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FROMDistribution authorized to U.S. Gov't.agencies and their contractors;Administrative/Operational Use; Aug 1964.Other requests shall be referred to HumanEngineering Lab., Aberdeen Proving Ground,MD 21010.

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Technical Note 4-64

I HIGH-INTENSITY IMPULSE NOISE:

A MAJOR PROBLEM

-~; : .. ,.,'4*

Robert F. Chaillet David C. Hodge

t C/) Georges R. Garinther Fred N. Newcomb

August 1964

HUMAN ENGINEERING LABORATORIES

ABERDEEN PROVING GROUND,Il oMARYLAND

f N( VI 2 t

DFXC-IHA C~

DDC Availability Notre

QUALIFIED REQUESTERS may obtain copies of this repcrt from DDC.

The findings i this report are not to be construed as an officialDepartment of the Army pusition, unless so designated by otherauthorized documents.

Technical Note 4-64

HIGH-INTENSITY IMPULSE NOISE:

A MAOR PROBLEM

Robert F. Chaillet David C. HodgeGeorges R. Garinther Fred N. Newcomib

August 1964

APPROVE91~NDWTS-

Hluian 1-nmgiw rik ig ohrotoriles

IS. ARMY HU MA~ NGIERNLBRAOT

I CONTENTS

ABSTRACT . ..........

INTRODUCTION . .............

PROBLEM . .............................. 3

IPOSSIBLE SO LUTIONS .

DISCUSSION. .. ... . .................... . .... 23

jREFERENCES .. .. ............... . ........... 32

FIGURES

1. A Typical Impulse Sound-Pressure Wave Form. .. ........... 2

2. Relationship Between Pressure in psi and Sound -Pressure Level in dBl . 4

3. Shock Wave Created When Weapon is Fired and the ProjectileShock Wave .. .. ........ . ................. 6

4. Standard M-16 Rifle....... .... .. .. .. .. .. ..........

5. M- 16 Rifle with Adjustable Muzzle-Brake. .. .. .............

6, Rifle withb MudLifed Muzzle-Pirake Compevie-ator .. .. ...... 9

7. M- 16 Rifle with Single-3xiaffbBe Combiination Brake-Silencer .

8. M- 16 Rifle with Double-Baffle Combination Brake-Silencer. .. .. ... 10

9. Sectionnl Drawing of Double-Baffle Combination Brake-Silencer 1.0

10. Standai.d 107mim Mortar . 12

v

11, 107nmm Mortar with .r. . ..intor ..... 13

12, Attenuator fdr 107mi- lortar.. ...................... .. .14

13. Homvitzer with an Impulse-Noise Shield ....... .............. 15

14. Overpressure Contours ....... ..................... .... 16

15, V-5iR Earplugs ......... ...................... . . .. 19

16. Typical Earmuffs ......... ........................ .. 20

17. Combat-Vehicle Crewman's T56-6 Helmet ... ............. ... 21

18. Attenuation Characteristics (in dH) of Various Types ofEar-Protective Devices ....... ..................... .... 22

19. Cross-Sectional Diagram of the Ear ..... ................ .. 24

20. Temporary Threshold Shift as a Function of Peak Sound-Pressure Level 26

21. An Example of Individual Differences in Susceptibility toTemporary Threshold Shifts ...... ................... .... 28

22. Mean Temporary Threshold Shift (ITS) as a Function ofPeak Sound-Pressure Level....... ..... . .... ..... 30

23. Temporary Threshold Shift (TTS) as a Function of Exposure Rate 31

vi

HIGH-INTENSITY IMPULSE NOISE:

A MAJOR PROBLEM

INTRODUCTION

During the past decade there has been a trend in weapon-system developmentthat has made these weapons psychologically unpleasant and physiologically danger-ous to our own forces. Three major changes have occurred to develop this trend:

First, weapons are being made as light as possible for air mobility. To lightenthe weapons, tubes are shortened, placig the origin of the impulse sound pressurecloser to the gun crew. Recoil mechanisms are of lighter construction and capableof absorbing less energy, necessitating the use of a muzzle brake which, in turn,deflects the impulse sound pressure back toward the crew.

Second, with the advent. of nuclear and other sophisticated ammunition, it hasbecome advantageous to propel our projectiles to greater distances, which requireshigher pressures and results in an increased noise level.

Third, increased firepower results in a greater number of impulse sound pres-sure insults to the operator's ears per unit of time. Thus, increased firepower,with increased pressures, in shorter tubes, together result in exposing the gun crewto dangerous impulse sound pressures.

When weapons, such as rifles, mortars, cannons, and bombs, are fired ordetonated, several physical phenomena occur. First, the chemical reaction of theexplosion takes place, after which there is an extremely rapid one-way flow of thegaseous products of combustion from the detonation center outward. This expansionmoves faster than the speed of sound, thereby creating a shock wave in a mannersimilar to the way a shock wave is created by an aircraft moving at supersonicspeed. The spherical shock wave, which is the second physical phenomenon to occur,continues to move outward, producing an abrupt increase in pressure (rise time ofless than one microsecond). As the shock weve moves out, it begins to lose energy

and several chfngns olculr:

a. Velocity decreases

bh Peak pressure decreases

c. Impulse decreases

ci. Diwation imcrea.ses

When .ifts speed reaches olAi.c velocitv, it i:; cia.x,.rified as an iutpflri, eOSllld wivO.

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When dicussiug hearing dannnge to personnel firing weapons we are concernedwith the pressure patterns caused by shock and impulse waves. Both of thesephenomena are transient in nature mid may b-e distinguished from steady-state soundwavs -. I- .. fact that thelr n eak pressure levels are very high, compared to theirroot mean square (rms) pressure levels.

Two factors which distinguish shock waves from impulse sound waves forauditory perceptual purposes are amplitude and rate of pressure increase and decay.The amplitude of a shock is gr veater than that of an impulse sound wave, and the timerequired for the pressure to reach its maximum and to decay is shorter. Therefore,for the purpose of this paper, we shall refer to a transient pressure wave, regard-less of the phenomena causing it as an impulse sound-pressure wave. Also, themaximum pressure achieved will be referred to as the peak sound-pressure level.Figure 1 is a representation of a typical impulse sound-pressure wave form createdby small arms.

PROBLEM

The repeated high pressures developed by new weapons have their greatesteffect upon the ears of the personnel. By the time the pressure becomes so greatthat other bodily organs are affected, the unprotected ear will have been irreparablydamaged.

Even if the long-term, hearing-loss effects upon the man were to be disregarded,field commanders still have to consider the short-term lowered efficiency of partiallydeafened gun crewmen when these crewmen are assigned to other duties, such asnight perimeter guard.

Basic to the entire impulse-noise problem is the development: of a hearing-damage-risk criterion. Until the hearing-loss effects that Impulse noises have onman are deterimined accurately, the degree to which fltese pressures should bereduced cannot be specified.

Many parameters must be considered in establishin.g a damage-risk criterion.Among these are the followinfg:

a. Peak pressure

b, Sound frequency-energy spectrum

c. Rie time

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Areas which rostYR be ivestgated include the following:

a. How consistent Is temporary hearing loss within one Individual?

b. How consistent is the hearing loss among different people exposed to agiven condition'?

c. What is the relationship between temporary hearing loss and permanenthearing loss, caused by Impulse noise?

d. How do peak pressure, rise time, and duration Interact in causingieiii-porarV hearing loss?

Prominent among the weapons under development that produce pressures muchgreater than those experienced in the past are the 107mm mortar and the M102howitzer, which produce noise loud. enough to rupture the r~ardrum. Also underdevelopment are highly effective small arms that produce Impulse sound-pressurelevels substantially greater than the M- 14 rifle. Since the M- 14 is loud enough tocause both temporary and permanent hea ring loss to the unprotected ear, the develop-ment of these new small arms serves to increase the problem.

Peak sound-pressure levels may be reported in one of two ways:

a.. In pounds per i-rquare Inch (psi).

b. In decibels (dBi) above a reference point of 0.0002 dynes per square

centimeter.

Figure 2 depicts the relationship between these two measures.* In isolated cases,

pressures have been reported in pounds per square foot.

POSSIBLE, '()I, 101I~

'1 herc(_ are VO.YiOUB 1~VejA0e 012 approach which mray he taken In an attempt 1:

roWc.d o ooa cxew' - expo.sure to high jipulie sound presimre~s:

I. The i pli 1 c -outld lc i r w~iy he vniedeat its -0ui1 c '[1 Fo T couj i

Ili.; 1i:; wei W( 1ghf. nI.V Jo p piY iol.I 0 Uit: tisi t mati[to ik (w 1 eiea se p1 ojctiteA rangec

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bi. The opera.I:or may be sepa-,rated from the impulse -sound pressure t

source b either distance or a barrier. SamI

c. Ear-protective devices, such as earplugs or earmuffs, may beprovided for the crew. This category might also include conditioning the ear so

:. that it becomes less sensitive to noise.c

All three of these approaches are being explored.

The first approach is a difficult one, but the most desirable if achieved. Themechanical methods of attenuation involve placing a device on the muzzle to eitherdeflect the excessive impulse sound pressure away from the crew or reduce thepeak sound-pressure level, or both.

Figure 3 ii 4ater the phenomena that oucur at rnivzlp of q wponnn ne a

projectile leaves the muzzle. The expanding gases are released from the muzzle at5 speeds greater than the speed of sound, thereby causing a shock wave, or a "sonic

boom." Simultaneously, as the projectile emerges, It produces Its own shock wavebecause it also is moving faster than the speed of sound. Where a muzzle brake isused, the problem of keeping the expanding gases away from the crew and dissipatingthe "shock bottle" is intensified.

One solution is the development of a muzzle brake -silencer. A muzzle brake -silencer traps most of the exiting gases, then cools, expands, diffuses, and expelsthese trapped gases over an expanded time frame. Trapping these gases also reduces

the recoil momentum. A recent Human Engineering Laboratories (HEL) effort usedthis principle to increase firing stability of two light autornatic rifles -.- the M-16 andthe Stoner Assault rifle. Figure 4 shows the standard M-16 rifle. The development -phases of the M-16 muzzle devices are shown in Figures 4 through 9.

The standard M-16 rifle's peak sound-pressure level, at the. gunner's ears, is154 dB. Figure 5 shows an adjustable muzzle -brake. Although effective in thereduction of recoil impulse, it increased the peak sound-pressure level (SP.L) to over160 dl. * Figure 6 shows a similar device which wao more efficient, but also noisier --

the peak SPI, again exceeded 160 dB. In an attempt to attenuate the noise while takingadvantage of ti: recoil .rduction, aSingle -baffle eOubiation brake -compensatorwas made as shown in Figire 7. This device reduced the peakSPL 1:o 152 d[. Theclatenta design (Fig. 8I) features a1 docibic hrfle with srnaller-diametor outlet. holes thanIthe previous design. This arrangement gave the lowest peak SPI --- 148 d3. ThisSPL, was 6 .1B less thwa the standard M -- riil, yel: it provided c-.ellent stabiity.VFiginlre 9 dopicts Ih latest citsig. :atowing iih' t1w in In 1wf le, and gas-eollectiO) ar'e0

h I II 0I1..Il iIi it t I s ' in) I Isa sor'lln 1 Waq coIi[l ;iiwt(i[ 111 p to aid i h ilog !;P L n( o ; f e

ill; of' 160 t1l. 'be el( r lability of rci nthig';; overi 160 dB is t 'uionihi.' sld ln'i',IN intl i l;o IiI.

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The HEL ha,, recently been engageO il a crash program to pr:othce deviesimila:r to ihe one shown ji Figur1esg 3.and 9, to fit tile n-lizle of a pIew morta r. 'This

weapon, desiied 6 ong axefrnhdavr ~hmzl Ths~~w e.'V highmuzzle pressure, coupled with the close proximity of the loader's head to tile muzzle,presented a challenging human factors impulse-noise problem. FaUnre to reducethe overpressure would most probably have required a new, lengthy and costly pro-gram to develop a radically different-type round of ammunition.

An impulse sound-pressure attenuator was designed, fabricated, and tested by

the WE L. Test data indicate that this attcnuator. reduced the 10 psi overpressure by60 percent.

Since it appears that this device is quite effective on both rifle and mortar, it isreasonable to assume that a similar device would be effective on the M].02, or anyoilier weapnon cif -Rimila~r configuration° Figure 10 shows the mortar without attach-I

ment. Figure 11 shows the mortar with attachment. Figure 12 depicts a sectionaldrawing of the attachment.

The second approach involves placing a barrier between the source and the crew.

The most obvious mechanical shielding Is the gun shield itself. In the case of a tank,the very massiveness of the hull and turret provides an effective shield for the crewin the tank, but this mass provides little if any protection to the infantryman walkingbeside the tank when the major caliber weapon is fired, or to the tank commander,whose head may be outside the cupola at the time.

For use with field pieces, shields, as such, have very minimal impulse sound-pressure-deflecting properties. A case in point is the M102, 105mm Howitzerequipped with an impulse-noise shield (Fig. 13).

This weapon was recently tested by HEL in the following manner: transducerswere arranged behind the weapon to provide data for plotting equal peak SPL contours.Measurements were made during firings at elevations of 00, 45', and 680; withcharges of 85 percent and 100 percent; with three different muzzle brakes and with -out any muzzle brake; and with and without a shield.

Figure 14 shows a few representative equal-pressure contour liues measured inthree of the many tests. The fireings depicted in Figure 14 were conducted wi:h a

1 0Oi percent charge mid an. 8t)0--m.II (450) angle of elevation, It was found tihat thc

shield did not significsatly reduce the peak SP, i) the crew area.

Placing the crew farther from the impulse soundtpressu.re souree would pro!ably): .,,,e to n.:m's offectJivear.es.s becau se ulol:tJlug A.: WOiald Loi|:.,l: take lonf, ero.

rCequire developieunt of complicated remote controls,

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'The third approach invoiveti using protective devices applied directly to the ea:C.IEThere axe three poasihilitlco: (1) placing a device in the ear; (2) placing a device

......... . -. ,,, the o tm t it lcomes less sensitive to noise,,11. . 1. -1.1. . t h e .... . . . .

The method of conditioning 'the oar appears to have promise°. It has proved quiteeffective in certain instances and may afford almost as much protection as an ear-plug. This method complements a naturni physiological protective mechanism of theear by eliciting, before firing, the contraction of certain ear muscles, thereby reduc-ing the transmission of impulse sound pressures. This activation is produced bygiving sharp pulses of sound over an intercommumication system for about 0.1 secondprior to firing the weapon. This device would probably be of greatest value to tankand self-propelled artillery crews.

Because so little is known about the physiological effects of impulse pressure onman's ear, there is no highly accurate Information about the pressure -attenuatingcharacteristics of the various types of ear protectors.

Personnel can wear several types of pressure -attenuating devices. The mostcommon and practical are:

a. The earplug: At present the Army is using the extremely effectiveV-51R earplug (Fig. 15).

b. The earmuff: At present the earmuff-is not available as a standardsupply item. Figure 16 shows representative samples of earmuffs.

c. The helmet: The standard combat-vehicle crewman's helmet -- CVC(T56-6) -- (Fig. 17), used in tanks and self-propelled howitzers, provides verylittle noise attenuation.

Typical ear-protective devices provide good attenuation at high frequencies(above 1000 cycles per second), but relatively poor protection nt lower frequencies.Figure 18 shows attenuation as a function of frequency for an average earmuff, forthe standard Army-issue V--51R earping, for the CVC helmet, and for the combna-lion of earplugs and earmuffs. .The nmber in parentheses below each device areentimates of the atteationu (in dB) that these devices give in an impulse-noiseOfigonfl.ent,, It will he noted, i Figure iM, that a. coinbhation of earmuff sni cur-plug was not as effective n.s the ,earmuff alone for frequencese' around 1200-2400 cpf (11).

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a. Qq.-n I- -- -l-, -94- -~t A. fl --E t 9--fl - 10 al. 01iu

shown in Figure 18.

b. Even though they have been properly inserted originally, they may workloose through jaw movement.

c. As with earmuffs, faint sounds carmot be heard. A speaker must raise

his voice to be understood. This last objection is especially significant since, in

many combat situations, it is vitally important to perceive faint auditory cues, and

loud talking cannot be allowed. One solution to this problem would be an ear-

protective device that attenuates loud noises but does not attenuate faint sounds. A

proposal to produce such a device has been submitted by an acoustical engineering

firm.

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DISCUSION

The Atomic Energy Commission has sponsored a considerable amount ofresearch to determine the effects that high-intensity shock waves, from actual, andsimulated nuclear explosions, have on animals and on men. This research hasestablished tentative lethality limits and thresholds of injury for various bodilyorgans. But, as has already been pointed out, man's hearing mechaiJum can betemporarily or permanently damaged by exposure to shock waves or impuse-rn!ieconditions which are far below the threshold limit for damage to the lungs or otherbodily organs. Thus the research programs which are being carried out by variouslaboratories are aimed at studying noise and shock-wave conditions which may causetemporary or permanent damage to hearing, or decrements in human performance,but which are not considered to carry the threat of death, or a threat of physiologicalharm other than to the hearing mechanism.

A short digression will clarify the sites of hearing damage. Figure 19 shows across -sectional view of the peripheral portion of the human hearing mechanism,including the external, middle, and inner ear. Impulse noise of high intensity (above180 dB) may cause rupture of the eardrum or damage to the chain of three ossicles(bones) In the middle ear. But most temporary or permanent changes in hearing arebelieved to be due to physiological damage inside the inner ear, or cochlea. In thiscase, airborne acoustic energy is transmitted to the eardrum, through the chain ofossicles or bones in the middle ear, and through the fluid inside the inner ear.Histological studies of animals exposed to high noise levels have shown that damageto the hair cells of the Organ of Corti inside the cochlea is characteristic, and thisdamage is believed to be responsible for temporary and permanent hearing loss.

The present state of our knowledge about the effects of impulse sound-pressurelevels on hearing is very sketchy -- partly because of a lack of research on theproblem, but also because of poor or inappropriate methods which have been used ina number of studies.

The first systematic studies of how impulse sound-pressure levels affect humanswere published in 1946 by Murray and Reid (7). These Australian scientists exposedenlisted men to a variety of small arms and artillery noises and, in spite of crudemethods and histrumentation, provided the first quantitative data about how impulsesound pressure affects hearing. One of the lnvestigators exposed himself in thisstudy and suffered a ruptured eardrum. Figur, 20 show.9 sume of the renults. Notethat exposing subjecl:s to ten rounds at a peak omjUd-pressikre level of about 188 dB ..comparable to noise in the crew area of a curreat U. S. Army 105ram howitzer --

Pylwei teulorary hearing lossco of 85 dB.

judging fr.om the literature, the itmpil e-.8oud .11(( j) u pcl obei areo wandorinalit from 1.940 un.ti. tsoleti.lie iii the 195 .- ioc: a;tudy of notw

)V li- hold and C reein in 1961 (4). These mren, .J-o"m iic Navl. Sciw] of Aviaii. .HMedic.in (Pen'i.;acoibt), estaJ, .hedthAt i: '.v omuc:- -,.i." thl-oroh lMa lge Coir: 'OpS ihacic

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spa rkcd coaiderable interest !n ithe hnpulbesound-plretur problem., in Spite ofome oLous metdo~gica shortcomin s- of the stiudy

In the past four years a considerable amount of research has been conducted onthe impulse-sound-pressure problem, much of it carried out or sponsored by Armyresearch laboratories. Again, methodological shortcomings cast some. doubt on theusefulness of much of this work.

Since it is impractical for many researchers to use firearms as noise sources,and equally impractical to fire weapons indoors under rigidly controlled laboratoryconditions, investigators have had to use other sources of impulse sound pressures.A number of artificial impulse-sound-pressure generators have been constructed,but all those known have the same limitation: the acoustic pulses these generatorsproduce are sufficiently unlike those produced by Army weapon systems that there issome doubt about the usefulness of data obtained with them (5). In other words, theamounts of temporary hearing change, i.e., temporary threshold shift (TTS), whichhave been attributed to certain noise conditions, are often questionable. However,the qualitative relationships among various exposure conditions are probably valid.

Assuming, then, that at least the qualitative relationships are valid, our presentknowledge in this area may be summarized as follows:

a. There are very large individual differences in susceptibility to impulse-sound-pressure effects, both in the Army population and in the population of Ameri-cans in general. The data in Figure 21, from a study by Carter and Kryter (1),illustrates this wide variability. It can be seen that the subject represented by thetop curve sustained a TTS of 41 dB from exposure to 20 impulses at a peak sound-pressure level (SPL) of 156 dB, while another subject represented by the bottomcurve sustained a TTS of only about 2 dB after exposure to 40 impulses -- twice asmany -- of a louder sound with a peak SPL of 168 dB. The impulses used in thisstudy were geierated by an artificial impulse-soud-presmine source, but similarvariation in susceptibility has been reported by Smith and Goldstone (8) and Donley(2) usingtl)eM-14 rifle as a noise source in studies at the IIEL. It has been estimatedthat at least five percent of the Army population are extremely susceptible to impulsesomd-pressure effects, while at least five percent are extremely resistant to theseeffects.

h. Other conditions eqa)lq.1, it appear8 that the highe:r the peak :iL, the

g.reater the reslfting TTS will be, This icena'ionship hau already been llustratedwith the Murray aud' Reid data in Figure 20, The utkniown qluantity, however, 1s thelowest peak 8.L, whici will cause a weasnrable TI' in] d e s ubject.. Figure22 shows soule da.it fioa Ward, Selte: :8, and (1 orig (J.0), bi wiO ) me.amsrableT'TS? w:i. p.|:odlued by exposure to 75 willm)i w ni a peak oPf oci dy 132 di, It, caualo be seen that ahoii 1.2 dri of 'ITS w'ai produed when the peak SPkl. wa ,s 141. dl,

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On the other hand, experiments now h') pt.ogrens at HE1J. have shon Idnegligible TTS after expo.lre to 100 gunfire impuilseri with A. nontrolled peak SPL of140 dk. 'Tius, wbiln it i.s logical to assume some relationship between peak SPL andamount of TTS produced, existing data are neither sufficient nor adequate to answerthe question: "What is the critical peak SPL where hearing damage can be expectedto begin to occur?"

c. The rate of exposure has been shown to be an important variable.Ward (9) demonstrated, as shown in Figure 23, that when the rate of exposure wasbetWeen one impulse per second and one impulse each nine seconds, there was nosignificant difference in the amount of TTS produced. However, when the rate wasdecreased to one impulse each 30 seconds, the TTS was considerably less, indicat-ing that some recovery occurred in the 30 seconds between successive impulses.

A number of studies by-the 'Amy Medical Research Laboratory (Ft.Knox) have shown that, as the rate is increased to more than one impulse per second,the TTS decreases. This and other evidence seems to indicate that the acousticreflex of the middle ear muscles is activated and sustained in activation, thus pro-viding a certain amount of protection from the subsequent impulse sound pressure (3).

d. There is also some evidence to indicate that the more impulses theperson is subjected to, for a given peak SPL and rate of exposure, the larger the TTSwill be, but more study is required to clarify this relationship.

Briefly, then, here is a summary of the knowledge available today:

a. There are large individual differences in susceptibility;

b. higher peak sound-pressure levels mean more hazard to hearing;

e. rate of fire is important; and

d. number of impulses is important.

There are few data to indicate how these variables interact, or what type oftrade-offs can be made between, or among, impulse-sound-.pressure parameters.

Also, there is no information about the effects of rise time or duration ofindividual impulses because, at the present time, it is not possible to ge.erate tfetype of acoustic impulses needed for research. Existing Instrumen tatia-n f.oaits

sea:rch to the use of either:

a. Arificial sources whi..,, while giving sme control over risC time,duration, peak SlIL, anwl rpel ition re,111:(" gellerate implllfser which a.re qtl. itoe unlike

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teristics are, in general., invariant and cen le modified only by placing the nhiectat various distances from the muzzle

In the former case only qualitative data can be obtained, while in the latter caseonly the hazards associated with specific weapons can be established. It is difficultat best to generalize the data to Intermediate noise conditions, and similarly difficultto attack the problems of the importance of rise time and duration. Duration, inci-dentally, was a very significant variable in the high-intensity, shock-wave studiescarried out by the Atomic Energy Commission.

In the impulse-sou: td-pressure studies now in progress at HEL, two approachesare being pursued,' both of which generate impulse sound. pressures by firing weapons.One approach is a very systematic examination of some of the problems in this area,while the other approach sacrifices a certain amount of precision in order to acquiresufficient data for the publication of an interim impulse sound-pressure-level expo-sure standard. The latter approach will generate an interim damage-risk criterion,but the conclusions drawn from studies conducted in this manner will require eventualsystematic verification before a final criterion can be established.

Until this research is complete, the nearest approach to a damage-risk criterionis that recommended by the National Research Council's Committee on Hearing, Bio-

7. Acoustics and Bio-Mechanics (CHABA). This committee recommended that theunprotected ear should not be subjected to peak sound-pressure levels above 140 dB(.03 psi). Every standard weapon that the Army uses, including small arms, exceedsthis level. Therefore the armed services are faced with the formidable problems ofdetermining: how hazardous to the user are the various weapons; how much must thesound-pressure level of these weapons be reduced; and how can this reduction be-ccomplished.

The United States is not the only nation concerned about this problem. Germanmedical and acoustical specialists met with weapon developers on 18 - 19 April 1962at Meppen, Germany, to start studying the problem of ear injuries in artillery crews.Their initial approach to this problem was to evaluate the pressure patterns for allweapons and firing conditions. The type of ear protection to be used with theseweapons will then le determined, after which injury -producing levels will be studiedand defined. Ti Germans stated at this meeting that they would like allied countriesto c(impe nve in estab!i.hing - , tndard criteri.a and rrquirementls for ear protectors.

It is obviouls that., in one more field, teclology has caught up with and exceeded*u,.psychological and physiological lnitilatiotis. T. o restore It safe linlane between

1111 "atl ntd ar).hjne, 1imn falctorsq resear~ch :Affoi:. niust heg acce;lerated.

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RE FERENCES

1. Carter, N. L., & Krytcr, K. D. Studies of temporary threshold shift caused byhigh Intensity acoustic transients. Bolt Beranek and Newman, Inc., Cambridge,Mass., Report No. 949, 1962.

2. Donley, R. Unpublished data. U. S. Army Human Engineering Laboratories,Aberdeen Proving Ground, Md., 1962.

3. Fletcher, J. L. Reflex response of the middle ear muscles: Protection of theear from noise. Sound, 1962, 1, (2), 17-23.

4. Harbold, G. I.,& Greene, J. W. A study of the effects of gunfire and otherinfantry combat training noises on the hearing acuity of U. S. Marine Corpsrecruits. USN School of Aviation Medicine, Pensacola, Fla. Project MR005. 13-2005, Subtask 1, Report No. 10, 1961.

5. Hodge, D. C., & Soderholm, R. Wo A survey of instrumentation for producingimpulse noise. Technical Note 3-64, U. S. Army Human Engineering Labora-tories, Aberdeen Proving Ground, Md., March 1964.

6. Holland, H. H. Muzzle blast measurements on Howitzer 105mm XMi03E1.Technical Memorandum 23-62, U. S. Army Human Engineering Laboratories,Aberdeen Proving Ground, Md., September 1962.

7. Murray, N. E., & Reid, G. J. Temporary deafness due to gunfire. J. Iryngot.,1946, 61, 92-121.

. ' 8. Smith, M. G., & Goldstone, G. A pilot study of temporary threshold shiftsresulting from exposure to high-intensity impulse noise. Technical Memoran-dum 19-61, U. S. Army Human Engineering Laboratories, Aberdeen ProvingGroand, Md., September 1961.

9. Ward, W. D. Effect of temporal spacing on temporary threshold shift fromimpulses. J. acoust. Soc. Amer., 1962, 34, 1230-1232.

10. Ward, W. D., Selters, W., & Glorig, A. Exploratory studies on temporarythreshold shift from impulses.. acoust. Soc. Amer., 1961, 33, 781-793.

11. Webster, 1. C., & Rubin, E. R. Noise attenuation of ear protective devices.Soun! 1962, 1, (5), 34.

IFE

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