important parametersin the specificationof the conditionsof exposure.

Beforeconcludingthis section,it shouldbe notedthat 194

arecommonlyavailable.Aliasingproblemsthat canoccurin analog-to-digital(A/D) conversioncan be avoidedif the amplifiedsignalsfrom the transducersare low-passfiltered prior to digitizing.The cutofffrequencyof this anti-aliasing filter is normally set at approximately1/3 of the sampling frequency.A filter rolloff of 24 dB/oct or greateris recommended. In order to insure sufficientaccuracy,the resolution of the A/D convertershouldbe 12 bit or greater. It shouldalsobe noted that, sincethe Fourier pressurespec-


i.e., IP(o)l - IP( - o)l. If a discreteFourier transform (DFT)

[or a fast Fourier transform (FFT) ] is used, the

frequencyrangeof the spectrumvariesfrom -- fs/2 (Hz) to + fs/2 (Hz), wherefs is the samplingfrequency.This type of Fourier spectrumis normallyreferredasa two-sidedspectrum. The one-sidedspectrumis calculatedby adding the negativefrequencycomponentsto their positivecounterparts (i.e., doublingthemexceptfor the dc component). The one-sidedspectrum is commonly used becauseit correspondsto measurements madeby traditionalspectrumanalyzers.When a relative (dB) spectrumis presented,it is important to specifythe referencequantity used so that the absolutequantity that is actually measuredcan always be retrieved.In addition, the samplingfrequency,f•, and the numberof pointswithin an FFT windowNneedsto be specifiedsothat the frequencyresolution(bandwidth,Af) canbe determined,where Af=f•/N. Note that the DFT yields a discreteFourierpressurespectrumthat represents a seriesof meanpressures over the bandwidthAfacrossthe frequency analysisrange (i.e., from --f•/2 to + f•/2). A better approximationof the continuousFourier transformofp (t) can be achievedby reducingthe Afused in the DFT. FrequencyR.P. Hamernikand K. D. Hsueh:Impulsenoisedefinitions

194


2O

slow,low-frequencybuildupof pressureuponwhich reflected wave componentsare superimposedprior to a decayto

16

ambient conditions. In such cases,a time-domain definition 12

of the impulseparametersis of questionableutility. In the frequencydomainthe impulsesmay becomemore amenable to quantification. While the variousclassesof impulsesdiscussedabove often occur as isolated events, there are also common situa-

4O 30

20 10 0

10

20

8O

6O 4O

20

tionsin whichthey are presentasa frequentcomponentof a morecomplex,high-level,non-Gaussiannoiseenvironment. For such environments,peak SPL, and rms measuresor evenconventionalfrequency-domain measuresmay not be adequateto definethe hazardsto hearing.A varietyof establishedanalyticaltechniques,which have beenmade practicalbecauseof high-speed digitaltechnology,maybesuitable for applicationto suchnon-Gaussiannoiseenvironments. For example,the conceptof kurtosis (Erdreich, 1986; and Dwyer, 1984) in both the time and frequencydomainsmay be usefulin quantifyingthe impulsivenature of the time signalor identifyingparticularfrequencycomponents of the signalthat are subjectto rapid, high-levelfluctuations.Another promisingapproachmay be that of complexcepstral analysis(Childerset al., 1977), which is usefulin quantifying someof the temporalvariablesof a non-Gaussian signal that are "lost" in a conventional Fourier transform as a re-

10

20

Time (msec) FIG. 7. Three examplesof impulsesrecordedwithin a hard-walledenclosure.

sult of our inability to effectivelydeal with the information containedin the phase spectrum (Oppenheim and Lim, 1981). Analytical techniquessuchasthese,aswell asothers usedin signaldetectionresearch,may prove usefulin the future.

ACKNOWLEDGMENTS

weightingfunctionsthat are oftenappliedto acousticdata shouldnot be appliedprior to the recordingor digitizing stageof the analysissystem.Otherwise,the originalwaveform will be difficultto retrieve.Windowingfunctionsoften usedwhencontinuous signalsareanalyzed,needto beselected with care when transientsare being analyzed.For impulsesa rectangularsamplingwindowis normallyrecommended. Other windowing functions, for example, a Hanningwindowcanintroduceinaccuracies in theresulting

Akay, A. (1978). "A reviewof impactnoise,"J. Acoust.Soc.Am. 64, 977987.

ANSI (1986). ANSI S1.42-1986,Designresponse of weightingnetworks for acoustical measurements (Acoustical Society of America, New

analysis.

York).

VII. REALISTIC

IMPULSE

SIGNATURES

In practice,realisticimpulsesdifferin varyingdegrees from the ideal Friedlandertype of impulse.The effectsof reflectingsurfacesproducesecondaryreflectedwavesthat reachthe recordingstation.The time interval betweenthe primaryand secondary wavesis dependent on the relative distancesbetweenthe reflectingsurfacesand the measuring location.In addition,the peakand the spectrumof the reflectedcomponents are alteredby the impedancecharacteristicsof the reflectingsurface.Figure 7 illustratesthep(t) for three differentimpulsesmeasuredwithin a confined space.Variablesfor suchcomplexsignatures suchasthedurationsdefinedin Fig. 5 cannotalwaysbe established without ambiguity,and theremay be more than a singlepeak pressure. Furthermore,within enclosures, thereis oftena 195

The supportof the U.S. Army Medical Researchand DevelopmentCommandthroughContractsDAMD-17-86C-6139 and DAMD- 17-86-C-6172 is gratefully acknowledged.The authorswould alsolike to thank G. Turrentine for his patienceand skill in preparingthe figuresand Renee Johnstonfor preparingthe manuscript.


Beranek, L. L. (1988). AcousticalMeasurements(Acoustical Society of America,New York) pp. 808 and 809. Childers, D. G., Skinner, D. P., and Kemerait, R. C. (1977). "The cep-

strum:A guideto processing," Proc.of IEEE 65(10), 1428-1443. Coles,R. R. A., Garinther, G. R., Rice, C. G., and Hodge,D.C. (1968). "Hazardousexposureto impulsenoise,"J. Acoust.Soc.Am. 43, 336343.

Crocker, M. J., and SutherlandL. C. (1968). "Instrumentation requirements for measurement of sonic boom and blast waves--A

theoretical

study,"J. SoundVib. 7, 351-370.

Dwyer,R. F. (1984). "Useofthekurtosisstatistics in thefrequency domain asan aid in detectingrandomsignals,"IEEE J. Ocean.Eng. OE-9 (2), 85-92.

Erdreich,J. (1986). "A distributionbaseddefinitionof impulsenoise,"J. Acoust. Soc. Am. 79, 990-998.

Friedlander,F. G. (1946). "The diffractionof soundpulses,"Proc. R. Soc. London Ser. A, 322-367.

Garinther, G. R., and Moreland, J. B. (1965). "Transducertechniquesfor

measuring theeffectofsmall-arms noiseonhearing,"U.S.H.E.L. TM 1165 AberdeenProvingGround,MarylandAMCMS Code5011.11.84100. R.P. Hamernikand K. D. Hsueh:Impulsenoisedefinitions

195


Kryter, K. D. (1970). The Effectsof Noise on Man (Academic, New York).

Lemche, V. (1987). "Effects of impulse noise," NATO

document

AC/1245 (Panel 8/RSG. 6)D/9.

Morse, P.M., and Ingard, K. U. (1968). TheoreticalAcoustics(McGrawHill, New York).

Oppenheim,A. V., and Lim, J. S. (1981). "The importanceof phasein signals,"Proc. IEEE 69(5), 529-541. Patterson, J., Coulter, G. A., Kalb, J. et al. (1980). "Standardization of Muzzle blastoverpressure measurements,"SpecialPublicationARBRL

SP-00014BallisticRes. Lab., AberdeenProvingGround, Maryland. Pfander, F., Bongartz,H., Brinkmann, H., and Kietz, H. (1980). "Danger

196


of auditoryimpairmentfrom impulsenoise:A comparativestudyof the CHABA damage-riskcriteriaand thoseof the FederalRepublicof Germany," J. Acoust. Soc. Am. 67, 628-633.

Shapiro,A. H. (1953). TheDynamicsand Thermodynamics of CompressibleFluid Flow (Wiley, New York), Vol. 1. Smoorenburg, G. F. (1982). "Damagerisk criteria for impulsenoise,"in New Perspectives onNoise-induced HearingLoss,editedby R. P. Hamernik, D. Henderson,and R. Salvi (Raven, New York), pp. 471-490. Webster,D. A., and Blackstock,D. T. (1977). "Finite-amplitudesaturation of planesoundwavesin air," J. Acoust.Soc.Am. 62, 518-523. Young, R. W. (1970). "On the energytransportedwith a soundpulse,"J. Acoust. Soc. Am. 47, 441-442.


196


Physical agents and occupational disease compensation: noise, vibration, radiation, and other physical agents.

Acoustics of fish shelters: background noise and signal-to-noise ratio.

Retinoblastoma: some considerations.

Infanticide. Some medicolegal considerations.

Placebos: some ethical considerations.

Impulse noise: can hitting a softball harm your hearing?

Asymptotic threshold shift in chinchillas exposed to impulse noise.

Other asthma considerations.

Nonauditory injury threshold for repeated intense freefield impulse noise.

Hearing loss from simulated work-week exposure to impulse noise.

Sexual polyploidization and depolyploidization: some terminology and definitions.

Effects of noise and acoustics in schools on vocal health in teachers.

Some philosophical considerations on pain.

Rethinking Medicare: some rural considerations.

Some considerations of ocular rotations.

Homosexuality: some diagnostic perspectives and dynamic considerations.

Some Medical Considerations of Sports and Games.

Some considerations on cerebral vasospasm.

Immobilization error: some theoretical considerations.

Briquet's syndrome--some diagnostic considerations.

Definitions, Foundations and Associations of Physical Literacy: A Systematic Review.

Occupational noise and myocardial infarction: considerations on the interrelation of noise with job demands.

The biological membrane potential some theoretical considerations.

The chartless office: some practical considerations.