Blast Noise of the L119 Light Gun - The Defence … Report 406 NR 1687 BLAST NOISE OF THE L119 LIGHT...

DTA Report 406

NR 1687

Blast Noise of the L119 Light Gun

Nathaniel de Lautour

October 2015

DTA Report 406NR 1687

BLAST NOISE OF THE L119 LIGHT GUN

Nathaniel de Lautour

October 2015

ABSTRACT

Blast noise measurements were carried out on the New Zealand Army L119 LightGun by the Defence Technology Agency in June 2015 at Exercise Brimstone. Thepurpose of the assessment was to determine the sound pressure levels near thegun crew and estimate the effectiveness of a number of hearing protectors. Theacoustic pressure was measured slightly forward of the loader’s position, approx-imately three metres back from the muzzle at zero barrel elevation, one metrefrom the gun centreline, and 1.6 metres from the ground. The peak sound pres-sure level of the muzzle blasts was in the range 173–176 dB, the A-weightedsound exposure level was 137–146 dB, and the A-durations were 0.65–2 mil-liseconds. Double hearing protection (earmuffs and earplugs) should reduce thepeak sound pressure level to 138 dB, which is compliant with the Health andSafety in Employment Act.

DTA Report 406ISSN 1175-6594 (Print)ISSN 2253-4849 (Online)

Published byDefence Technology AgencyPrivate Bag 32901DevonportAuckland 0744New Zealand

c©Crown Copyright 2015

EXECUTIVE SUMMARY

BACKGROUNDIn 2003 noise level measurements were made by a private contractor on theL119 105 mm gun. The measurements were not calibrated and at the requestof the New Zealand Army a review of the 2003 data was carried out by the De-fence Technology Agency (DTA). DTA recommended that the measurements berepeated with a calibrated microphone for different weapon configurations, andthat the hearing protection recommendations in the report be reviewed with thenew data.

AIMTo measure the muzzle blast pressure of the L119 and estimate the acousticperformance of hearing protectors based on the new data.

RESULTSDTA carried out blast noise measurements on the New Zealand Army L119 105mm gun in June 2015 at Exercise Brimstone. A computer simulation model ofhearing protector performance for impulse noise was developed, using manufac-turer supplied octave band attenuation data.

This report has focussed specifically on blast noise from heavy calibre weapons,but the methodologies and techniques could be applied to small arms.

The report considers only the acoustic performance of hearing protectors. Issuesof ergonomics, communication and cost were outside the scope of this study andhave not been addressed.

The main findings regarding sound levels near the L119 are:

1. The peak sound pressure level (SPL) was 173–176 dB at charge 6 and175–176 dB at charge 7. These pressure levels are very high and hearingprotection is essential;

2. There is a double peak in the acoustic pressure of the charge 7 shots thatmay be due to delayed ignition of one of the propellant charge bags.

3. The sound exposure level (SEL) was 137–139 dB SEL(A) for charge 6 shotsin open air increasing to 144–146 dB SEL(A) for charge 7 shots under thecamouflage net. The higher acoustic energy levels of the latter shots maybe due to reverberation caused by the camouflage net;

4. The computer simulation model indicates that single hearing protectors canreduce the peak SPL to 145–155 dB, which is not compliant with the Healthand Safety in Employment (HSE) Act limit of 140 dB;

DTA Report 406 i

5. The model indicates that double hearing protection (earmuffs and earplugs)can reduce the peak SPL to 138 dB for charge 7 shots. However, doubleprotection may hamper communication of the gun crew;

6. For double hearing protection the maximum daily exposure is 160 charge 7shots under the HSE Act limit of 85 dB on LAeq,8h;

7. The NATO RSG-029 study group on impulse noise hazard found that longduration impulse noise (0.9–3 ms) is less damaging and recommended adaily exposure limit of 98 dB on LAeq,8h. This equates to 20 times as manyshots per day as an 85 dB limit. However, RSG-029 also recommended areduced limit of 80 dB for small arms fire, which is more restrictive than theHSE Act. Subjective experience supports the more permissive RSG-029recommendations for the L119;

8. The NATO RSG-029 study group did not recommend a limit on peak SPLfor impulse noise. Instead, they proposed limits on the SEL(A) for singleimpulses of 135 dB SEL(A) for long duration blasts and 116 dB SEL(A) forsmall arms fire.

SPONSOR

DTA Project J1342

DTA Report 406 ii

ABBREVIATIONS

APV . . . . . . . . . . . . Assumed Protective Value

B&K . . . . . . . . . . . . Brüel & Kjær

dB . . . . . . . . . . . . . . Decibels

DTA . . . . . . . . . . . . Defence Technology Agency

HSE . . . . . . . . . . . . Health and Safety in Employment

IL . . . . . . . . . . . . . . . Insertion Loss

ms . . . . . . . . . . . . . milliseconds

MIRE . . . . . . . . . . . Microphone In Real Ear

NIOSH . . . . . . . . . National Institute for Occupational Safety and Health

NR . . . . . . . . . . . . . Noise Reduction

NRR . . . . . . . . . . . . Noise Reduction Rating

NZDF . . . . . . . . . . . New Zealand Defence Force

REAT . . . . . . . . . . . Real Ear Attenuation at Threshold

RSG . . . . . . . . . . . . Research Study Group

SEL . . . . . . . . . . . . Sound Exposure Level

SEL(A) . . . . . . . . . Sound Exposure Level with A-type frequency weighting

SPL . . . . . . . . . . . . Sound Pressure Level (with reference pressure 20 µPa rms)

TFOE . . . . . . . . . . . Transfer Function of the Open Ear

TL . . . . . . . . . . . . . . Transmission Loss

TTS . . . . . . . . . . . . Temporary Threshold Shift

DTA Report 406 iii

CONTENTS

1 Introduction 11.1 The L119 light gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Acoustic measurement equipment 2

3 Hearing protector performance in impulse noise 33.1 Using interpolated octave band attenuations to model the effect of

hearing protection on impulse noise . . . . . . . . . . . . . . . . . . 43.2 Validation of the DTA method on 5.56 mm rifle shot data . . . . . . 6

4 Measurements 74.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.2 Sound pressure levels . . . . . . . . . . . . . . . . . . . . . . . . . . 94.3 Acoustic pressure waveforms . . . . . . . . . . . . . . . . . . . . . . 94.4 Octave band levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Legislative limits on occupational noise exposure 125.1 Origin of the 140 dB limit on peak pressure . . . . . . . . . . . . . . 12

6 Hearing protector performance estimates 136.1 Reduction of the peak SPL . . . . . . . . . . . . . . . . . . . . . . . 136.2 Daily limit on the number of shots . . . . . . . . . . . . . . . . . . . 14

7 The NATO RSG-029 recommendations on impulse noise exposure 15

8 Summary and conclusions 17

Appendix A Pressure waveforms and spectra 19

Appendix B Sound level metrics 25B.1 Sound exposure level . . . . . . . . . . . . . . . . . . . . . . . . . . 25B.2 Equivalent continuous sound level . . . . . . . . . . . . . . . . . . . 25B.3 Eight hour equivalent continuous sound level . . . . . . . . . . . . . 25B.4 Equivalent continuous level for N impulses . . . . . . . . . . . . . . 26B.5 Noise exposure limits . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Appendix C The Class rating system 27

References 28

DTA Report 406 iv

1 INTRODUCTION

Blast overpressure measurements of the L119 105 mm light gun were carriedout at the Waiouru Military Camp, during Exercise Brimstone, on 9 June 2015.The exercise involved the 161st and 163rd Batteries of the New Zealand Army’s16th Field Regiment. Exercise Brimstone is an annual event during which artillerygun crews undergo qualification testing for the L119. These circumstances arenot ideal for making blast noise measurements, but it is difficult to organize trialsspecifically for scientific purposes due to ammunition costs.

The aims of the work discussed in this report were:

• to check previous measurements carried out in 2003;

• to estimate performance of hearing protectors based on the new data.

The original 2003 measurements were performed by a private consultant usinga hydrophone as a pressure sensor [1]. The measurement system was not cal-ibrated and some uncertainty remains as to the actual sound levels. At the re-quest of the New Zealand Army the Defence Technology Agency (DTA) carriedout a review of the consultant’s report [2]. It was recommended that the mea-surements be repeated with a calibrated microphone for different weapon config-urations, and that the original hearing protection recommendations in the 2003report be reviewed in light of the new data.

DTA has measured muzzle blast noise from small arms since 2010. Initially, mea-surements were made using a variety of microphone and hydrophone sensorsand data acquisition systems. DTA now uses a Bruel & Kjaer (B&K) 4941 highpressure microphone and a piston-phone calibrator to provide calibrated blastnoise measurements.

1.1 The L119 light gun

The L119 light gun used by the New Zealand Army is a 105 mm howitzer witha 3.17 m monobloc type barrel, and is fitted with a single baffle muzzle brake(Fig. 1). The barrel elevation of the L119 is adjustable from -100 mils to 1244mils, and the barrel azimuth movement is limited to between -100 to +100 mils1.

To engage targets outside this azimuth range the entire gun must be rotated onits platform. Movement of the barrel in azimuth has a relatively insignificant effecton the acoustic measurements because of the limited range of movement. It wasnot possible to control this variable during the exercise and barrel azimuth wasnot recorded.

The propellant charge for the shells is contained in a brass cartridge case inseven pre-packed bags. The total propellant charge may be reduced by removing

1Barrel angles are usually stated in mils rather than degrees, and there are 6400 NATO standardmils per 360 degrees.

DTA Report 406 1

bags from the case. Before the gun is fired the commander orders the crew toprepare the required projectile types and propellant cases with a specified chargelevel.

Figure 1: Left: A New Zealand Army 161 Battery gun crew preparing theL119 during Exercise Brimstone, June 2015. Right: The gun muzzle witharrows indicating the locations of the left and right ports of the muzzle brake.

2 ACOUSTIC MEASUREMENT EQUIPMENT

The muzzle blast overpressure was recorded using a B&K 4941 0.25-inch micro-phone. The upper sound pressure level (SPL) limit for this microphone is 184decibels (dB) relative to a reference pressure of 20 micropascals. The peak SPLdid not exceed 177 dB and so the measurement system was operating well withinits linear dynamic range throughout the trial.

The microphone and preamplifier assembly are placed in a sleeve of sound ab-sorbing foam inside a section of PVC tube with a longitudinal cut. The PVC tubeis mounted on a plastic block which attaches to the tripod. The foam sleeve isnecessary to reduce the effects of tripod vibration on the microphone. The cableruns out through a groove in the foam sleeve (Fig. 2).

The pressure signal was conditioned and digitized using a B&K 2250 sound levelmeter. The pressure waveforms were recorded onto the sound level meter’s ex-ternal Secure Digital (SD) card in 24-bit format at a sampling rate of 48 kHz. Thisrate is sufficiently fast to record sound over the human auditory frequency range,which extends from roughly 20 Hz to 20 kHz.

The system was calibrated using a B&K 4228 piston-phone calibrator which gen-erates a 250 Hz tone, nominally at 124 dB. There is a small ambient pressurecorrection to the stated calibration level. The barometric pressure at the startof the trial was 925 hPa, measured using a portable barometer. Recordings of

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the 250 Hz calibration tone were used to estimate the scale factor for convertingdigitized sample values to acoustic pressure in Pascals. The equipment used tomake the acoustic measurements and the dates of calibration are given below inTable 1.

Figure 2: Left: The B&K 4941 microphone used for the measurementswas placed in a foam sleeve and mounted in a short section of PVC tube.Right: The PVC tube attached to the tripod.

Item Description Calibrated

B&K 2250 Sound Level Meter Nov 2014

B&K 4941 0.25-inch microphone Nov 2014

B&K 4228 Piston-phone calibrator Dec 2014

Table 1: Acoustic measurement equipment used in the trial.

3 HEARING PROTECTOR PERFORMANCE IN IMPULSE NOISE

There is no generally accepted standard available to predict the attenuation achiev-able by a hearing protector against impulse noise. The octave band method [3,p208] is recommended for use with continuous broadband noise, but this methodcannot be directly applied to predict the peak level reduction against impulsenoise sources. This difficulty was noted by the author of the 2003 report on thenoise levels of the L119.

At present, hearing protector attenuation is quantified using a procedure knownas Real Ear Attenuation at Threshold (REAT). The method measures a quantityknown as insertion loss (IL) which is the difference in noise level at the eardrumlocation between the unprotected and protected ear. The REAT method deter-

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mines the IL by measuring hearing thresholds for the unprotected and protectedconditions across the sample of test subjects.

The quantity used in practical noise assessments is the mean attenuation minusone standard deviation, which is called the assumed protective value (APV). Thisresults in a value of attenuation that should be achieved for 84% of users [4].

The U.S. Army Research Laboratory has proposed a model for predicting impulsenoise inside a protector based on free-field measurements of the acoustic pres-sure [5]. The method works by fitting a transfer function model to octave bandattenuations measured by the REAT method. DTA experimented with this modelbut was unable to obtain a satisfactory fit with all of the hearing protectors ofinterest in this study.

Other researchers have used Shaw’s earmuff transfer function model to predictimpulse noise inside a range of earmuffs in response to pistol shots [6]. Whilethe fit of the model was very good in certain cases, the error was as high as 7dB in others. DTA also experimented with the Shaw model, but again was unableto obtain a satisfactory fit with the available hearing protector attenuation data.

3.1 Using interpolated octave band attenuations to model the effect of hear-ing protection on impulse noise

The effect of a hearing protector on impulse noise was estimated from cubicspline interpolation of REAT attenuations onto an evenly spaced grid of frequen-cies in the range from 0 to 24 kHz (the Nyquist frequency). The interpolatedfrequency response is used to generate a transfer function that transforms a free-field acoustic pressure measurement into a signal inside the hearing protector.

The transfer function of the open ear (TFOE) has been neglected in these cal-culations, that is, the REAT IL values are used as transmission losses2. This isjustifiable as the majority of the acoustic energy content lies below 2 kHz and theTFOE has little effect in this band.

The REAT test values are usually only available at frequencies between 125 Hzand 8 kHz. The response outside this range was estimated by using the attenua-tion at 125 Hz for frequencies down to 0 Hz as suggested by Berger [7], and the8 kHz value up to 24 kHz. There is negligible energy in L119 shots above 4 kHz,so it is only the low frequency response of the protector that has a significanteffect on the outcome.

The hearing protector phase response is not measured in the REAT test proce-dure. To estimate phase and amplitude response of earmuffs over a broad fre-quency range Mlynski and Kozlowski applied the Shaw earmuff transfer functionmodel to a number of earmuffs [6].

2The TFOE transforms free-field acoustic pressure into pressure at the ear drum location.

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A generic phase curve based on those given by Mlynski and Kozlowski [6, Fig. 6b]was constructed to explore the effect of changes in phase on the peak soundpressure level (SPL) and sound exposure level (SEL) under the protector. Theerror caused by neglect of the generic phase response was in the range 1–2 dB.This is of the same order of magnitude as typical errors in octave band atten-uation measurements. It is also similar to the change in attenuation caused bynon-linearities in response to impulse noise [8]. Taking into account the othersources of error it seems justifiable, for the purposes of estimating peak SPL andSEL, to ignore the phase response. Consequently, all hearing protectors havebeen assumed to have zero phase shift.

To validate the approach taken the method was applied to some hearing protectorattenuation measurements obtained using 5.56 mm rifle shots [8], and adjustedto provide a reasonable fit to the attenuations. The performance of the model onthis data set, and the model fitting procedure, is discussed next in Section 3.2.The interpolated attenuation curves for the hearing protectors investigated in thisstudy are given below in Fig.3.

Frequency (Hz, log scale)102 103 104

Atte

nuat

ion

(dB

)

5

10

15

20

25

30

35

40

45

H10A

SportTac

Tactical XP

ProTac II

ComTac XPI-FB

ComTac XPI-FB + EAR

EP7 Closed Cap

Combat Arms - Closed

Figure 3: Hearing protector attenuation curves assumed in this study. Thepoints on the curves are the manufacturer supplied assumed protective val-ues at octave band centre frequencies. The curves are generated by cubicspline interpolation between these values.

DTA Report 406 5

3.2 Validation of the DTA method on 5.56 mm rifle shot data

In Section 3.1 a method of estimating hearing protector performance against im-pulse noise based on interpolated octave band attenuations was proposed. Somedata on protector performance against 5.56 mm rifle shots is available and thisdata set has been used to provide a partial validation of the proposed method.The rifle shot data and the DTA method predictions for this data set are describedin the remainder of this section.

Hearing protectors are not routinely tested for effectiveness against impulse noise.It is not possible to use the standard REAT method since this would expose testsubjects to harmful levels of noise. As an alternative, measurements have beenmade on a model of the human head and ear canal, known as an acoustic testfixture (ATF). Because a single ATF does not capture the diversity of human phys-iology it is not generally used as an assessment method. However, for high levelimpulse noise it is currently the only means of measuring insertion loss and esti-mating sound pressure level in the ear canal [9].

In 2012, Murphy et al. reported test data on a number of hearing protectorsagainst impulse noise using an ATF consisting of a solid acrylic head equippedwith a shock isolated B&K ear simulator and a high pressure microphone [8]. Thefree-field acoustic waveforms measured in the experiment are not available butDTA has a large number of calibrated recordings of 5.56 mm rifle shots obtainedusing a B&K 4941 microphone.

The tests included three nominally linear protectors for which octave band attenu-ation data is available: the 3M EAR Pod Express earplug; the single-ended Com-bat Arms earplug in the closed position; and the Etymotic Research EB1. Exper-iments were performed at 130, 150 and 170 dB peak impulse levels. Althoughthese protectors are not designed to have a non-linear characteristic, each ex-hibited an increase in attenuation with rising impulse level. It was already knownbefore this study that the attenuation of non-linear earplugs and earmuffs canincrease significantly from 140 to 170 dB peak SPL [10, 11].

The measured IL for these protectors were obtained from Murphy et al. in Ref. [8].These measurements, together with the predicted IL from the interpolation methoddescribed in the previous section, are given in Table 2 below.

When the mean attenuation levels for each hearing protector were used in theDTA interpolation method, the predicted IL was significantly higher than the mea-sured levels. To adjust the model to better agree with measurements the APVs foreach protector were used instead, i.e. the mean minus one standard deviation.

The estimates obtained from the interpolation method lie between the measure-ments for the 130 dB and 170 dB peak impulse levels. On the basis of thiscomparison the method should provide slightly conservative estimates of IL forpeak impulse levels of around 170 dB.

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Protector Measured IL@ 130 dB

Measured IL@ 170 dB

Predicted byDTA method

EAR Pod Express 33.5 37.7 35.0

Combat Arms - Closed 28.9 33.2 31.5

Etymotic Research EB1 33.3 36.8 34.8

Table 2: The insertion loss (IL) for the indicated hearing protectors against

5.56 mm rifle shots. Measurements given in [8] at 130 and 170 dB peak lev-

els are compared with predictions from the interpolation method proposed

here.

None of the protectors considered in this study have been tested in low frequencyblast noise similar to the L119 muzzle blast. Until test measurements becomeavailable there will remain a degree of uncertainty as to the actual performanceof the protectors against the L119 and similar blast noise sources.

4 MEASUREMENTS

4.1 Setup

The microphone was always positioned on the right hand side of the gun, outsidethe trails, slightly forward of the loader’s position. It was not possible to place themicrophone inside the trails since this would interfere with the operation of thegun crew.

It was not possible to measure the position of the microphone with accuracy dur-ing the trial, although some measurements with a tape measure were attempted.In serial 1 the microphone was displaced to the side about 1.4 m from the cen-treline of the barrel, and was about 3.5 m back from the end of the muzzle (mea-sured at zero barrel elevation). In serials 2 and 3 it was necessary to move themicrophone to a position about 1 m from the barrel centreline and about 3 m backfrom the muzzle. These approximate positions are indicated in Fig. 4. The micro-phone was approximately 1.6 m above the ground in all serials, and oriented inthe vertical direction.

Pictures of the actual microphone locations are shown in Fig. 5. Because themicrophone position differed between serial 1 and serials 2 and 3 it is difficultto directly compare these acoustic measurements. The hearing protector perfor-mance estimates in this study made use of the serial 3 measurements which hadthe highest pressure levels.

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Serial 1

Serial 2,3

Figure 4: Approximate microphone positions during serial 1 and serials 2and 3. The microphone was approximately 1.6 m above ground.

Figure 5: Top: the microphone position for serial 1, with the tubular trailsof the L119 indicated by the arrows. The microphone is mounted in thewhite PVC tube on the top of the tripod. Centre: the microphone positionfor serial 2. Bottom: the microphone position for serial 3.

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4.2 Sound pressure levels

Three sets of acoustic pressure measurements were performed using differentcharge levels and barrel elevations. In each serial the gun was mounted in afixed position. The gun barrel elevation and azimuth varied only marginally withina serial. The range of peak SPL for each serial is shown in Table 3. The 2003measurements obtained by the contractor are included for comparison.

In the 2003 trial a peak SPL of 170 dB and an SEL(A) of 131 dB were recorded.The location of the 2003 peak SPL reading was given as “at source", while theSEL(A) measurement was obtained at a distance of “five metres”. In addition, thebarrel elevation and propellant charge level in the 2003 shots were not stated.Without this information it is difficult to directly compare the 2003 and 2015 mea-surements. The 2015 measurements were considerably higher. This may be dueto closer proximity of the microphone to the muzzle, a calibration error in the 2003measurements, or a higher propellant charge level.

In the serial 2 and 3 measurements the microphone was positioned close towhere the crew were expected to be exposed to the highest blast levels, namely,slightly forward of the loader’s position and about one metre from the centrelineof the barrel. It was not possible to make measurements inside the trail assem-bly where the gun crew operates. However, based on subjective experience theblast level inside the trails is lower than at the microphone location. Sound levelmeasurements at the crew positions, as well as more precise measurements ofthe microphone location, would require a dedicated trial.

Year Serial Charge No. ofShots

Elevation(mils)

SPL(peak) SEL(A)

2015 1 6 3 1159–1179 174–176 137–139

2015 2 6 3 369–491 173–174 141–146

2015 3 7 8 295–461 175–176 144–145

2003 - ? 3 ? 168–170 131

Table 3: L119 configuration and acoustic pressure metrics for each serialin the 2015 measurements. The barrel elevation range in mils is also given(1 mil = 0.05625 degrees). The 2003 results have been included for com-parison.

4.3 Acoustic pressure waveforms

Acoustic pressure waveforms for one sample shot in each serial are shown inFig. 6. The serial 1, charge 6 shot (Fig. 6, Top) has fairly typical characteristicsexpected of blast noise: a rapid onset leading to peak overpressure; a slow decayback to a partial vacuum; a slow increase back to positive pressure.

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The peak pressure is followed by secondary peaks at about 0.4 and 0.6 ms af-ter the first arrival. The secondary peaks arrive too soon to be associated withreflections from the ground, and are also unlikely to be reflections from parts ofthe gun. The propellant charge is stored in a number of separate bags and isdetonated by a charge in a central column. The secondary peaks in the acousticpressure may be caused by uneven ignition of the propellant bags.

The pressure waveform for the serial 2 charge 6 shot is similar to the serial 1shots except that the peak pressure is not reached until about 0.6 ms after themain blast arrival (Fig. 6, Centre).

The pressure waveform for the charge 7 shot has two distinct impulses in theinterval from 10 to 11 ms (Fig. 6, Bottom), and in between these two impulsesthe pressure decays almost back to the ambient level. This feature is present tovarying degrees in all the charge 7 shots. The double peak in these shots maybe due to a late detonation of the final propellant bag.

8 9 10 11 12 13 14 15 16 17 18

Pre

ssur

e (k

Pa)

-10

0

10

Charge 6, Elevation 1159

8 9 10 11 12 13 14 15 16 17 18

Pre

ssur

e (k

Pa)

-10

0

10


time (millisecs)8 9 10 11 12 13 14 15 16 17 18

Pre

ssur

e (k

Pa)

-10

0

10


Figure 6: The initial part of the recorded acoustic pressure waveforms.Top: serial 1, shot 1; Centre: serial 2, shot 1; Bottom: serial 3, shot 1.The double peak at the onset of the charge 7 waveform may be due todelayed ignition of the final propellant bag.

Plots of the acoustic pressure waveforms and spectra for all shots in serials 1and 2, and the first six shots of serial 3, are given in Appendix A. The shots inserials 2 and 3 were carried out under a camouflage net, and reverberation dueto the net is likely to be the cause of the noisy tails in these waveforms. This

DTA Report 406 10

increases the amount of acoustic energy the operators are exposed to, althoughit does not alter the peak SPL.

The estimates of hearing protection effectiveness given in Section 6 made useof the charge 7 data since these shots were the most acoustically energetic. Forfuture reference, pressure waveforms and spectra for all three shots of serials 1and 2, as well as the first six shots in serial 3, are given in Appendix A.

4.4 Octave band levels

The mean unweighted octave band energy levels in the shots are plotted in Fig. 7,and the A-weighted energy levels are given in Fig. 8. The error bars in thesegraphs range from the minimum to the maximum SEL value measured in eachband. The octave band levels can be used to estimate the total noise level experi-enced by a listener under a hearing protector using octave band attenuation data.This is important since there is a daily limit on the amount of acoustic energy thata worker may be exposed to. However, octave band energy levels cannot beused on their own to predict the peak SPL inside a protector. A measurement ofthe free-field acoustic pressure history is required for this purpose.

Octave band (Hz)31.25 62.5 125 250 500 1000 2000 4000 8000

SE

L (d

B)

120

125

130

135

140

145

150

131130

138

136

133132 131

129

126

134133

136136

136

140

136

130

127

137

136

137 136 135

141

139

134

128

Octave Band Levels (unweighted)

Charge 6, 1159-1185 milsCharge 6, 369-491 milsCharge 7, 288-315 mils

Figure 7: The mean SEL of the L119 shots in octave bands. The error barsshow the minimum and maximum levels over all shots.

DTA Report 406 11

Octave band (Hz)31.25 62.5 125 250 500 1000 2000 4000 8000

SE

L(A

) (d

B)

120

125

130

135

140

145

150

122

127129

131132

130

125

122

127

134

140

137

131

125123

127

133

141

140

135

127

Octave Band Levels (A-weighted)

Charge 6, 1159-1185 milsCharge 6, 369-491 milsCharge 7, 288-315 mils

Figure 8: The mean A-weighted SEL of the L119 shots in octave bands.

5 LEGISLATIVE LIMITS ON OCCUPATIONAL NOISE EXPOSURE

In New Zealand limits on workplace noise exposure are given in Regulation 11 ofthe Health and Safety in Employment (HSE) Act. Regulation 11 requires employ-ers to take all practicable steps to ensure that no employee is exposed to noiseabove the following levels:

(a) Eight-hour equivalent continuous A-weighted sound pressure level, LAeq,8h,of 85 dB(A)3; and

(b) Peak sound pressure level, Lpeak, of 140 dB, –

whether or not the employee is wearing a personal hearing protector.

In the following section the performance of the hearing protectors listed in Fig. 3will be evaluated by the computer simulation model discussed in Section 3.1. Themodel will estimate both the peak SPL and the SEL(A) metrics. Using the SEL(A)for a single shot the LAeq,8h can be obtained for a given number of shots, usingthe formula given in Section B.4. This can be used to find a limit on the numberof shots that can be fired in one day that is compliant with the 85 dB limit inregulation 11(a).

5.1 Origin of the 140 dB limit on peak pressure

The 140 dB limit recommended for the peak SPL is traceable to a 1968 reportfrom the U.S. National Research Council Committee on Hearing, Bioacoustics

3The 85 dB(A) limit on LAeq,8h is equivalent to a limit on the time integral of the squared A-weighted acoustic pressure of 3600 Pa2s.

DTA Report 406 12

(CHABA) [12, 13]. The limit for impulse noise originally recommended in theCHABA report was defined in terms of peak pressure and impulse duration - theshorter the impulse, the higher the permitted peak pressure [14, p141].

However, subsequent U.S. Army standards limiting impulse noise exposure sim-plified the CHABA report recommendations by setting a limit of 140 dB on thepeak pressure irrespective of impulse duration [13]. This may have been moti-vated by the difficulty in measuring duration with equipment available at the time.This limit has since entered into civilian noise exposure legislation around theworld.

The role of peak pressure in determining auditory damage is discussed by Patter-son et al. [13]. The authors concluded that peak pressure alone is not sufficientto predict auditory hazard. The duration, frequency content and energy of im-pulse noise are also important in determining risk to hearing.

6 HEARING PROTECTOR PERFORMANCE ESTIMATES

The effect of hearing protection on the L119 blast noise was simulated usingthe recorded pressure signals for each serial and manufacturer supplied datafor a number of hearing protectors. For each recorded pressure signal the A-weighted sound exposure level (SEL) and peak SPL inside the hearing protectorwas estimated. The results, with error bars, are shown in Fig. 9.

The error bars shown on the graph are from the round-to-round variations in blastnoise level. The method used here to predict peak level has not been validatedfor low frequency blast noise. Existing experimental data can provide only a par-tial validation against blast noise from rifle shots. Improved confidence in thesepredictions can come from additional experimentation.

6.1 Reduction of the peak SPL

The model predicts that the ComTac XPI-FB + EAR option (earmuff and earplugs,i.e. double protection) is sufficient to bring the average peak SPL to 138 dB forcharge 7 shots. The predicted performance of the earmuff/earplug combinationis much better than the single protectors due to the additional blocking of lowfrequency noise. For all other protectors the peak SPL in the ear canal is in the145–155 dB range, which is significantly higher than the limit of 140 dB stipulatedin the HSE Act.

DTA Report 406 13

dB135 140 145 150 155 160

155

151

149

152

145

149

148

138ComTac XPI-FB + EAR

ProTac II

H10A

Combat Arms - Closed

Tactical XP

EP7 Closed Cap

SportTac

ComTac XPI-FB

SPL(peak)

dB105 110 115 120 125

120

119

118

116

115

114

114

107

SEL(A)

Figure 9: Simulated performance of selected hearing protectors. All arePeltor branded protectors except for the EP7 which is manufactured bySurefire. The error bars are obtained from the round-to-round variationsin noise level. The protectors are ranked based on the A-weighted soundexposure level (SEL(A)) inside the protector (lower is better).

6.2 Daily limit on the number of shots

The HSE Act places a limit on the LAeq,8h of 85 dB, and this requirement setsa limit on the number of shots the crew can be exposed to in one day. Theestimated maximum, based on charge 7 data and assuming identical pressurewaveforms, is given in Table 4. The maximum numbers that satisfy the morepermissive NATO RSG-029 limit of 98 dB for LAeq,8h are a factor of 20 higher, aconsequence of the increase of the daily limit by 13 dB (see Appendix B). Forcomparison, these numbers are also given in the final column. The RSG-029recommendations are further discussed in Section 7.

The HSE limit of 85 dB leads to very low numbers of shots that may be firedduring the course of a day. If these limits are adopted they may have an impacton training outcomes.

The HSE limit also seems overly restrictive in light of subjective experience. Dur-ing the Exercise Brimstone measurements the author used the H10A earmuffs,and was positioned where the blast noise levels were at least as high as thoseexperienced by the gun crew. The maximum number of shots allowed in one dayfor the H10A was estimated at 33 (see Table 4). In the course of the measure-ments the author was exposed to about 25 shots, including 14 at charge 7. At thisexposure level, the author did not discern any hearing discomfort or temporaryloss even though it was approaching the HSE limit.

Although significant reductions in noise exposure can be achieved with the use

DTA Report 406 14

Protector SPL(peak) SEL(A) HSE ActShots/Day

RSG-029Shots/Day

ComTac XPI-FB + EAR 138 107 190 3800

ProTac II 148 114 33 670

H10A 149 114 33 660

Combat Arms - Closed 145 115 31 620

Tactical XP 152 116 21 420

EP7 Closed Cap 149 118 15 290

SportTac 151 119 10 210

ComTac XPI-FB 155 120 9 180

Table 4: Estimated SEL(A) and SPL(peak) levels under the listed protec-

tors. The maximum number of shots per day is given for (a) the New

Zealand HSE Act limit on LAeq,8h of 85 dB, and (b) the NATO RSG-029

recommended limit on LAeq,8h of 98 dB.

of double hearing protection there will be a loss in ability to communicate. Thiscan be mitigated to some extent with the use of active earmuffs that preferen-tially amplify low level sounds, to ensure they are audible through earplugs, whilerejecting high level impulse noise.

7 THE NATO RSG-029 RECOMMENDATIONS ON IMPULSE NOISE EX-POSURE

Research in NATO into the effects of impulse noise on hearing began in 1979with the establishment of the Research Study Group RSG-6 titled “On the Effectsof Impulse Noise” [15]. The group came to the conclusion that exposure limitcriteria in use at the time were probably overprotective for large calibre weapons,and recommended that new blast data should be collected. The US represen-tatives to RSG-6 proposed new experiments, known as the Blast OverpressureProject (BOP), designed to measure the temporary threshold shift of test subjectsexposed to blasts with varying duration and intensity.

By 1994 the US BOP had been completed and additional impulse noise datahad also been collected in France and Germany. A new NATO Research StudyGroup, RSG-029, was formed to reconsider the effects of impulse noise in hu-man hearing in light of the new data. In 2003 the RSG-029 published the report“Reconsideration of the Effects of Impulse Noise” [15], which contained the con-sensus recommendations of the group on safe exposure limits for impulse noise.

The RSG-029 recommended exposure limits for impulse noise are based on the

DTA Report 406 15

SEL (also written LE) with A-weighting4, rather than the peak SPL. All integratingsound level meters, including those in use by the NZDF, are capable of directlymeasuring the SEL.

On the basis of the available data the RSG divided impulse sources into twocategories, based on the blast duration: short impulses, with A-durations of 0.2–0.3 ms typical of rifle shots; and long impulses with A-durations in the range 0.9–3ms characteristic of blasts from heavy calibre weapons and explosives. The A-duration is defined as the length of the initial positive phase of a blast pressurewave.

For both sources there is a critical exposure level that should not be exceededfor a single impulse: for short duration impulses the limit is 116 dB SEL(A); forlong duration impulses the limit is 135 dB SEL(A).

For multiple blasts, the daily noise exposure LAeq,8h should not exceed 80 dB forshort impulses, and 98 dB for long duration blasts.

The daily exposure limit of 80 dB on LAeq,8h for short duration impulses is more re-strictive than the current New Zealand limit of 85 dB. However, the 98 dB limit onlong duration blast noise is significantly more permissive - the number of identicalblasts that can be safely tolerated is a factor of 20 greater with a 98 dB limit onLAeq,8h than an 85 dB limit. For convenience, the NATO RSG-029 recommendedimpulse noise exposure limits are summarized here in Table 5 [15, 1.7.4].

Source A-duration(msec)

Single impulselimit on SEL(A)

Daily limit onLAeq,8h

Small arms 0.2–0.3 116 80

Blasts 0.9–3 135 98

Table 5: Recommended impulse noise exposure limits given by NATORSG-029 in 2003 [15]. The “Blasts” category includes muzzle blast fromheavy calibre weapons and blast noise from explosive charges.

Given the unique nature of impulse noise it is worth considering whether a newstandard, specifically for muzzle blast and explosive noise sources, could be im-plemented for the NZDF. The recommendations of the RSG-029 are an appro-priate place to begin, since they are based on the limited data that is availableregarding physiological responses to impulse noise. Moreover, the metric thatwas proposed - the A-weighted SEL - can be measured with modern integratingsound level meters.

4The A-weighted SEL is usually written SEL(A) or LAE .

DTA Report 406 16

8 SUMMARY AND CONCLUSIONS

Blast noise measurements were carried out on the New Zealand Army L119 LightGun by the Defence Technology Agency in June 2015 at Exercise Brimstone. Thepurpose of the assessment was to determine the sound pressure levels near thegun crew and estimate the effectiveness of different types of hearing protection.

The acoustic pressure levels were recorded using a Bruel & Kjaer 4941 highpressure microphone. The microphone was placed approximately three metresback from the muzzle at zero barrel elevation, one metre from the centreline ofthe gun, and 1.6 metres from the ground.

Shots using charge 6 and charge 7 (maximum) propellant loads were measured.Charge 6 shots were measured at both low and high barrel elevations, whilecharge 7 shots were measured at low elevation only. The second two serialswere carried out with the gun under a camouflage net.

Impulse noise testing of hearing protectors is not yet routine. Instead, a pro-cedure based on manufacturer supplied octave band attenuations was used topredict noise levels in the ear canal for a number of candidate hearing protectors.

For each protector the octave band attenuations were cubic spline interpolatedonto a regular frequency grid. This curve was used to generate a transfer functionthat transforms the free-field acoustic pressure into the pressure in the ear canal.Measurements of the phase response of hearing protectors are not generallyavailable, but numerical experiments indicate the maximum effect of the unknownphase shift to be 1–2 dB.

No blast noise data is available for model validation using longer duration impulsenoise (0.9–3 ms). However, the method was partially validated using attenuationdata obtained from 5.56 mm rifle shots and an artificial head.

Based on the measurements and DTA’s hearing protector model the main findingsand conclusions regarding the L119 sound pressure levels are:

1. The peak sound pressure level (SPL) was 173–176 dB at charge 6 and175–176 dB at charge 7. These pressure levels are very high and hearingprotection is essential;

2. There is a double peak in the acoustic pressure of the charge 7 shots thatmay be due to delayed ignition of one of the propellant charge bags.

3. The sound exposure level (SEL) was 137–139 dB SEL(A) for charge 6 shotsin open air increasing to 144–146 dB SEL(A) for charge 7 shots under thecamouflage net. The higher acoustic energy levels of the latter shots maybe due to reverberation caused by the camouflage net;

4. The computer simulation model indicates that single hearing protectors canreduce the peak SPL to 145–155 dB, which is not compliant with the Healthand Safety in Employment (HSE) Act limit of 140 dB;

DTA Report 406 17

5. The model indicates that double hearing protection (earmuffs and earplugs)can reduce the peak SPL to 138 dB for charge 7 shots. However, doubleprotection may hamper communication of the gun crew;

6. For double hearing protection the maximum daily exposure is 160 charge 7shots under the HSE Act limit of 85 dB on LAeq,8h;

7. The NATO RSG-029 study group on impulse noise hazard found that longduration impulse noise (0.9–3 ms) is less damaging and recommended adaily exposure limit of 98 dB on LAeq,8h. This equates to 20 times as manyshots per day as an 85 dB limit. However, RSG-029 also recommended areduced limit of 80 dB for small arms fire, which is more restrictive than theHSE Act. Subjective experience supports the more permissive RSG-029recommendations for the L119;

8. The NATO RSG-029 study group did not recommend a limit on peak SPLfor impulse noise. Instead, they proposed limits on the SEL(A) for singleimpulses of 135 dB SEL(A) for long duration blasts and 116 dB SEL(A) forsmall arms fire.

DTA Report 406 18

Appendix A PRESSURE WAVEFORMS AND SPECTRA

The initial part of the acoustic pressure waveforms are plotted in Fig. 10, in a10 ms time window. There is reverberation following the initial blast wave for theshots under the camouflage net (centre and bottom), particularly for the charge 7shots. The initial blast wave for charge 7 is split into two peaks; this was alreadyshown for the first shot in serial 3 in Fig. 6. The double peak in the charge 7shots may be due to uneven ignition of the propellant bags.

The full acoustic pressure waveforms, and amplitude spectra, are shown in Figs. 11–14. The charge level and barrel elevation of the shot are indicated in each graph.All three shots from serial 1 and 2 are shown in Figs. 11 and 12, while the firstsix of the eight shots of serial 3 are given in Figs. 13 and 14.

A 55 ms time window is used to display the acoustic waveform, which includesessentially all the energy in a shot. The window begins 5 ms before the peakimpulse level. The waveform is scaled so that the acoustic pressure in kilopascalsis plotted. The amplitude spectrum of each shot is shown next to the waveform.A frequency window of 1400 Hz has been used since this contains most of theenergy.

The waveforms shown in Fig. 11 were taken at high elevation angles in openair (see Fig. 5, Top). These waveforms clearly contain the features of the idealFriedlander wave, i.e. a rapid rise to the impulse peak, then a slower decayfollowed by a partial vacuum. Slightly before 20 ms there is what appears to bea reflection of the initial part of the shock wave. Since this arrives about 10 msafter the first arrival it is likely to be a ground reflection of the muzzle blast.

The waveforms in Fig. 12 were also from charge 6 propellant loads, but wereat low barrel elevations and were fired from beneath camouflage netting (Fig. 5,Centre). These waveforms have a substantial amount of reverberation not presentin the open air shots in serial 1. The reverberation in these recordings is probablydue to partial reflections from the camouflage net.

The pressure waveforms for the charge 7 shots (Figs. 13 and 14) also have anoisy tail, which again is likely due to reverberation inside camouflage netting(Fig. 5, Bottom). Note also the substantial increase in the acoustic energy below100 Hz in the spectrum; this is probably associated with the reverberation.

Inevitably, there are reflections from the ground and nearby obstacles which tendto interfere with the recording of the direct path muzzle blast. It is also importantto understand the effects of the environment on the acoustic pressure waveform.However, in the interests of recording a clean pressure signal it would be prefer-able for future shots to be recorded in open air.

DTA Report 406 19

5 10 15 20 25 30 35 40 45

Pre

ssur

e (k

Pa)

-10

-5

0

5

10

15Charge 6, Mean Elevation 1174 mils

5 10 15 20 25 30 35 40 45

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


time (millisecs)5 10 15 20 25 30 35 40 45

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


Figure 10: A close-up on the initial part of the recorded acoustic pressurewaveforms. Top: serial 1 (three shots); Centre: serial 2 (three shots);Bottom: serial 3 (eight shots).

DTA Report 406 20

Pre

ssur

e (k

Pa)

-10

-5

0

5

10

15Charge 6, 1159 mils

Acoustic Pressure

102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30Spectrum

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30

time (millisecs)20 40 60 80

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


frequency (Hz)102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30

Figure 11: The acoustic pressure and spectrum for the shots in serial 1.

DTA Report 406 21

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


Acoustic Pressure

102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30Spectrum

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30


Pre

ssur

e (k

Pa)

-10

-5

0

5

10



Am

plitu

de (

Pa-

sec)

0

10

20

30

Figure 12: The acoustic pressure and spectrum for the shots in serial 2.

DTA Report 406 22

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


Acoustic Pressure

102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30Spectrum

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30


Pre

ssur

e (k

Pa)

-10

-5

0

5

10



Am

plitu

de (

Pa-

sec)

0

10

20

30

Figure 13: The acoustic pressure and spectrum for the first three shots inserial 3.

DTA Report 406 23

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


Acoustic Pressure

102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30Spectrum

Pre

ssur

e (k

Pa)

-10

-5

0

5

10


102 104

Am

plitu

de (

Pa-

sec)

0

10

20

30


Pre

ssur

e (k

Pa)

-10

-5

0

5

10



Am

plitu

de (

Pa-

sec)

0

10

20

30

Figure 14: The acoustic pressure and spectrum for the second three shotsin serial 3.

DTA Report 406 24

Appendix B SOUND LEVEL METRICS

B.1 Sound exposure level

The sound exposure level (written SEL or LE) is the constant sound level thathas the same amount of energy in one second as a transient noise event. TheSEL is a measure of the total energy in a sound. It is defined by [16]

SEL = LE = 10 log10

∫ T

0

p2

p20dt

where p is the acoustic pressure and p0 is the reference pressure. When anA-weighting is applied to the pressure the sound exposure level is denoted bySEL(A) or LAE.

B.2 Equivalent continuous sound level

The quantity LAeq is known as the A-weighted equivalent continuous sound leveland is defined by [16]

LAeq = 10 log10

(1

T

∫ T

0

p2A

p20dt

)where pA is the A-weighted acoustic pressure and p0 = 20 µPa is the referencepressure. This is the RMS sound level with the measurement duration used asthe averaging time.

B.3 Eight hour equivalent continuous sound level

It is common for health and safety legislation to prescribe limitations on the totalsound energy received over a working day, rather than the RMS sound level.For this purpose the A-weighted 8-hour equivalent continuous sound level wasintroduced. It is defined by

LAeq,8h = 10 log10

(1

28800

∫ T

0

p2A

p20dt

)= SEL(A) − 44.6

where T is the duration of the noise in seconds and pA is the A-weighted acousticpressure. It is related to the equivalent continuous sound level by

LAeq,8h = LAeq + 10 log10

(T

28800

).

The LAeq,8h is also known as the daily personal noise exposure and may also bewritten as Lex,8h or LEP,d.

DTA Report 406 25

B.4 Equivalent continuous level for N impulses

For N identical impulse waveforms the LAeq,8h is

LNAeq,8h = L1

Aeq,8h + 10 log10N

= SEL1(A) − 44.6 + 10 log10N

where L1Aeq,8h is the value for a single impulse. This statistic was found to be the

best predictor of TTS for impulse noise in the study of blast overpressure data byMurphy, Khan and Shaw [17].

B.5 Noise exposure limits

In New Zealand the limit on daily noise exposure level LAeq,8h is 85 dB, which isequivalent to a limit on the integral of the squared A-weighted acoustic pressureof 3600 Pa2s. The daily noise exposure level is related to the integral of thesquared A-weighted pressure by∫ T

0

p2A(t) dt = 1.152× 10−5 × 10LAeq,8h/10.

Using this relation we can re-express level limits on the LAeq,8h metric into limitson the squared pressure integral. Values for some common noise exposure limitsare given in Table 6.

LAeq,8h (dB) Pa2s Pa2h

80 1100 0.3

85 3600 1

98 72000 20

Table 6: Exposure limits on the LAeq,8h in dB, and the equivalent limits for

the time-integral of the A-weighted squared pressure in units of Pa2s and

Pa2h.

DTA Report 406 26

Appendix C THE CLASS RATING SYSTEM

In the Class method hearing protectors are now assigned to one of five hearingprotector classes according to their acoustic performance. The Class rating isclosely related to the SLC80 which is defined by

SLC80 = 100− 10 log10∑f

10[B(f)−A(f)]/10

where the sum is over all octave band frequencies f and A = A(f) is the as-sumed protective value at octave band frequency f . The values B are certainspecified band levels and are given in Table 7.

f (Hz) B (dB) f (Hz) B (dB)125 71 2000 95250 81 4000 93500 89 8000 86

1000 93

Table 7: Specified band levels B for the SLC80 calculation.

A personal hearing protector should be selected on the basis of the LAeq,8h towhich a worker is exposed during a working day. The classes are defined inTable 8 [18, p46].

SLC80 Class LAeq,8h

10–13 1 < 9014–17 2 90–9518–21 3 95–10022–25 4 100–105≥ 26 5 105–110

Table 8: The relationship between the SLC80 and the Class rating. Theselection of a hearing protector by the Class method is based on the LAeq,8h

of the noise.

DTA Report 406 27

REFERENCES

[1] “Noise Assessment Program on the 105 mm Artillery Gun,” NZDF internalreport, New Zealand Defence Force, September 2003.

[2] N. de Lautour, “A Review of Noise Measurements on the L119 Howitzer,”DTA Technical Report DTA-391, Defence Technology Agency, 2014.

[3] T. South, Managing Noise and Vibration at Work: A Practical Guide to Assess-ment, Measurement and Control. Oxford: Elsevier Butterworth-Heinemann,2004.

[4] E. H. Berger, “Preferred methods for measuring hearing protector attenu-ation,” in The 2005 congress and exposition on noise control engineering,International Institute of Noise Control Engineering, 2005.

[5] J. T. Kalb, “An Electroacoustic Hearing Protector Simulator That AccuratelyPredicts Pressure Levels in the Ear Based on Standard Performance Met-rics,” Technical Report ARL-TR-6562, Army Research Laboratory, 2013.

[6] R. Mlynski and E. Kozlowski, “Determining attenuation of impulse noise withan electrical equivalent of a hearing protection device,” International Journalof Occupational Safety and Ergonomics, vol. 19, no. 1, pp. 127–141, 2013.

[7] E. Berger, “EARLog 14 - Protection from Infrasonic and Ultrasonic NoiseExposure,” Am. Ind. Hyg. Assoc. J., vol. 45, no. 9, pp. B32–B33, 1984.

[8] W. J. Murphy, G. A. Flamme, D. K. Meinke, J. Sondergaard, D. S. Finan, J. E.Lankford, A. Khan, J. Vernon, and M. Stewart, “Measurement of impulsepeak insertion loss for four hearing protection devices in field conditions,”International Journal of Audiology, vol. 51, no. S1, pp. S31–S42, 2012.

[9] K. Buck and G. Parmentier, “Artificial heads for high-level impulse soundmeasurement,” Tech. Rep. PU 341/99, DTIC Document, 1999.

[10] A. Dancer, K. Buck, P. Hamery, and G. Parmentier, “Hearing protection inthe military environment,” Noise and Health, vol. 2, no. 5, pp. 1–15, 1999.

[11] J. Zera and R. Mlynski, “Attenuation of high-level impulses by earmuffs,” J.Acoust. Soc. Am., vol. 122, no. 4, pp. 2082–2096, 2007.

[12] “Proposed damage risk criterion for impulse noise (gunfire),” Report of Work-ing Group 57, NAS-NRC committee on Hearing, Bioacoustics and Biome-chanics, Washington, D.C., 1968.

[13] J. H. Patterson Jr., I. M. Gautier, D. L. Curd, R. P. Hamernik, R. J. Salvi, C. E.Hargett, Jr., and G. Turrentine, “The role of peak pressure in determining theauditory hazard of impulse noise,” USAARL Report 86-7, 1986.

DTA Report 406 28

[14] E. H. Berger, ed., The Noise Manual. American Industrial Hygiene Associa-tion, 5th ed., 2003.

[15] “Reconsideration of the Effects of Impulse Noise,” tech. rep.

[16] D. A. Bies and C. H. Hansen, Engineering Noise Control: Theory and Prac-tice. CRC Press, fourth ed., 2009.

[17] W. Murphy, A. Khan, and P. Shaw, “An Analysis of the Blast OverpressureStudy Data Comparing Three Exposure Criteria,” Tech. Rep. EPHB 309-05h,U.S. Department of Health and Human Services, NIOSH, Cincinnati, OH,2009.

[18] “Approved Code of Practice for the Management of Noise in the Workplace,”OSH 3280, New Zealand Occupational Safety and Health Service, Depart-ment of Labour, 2002.

DTA Report 406 29

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