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DTA Report 406 NR 1687 Blast Noise of the L119 Light Gun Nathaniel de Lautour October 2015 DTA Report 406 NR 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 Light Gun by the Defence Technology Agency in June 2015 at Exercise Brimstone. The purpose of the assessment was to determine the sound pressure levels near the gun crew and estimate the effectiveness of a number of hearing protectors. The acoustic pressure was measured slightly forward of the loader’s position, approximately three metres back from the muzzle at zero barrel elevation, one metre from the gun centreline, and 1.6 metres from the ground. The peak sound pressure level of the muzzle blasts was in the range 173–176 dB, the A-weighted sound exposure level was 137–146 dB, and the A-durations were 0.65–2 milliseconds. Double hearing protection (earmuffs and earplugs) should reduce the peak sound pressure level to 138 dB, which is compliant with the Health and Safety in Employment Act. DTA Report 406 ISSN 1175-6594 (Print) ISSN 2253-4849 (Online) Published by Defence Technology Agency Private Bag 32901 Devonport Auckland 0744 New Zealand c Crown Copyright 2015 EXECUTIVE SUMMARY BACKGROUND In 2003 noise level measurements were made by a private contractor on the L119 105 mm gun. The measurements were not calibrated and at the request of the New Zealand Army a review of the 2003 data was carried out by the Defence Technology Agency (DTA). DTA recommended that the measurements be repeated with a calibrated microphone for different weapon configurations, and that the hearing protection recommendations in the report be reviewed with the new data. AIM To measure the muzzle blast pressure of the L119 and estimate the acoustic performance of hearing protectors based on the new data. RESULTS DTA carried out blast noise measurements on the New Zealand Army L119 105 mm gun in June 2015 at Exercise Brimstone. A computer simulation model of hearing protector performance for impulse noise was developed, using manufacturer 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. Issues of ergonomics, communication and cost were outside the scope of this study and have 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 and 175–176 dB at charge 7. These pressure levels are very high and hearing protection is essential; 2. There is a double peak in the acoustic pressure of the charge 7 shots that may 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 shots in open air increasing to 144–146 dB SEL(A) for charge 7 shots under the camouflage net. The higher acoustic energy levels of the latter shots may be due to reverberation caused by the camouflage net; 4. The computer simulation model indicates that single hearing protectors can reduce the peak SPL to 145–155 dB, which is not compliant with the Health and 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, double protection may hamper communication of the gun crew; 6. For double hearing protection the maximum daily exposure is 160 charge 7 shots under the HSE Act limit of 85 dB on LAeq,8h ; 7. The NATO RSG-029 study group on impulse noise hazard found that long duration impulse noise (0.9–3 ms) is less damaging and recommended a daily exposure limit of 98 dB on LAeq,8h . This equates to 20 times as many shots per day as an 85 dB limit. However, RSG-029 also recommended a reduced limit of 80 dB for small arms fire, which is more restrictive than the HSE Act. Subjective experience supports the more permissive RSG-029 recommendations for the L119; 8. The NATO RSG-029 study group did not recommend a limit on peak SPL for impulse noise. Instead, they proposed limits on the SEL(A) for single impulses of 135 dB SEL(A) for long duration blasts and 116 dB SEL(A) for small 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 1.1 The L119 light gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 2 Acoustic measurement equipment 2 3 Hearing protector performance in impulse noise 3.1 Using interpolated octave band attenuations to model the effect of hearing protection on impulse noise . . . . . . . . . . . . . . . . . . 3.2 Validation of the DTA method on 5.56 mm rifle shot data . . . . . . 3 4 Measurements 4.1 Setup . . . . . . . . . . . . . 4.2 Sound pressure levels . . . . 4.3 Acoustic pressure waveforms 4.4 Octave band levels . . . . . . 4 6 . . . . 7 7 9 9 11 5 Legislative limits on occupational noise exposure 5.1 Origin of the 140 dB limit on peak pressure . . . . . . . . . . . . . . 12 12 6 Hearing protector performance estimates 6.1 Reduction of the peak SPL . . . . . . . . . . . . . . . . . . . . . . . 6.2 Daily limit on the number of shots . . . . . . . . . . . . . . . . . . . 13 13 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 B.1 Sound exposure level . . . . . . . . . . . . . B.2 Equivalent continuous sound level . . . . . . B.3 Eight hour equivalent continuous sound level B.4 Equivalent continuous level for N impulses . B.5 Noise exposure limits . . . . . . . . . . . . . 25 25 25 25 26 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 carried out 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’s 16th Field Regiment. Exercise Brimstone is an annual event during which artillery gun crews undergo qualification testing for the L119. These circumstances are not ideal for making blast noise measurements, but it is difficult to organize trials specifically 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 using a hydrophone as a pressure sensor [1]. The measurement system was not calibrated and some uncertainty remains as to the actual sound levels. At the request of the New Zealand Army the Defence Technology Agency (DTA) carried out a review of the consultant’s report [2]. It was recommended that the measurements be repeated with a calibrated microphone for different weapon configurations, and that the original hearing protection recommendations in the 2003 report be reviewed in light of the new data. DTA has measured muzzle blast noise from small arms since 2010. Initially, measurements were made using a variety of microphone and hydrophone sensors and data acquisition systems. DTA now uses a Bruel & Kjaer (B&K) 4941 high pressure microphone and a piston-phone calibrator to provide calibrated blast noise measurements. 1.1 The L119 light gun The L119 light gun used by the New Zealand Army is a 105 mm howitzer with a 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 1244 mils, 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 on its platform. Movement of the barrel in azimuth has a relatively insignificant effect on the acoustic measurements because of the limited range of movement. It was not possible to control this variable during the exercise and barrel azimuth was not recorded. The propellant charge for the shells is contained in a brass cartridge case in seven pre-packed bags. The total propellant charge may be reduced by removing 1 Barrel angles are usually stated in mils rather than degrees, and there are 6400 NATO standard mils per 360 degrees. DTA Report 406 1 bags from the case. Before the gun is fired the commander orders the crew to prepare the required projectile types and propellant cases with a specified charge level. Figure 1: Left: A New Zealand Army 161 Battery gun crew preparing the L119 during Exercise Brimstone, June 2015. Right: The gun muzzle with arrows 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 microphone. The upper sound pressure level (SPL) limit for this microphone is 184 decibels (dB) relative to a reference pressure of 20 micropascals. The peak SPL did not exceed 177 dB and so the measurement system was operating well within its linear dynamic range throughout the trial. The microphone and preamplifier assembly are placed in a sleeve of sound absorbing foam inside a section of PVC tube with a longitudinal cut. The PVC tube is mounted on a plastic block which attaches to the tripod. The foam sleeve is necessary to reduce the effects of tripod vibration on the microphone. The cable runs out through a groove in the foam sleeve (Fig. 2). The pressure signal was conditioned and digitized using a B&K 2250 sound level meter. The pressure waveforms were recorded onto the sound level meter’s external Secure Digital (SD) card in 24-bit format at a sampling rate of 48 kHz. This rate 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 generates a 250 Hz tone, nominally at 124 dB. There is a small ambient pressure correction to the stated calibration level. The barometric pressure at the start of the trial was 925 hPa, measured using a portable barometer. Recordings of DTA Report 406 2 the 250 Hz calibration tone were used to estimate the scale factor for converting digitized sample values to acoustic pressure in Pascals. The equipment used to make the acoustic measurements and the dates of calibration are given below in Table 1. Figure 2: Left: The B&K 4941 microphone used for the measurements was 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 achievable by a hearing protector against impulse noise. The octave band method [3, p208] is recommended for use with continuous broadband noise, but this method cannot be directly applied to predict the peak level reduction against impulse noise sources. This difficulty was noted by the author of the 2003 report on the noise levels of the L119. At present, hearing protector attenuation is quantified using a procedure known as Real Ear Attenuation at Threshold (REAT). The method measures a quantity known as insertion loss (IL) which is the difference in noise level at the eardrum location between the unprotected and protected ear. The REAT method deter- DTA Report 406 3 mines the IL by measuring hearing thresholds for the unprotected and protected conditions across the sample of test subjects. The quantity used in practical noise assessments is the mean attenuation minus one standard deviation, which is called the assumed protective value (APV). This results 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 impulse noise inside a protector based on free-field measurements of the acoustic pressure [5]. The method works by fitting a transfer function model to octave band attenuations measured by the REAT method. DTA experimented with this model but was unable to obtain a satisfactory fit with all of the hearing protectors of interest in this study. Other researchers have used Shaw’s earmuff transfer function model to predict impulse noise inside a range of earmuffs in response to pistol shots [6]. While the fit of the model was very good in certain cases, the error was as high as 7 dB in others. DTA also experimented with the Shaw model, but again was unable to obtain a satisfactory fit with the available hearing protector attenuation data. 3.1 Using interpolated octave band attenuations to model the effect of hearing protection on impulse noise The effect of a hearing protector on impulse noise was estimated from cubic spline interpolation of REAT attenuations onto an evenly spaced grid of frequencies in the range from 0 to 24 kHz (the Nyquist frequency). The interpolated frequency response is used to generate a transfer function that transforms a freefield acoustic pressure measurement into a signal inside the hearing protector. The transfer function of the open ear (TFOE) has been neglected in these calculations, that is, the REAT IL values are used as transmission losses2 . This is justifiable as the majority of the acoustic energy content lies below 2 kHz and the TFOE has little effect in this band. The REAT test values are usually only available at frequencies between 125 Hz and 8 kHz. The response outside this range was estimated by using the attenuation at 125 Hz for frequencies down to 0 Hz as suggested by Berger [7], and the 8 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 significant effect on the outcome. The hearing protector phase response is not measured in the REAT test procedure. To estimate phase and amplitude response of earmuffs over a broad frequency range Mlynski and Kozlowski applied the Shaw earmuff transfer function model to a number of earmuffs [6]. 2 The TFOE transforms free-field acoustic pressure into pressure at the ear drum location. DTA Report 406 4 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 sound pressure level (SPL) and sound exposure level (SEL) under the protector. The error 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 attenuation measurements. It is also similar to the change in attenuation caused by non-linearities in response to impulse noise [8]. Taking into account the other sources of error it seems justifiable, for the purposes of estimating peak SPL and SEL, to ignore the phase response. Consequently, all hearing protectors have been assumed to have zero phase shift. To validate the approach taken the method was applied to some hearing protector attenuation measurements obtained using 5.56 mm rifle shots [8], and adjusted to provide a reasonable fit to the attenuations. The performance of the model on this data set, and the model fitting procedure, is discussed next in Section 3.2. The interpolated attenuation curves for the hearing protectors investigated in this study are given below in Fig.3. 45 40 30 ComTac XPI-FB + EAR Combat Arms - Closed 25 EP7 Closed Cap 20 SportTac ProTac II H10A 15 Attenuation (dB) 35 Tactical XP 10 ComTac XPI-FB 10 2 10 3 Frequency (Hz, log scale) 5 10 4 Figure 3: Hearing protector attenuation curves assumed in this study. The points on the curves are the manufacturer supplied assumed protective values at octave band centre frequencies. The curves are generated by cubic spline 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 impulse noise based on interpolated octave band attenuations was proposed. Some data on protector performance against 5.56 mm rifle shots is available and this data 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 described in 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 test subjects to harmful levels of noise. As an alternative, measurements have been made on a model of the human head and ear canal, known as an acoustic test fixture (ATF). Because a single ATF does not capture the diversity of human physiology it is not generally used as an assessment method. However, for high level impulse noise it is currently the only means of measuring insertion loss and estimating sound pressure level in the ear canal [9]. In 2012, Murphy et al. reported test data on a number of hearing protectors against impulse noise using an ATF consisting of a solid acrylic head equipped with a shock isolated B&K ear simulator and a high pressure microphone [8]. The free-field acoustic waveforms measured in the experiment are not available but DTA has a large number of calibrated recordings of 5.56 mm rifle shots obtained using a B&K 4941 microphone. The tests included three nominally linear protectors for which octave band attenuation data is available: the 3M EAR Pod Express earplug; the single-ended Combat Arms earplug in the closed position; and the Etymotic Research EB1. Experiments were performed at 130, 150 and 170 dB peak impulse levels. Although these protectors are not designed to have a non-linear characteristic, each exhibited an increase in attenuation with rising impulse level. It was already known before this study that the attenuation of non-linear earplugs and earmuffs can increase 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 method described in the previous section, are given in Table 2 below. When the mean attenuation levels for each hearing protector were used in the DTA interpolation method, the predicted IL was significantly higher than the measured levels. To adjust the model to better agree with measurements the APVs for each protector were used instead, i.e. the mean minus one standard deviation. The estimates obtained from the interpolation method lie between the measurements for the 130 dB and 170 dB peak impulse levels. On the basis of this comparison the method should provide slightly conservative estimates of IL for peak impulse levels of around 170 dB. DTA Report 406 6 Measured IL Measured IL Predicted by @ 130 dB @ 170 dB DTA 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 Protector 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 levels are compared with predictions from the interpolation method proposed here. None of the protectors considered in this study have been tested in low frequency blast noise similar to the L119 muzzle blast. Until test measurements become available there will remain a degree of uncertainty as to the actual performance of the protectors against the L119 and similar blast noise sources. 4 4.1 MEASUREMENTS Setup The microphone was always positioned on the right hand side of the gun, outside the trails, slightly forward of the loader’s position. It was not possible to place the microphone inside the trails since this would interfere with the operation of the gun crew. It was not possible to measure the position of the microphone with accuracy during 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 centreline of the barrel, and was about 3.5 m back from the end of the muzzle (measured at zero barrel elevation). In serials 2 and 3 it was necessary to move the microphone to a position about 1 m from the barrel centreline and about 3 m back from the muzzle. These approximate positions are indicated in Fig. 4. The microphone was approximately 1.6 m above the ground in all serials, and oriented in the vertical direction. Pictures of the actual microphone locations are shown in Fig. 5. Because the microphone position differed between serial 1 and serials 2 and 3 it is difficult to directly compare these acoustic measurements. The hearing protector performance estimates in this study made use of the serial 3 measurements which had the highest pressure levels. DTA Report 406 7 Serial 1 Serial 2,3 Figure 4: Approximate microphone positions during serial 1 and serials 2 and 3. The microphone was approximately 1.6 m above ground. Figure 5: Top: the microphone position for serial 1, with the tubular trails of the L119 indicated by the arrows. The microphone is mounted in the white PVC tube on the top of the tripod. Centre: the microphone position for serial 2. Bottom: the microphone position for serial 3. DTA Report 406 8 4.2 Sound pressure levels Three sets of acoustic pressure measurements were performed using different charge levels and barrel elevations. In each serial the gun was mounted in a fixed position. The gun barrel elevation and azimuth varied only marginally within a serial. The range of peak SPL for each serial is shown in Table 3. The 2003 measurements 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 the SEL(A) measurement was obtained at a distance of “five metres”. In addition, the barrel 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 measurements. The 2015 measurements were considerably higher. This may be due to closer proximity of the microphone to the muzzle, a calibration error in the 2003 measurements, or a higher propellant charge level. In the serial 2 and 3 measurements the microphone was positioned close to where 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 centreline of the barrel. It was not possible to make measurements inside the trail assembly where the gun crew operates. However, based on subjective experience the blast level inside the trails is lower than at the microphone location. Sound level measurements at the crew positions, as well as more precise measurements of the microphone location, would require a dedicated trial. Year Serial Charge 2015 1 6 2015 2 2015 2003 No. of Shots Elevation (mils) SPL(peak) SEL(A) 3 1159–1179 174–176 137–139 6 3 369–491 173–174 141–146 3 7 8 295–461 175–176 144–145 - ? 3 ? 168–170 131 Table 3: L119 configuration and acoustic pressure metrics for each serial in the 2015 measurements. The barrel elevation range in mils is also given (1 mil = 0.05625 degrees). The 2003 results have been included for comparison. 4.3 Acoustic pressure waveforms Acoustic pressure waveforms for one sample shot in each serial are shown in Fig. 6. The serial 1, charge 6 shot (Fig. 6, Top) has fairly typical characteristics expected of blast noise: a rapid onset leading to peak overpressure; a slow decay back to a partial vacuum; a slow increase back to positive pressure. DTA Report 406 9 The peak pressure is followed by secondary peaks at about 0.4 and 0.6 ms after the first arrival. The secondary peaks arrive too soon to be associated with reflections from the ground, and are also unlikely to be reflections from parts of the gun. The propellant charge is stored in a number of separate bags and is detonated by a charge in a central column. The secondary peaks in the acoustic pressure 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 1 shots except that the peak pressure is not reached until about 0.6 ms after the main blast arrival (Fig. 6, Centre). The pressure waveform for the charge 7 shot has two distinct impulses in the interval from 10 to 11 ms (Fig. 6, Bottom), and in between these two impulses the pressure decays almost back to the ambient level. This feature is present to varying degrees in all the charge 7 shots. The double peak in these shots may be due to a late detonation of the final propellant bag. Pressure (kPa) Charge 6, Elevation 1159 10 0 -10 8 9 10 11 12 13 14 15 16 17 18 15 16 17 18 15 16 17 18 Pressure (kPa) Charge 6, Elevation 378 10 0 -10 8 9 10 11 12 13 14 Pressure (kPa) Charge 7, Elevation 315 10 0 -10 8 9 10 11 12 13 14 time (millisecs) 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 to delayed ignition of the final propellant bag. Plots of the acoustic pressure waveforms and spectra for all shots in serials 1 and 2, and the first six shots of serial 3, are given in Appendix A. The shots in serials 2 and 3 were carried out under a camouflage net, and reverberation due to 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, although it does not alter the peak SPL. The estimates of hearing protection effectiveness given in Section 6 made use of the charge 7 data since these shots were the most acoustically energetic. For future reference, pressure waveforms and spectra for all three shots of serials 1 and 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 these graphs range from the minimum to the maximum SEL value measured in each band. The octave band levels can be used to estimate the total noise level experienced 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 that a worker may be exposed to. However, octave band energy levels cannot be used on their own to predict the peak SPL inside a protector. A measurement of the free-field acoustic pressure history is required for this purpose. Octave Band Levels (unweighted) 150 Charge 6, 1159-1185 mils Charge 6, 369-491 mils Charge 7, 288-315 mils 145 SEL (dB) 140 138 137 136 134 135 141 136 137 136 140 136 136136 139 135 136 134 133 133 132 131 131 130 129 130 130 128 126127 125 120 31.25 62.5 125 250 500 1000 2000 4000 8000 Octave band (Hz) Figure 7: The mean SEL of the L119 shots in octave bands. The error bars show the minimum and maximum levels over all shots. DTA Report 406 11 Octave Band Levels (A-weighted) 150 Charge 6, 1159-1185 mils Charge 6, 369-491 mils Charge 7, 288-315 mils 145 140 141 SEL(A) (dB) 140 140 137 134 135 133 135 132 131 130 127127127 130 131 129 127 125125 125 123 122122 120 31.25 62.5 125 250 500 1000 2000 4000 8000 Octave band (Hz) 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 of the Health and Safety in Employment (HSE) Act. Regulation 11 requires employers to take all practicable steps to ensure that no employee is exposed to noise above 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. 3 will be evaluated by the computer simulation model discussed in Section 3.1. The model 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, using the formula given in Section B.4. This can be used to find a limit on the number of shots that can be fired in one day that is compliant with the 85 dB limit in regulation 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 report from the U.S. National Research Council Committee on Hearing, Bioacoustics 3 The 85 dB(A) limit on LAeq,8h is equivalent to a limit on the time integral of the squared Aweighted acoustic pressure of 3600 Pa2 s. DTA Report 406 12 (CHABA) [12, 13]. The limit for impulse noise originally recommended in the CHABA report was defined in terms of peak pressure and impulse duration - the shorter the impulse, the higher the permitted peak pressure [14, p141]. However, subsequent U.S. Army standards limiting impulse noise exposure simplified the CHABA report recommendations by setting a limit of 140 dB on the peak pressure irrespective of impulse duration [13]. This may have been motivated by the difficulty in measuring duration with equipment available at the time. This limit has since entered into civilian noise exposure legislation around the world. The role of peak pressure in determining auditory damage is discussed by Patterson et al. [13]. The authors concluded that peak pressure alone is not sufficient to predict auditory hazard. The duration, frequency content and energy of impulse 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 using the recorded pressure signals for each serial and manufacturer supplied data for a number of hearing protectors. For each recorded pressure signal the Aweighted sound exposure level (SEL) and peak SPL inside the hearing protector was 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 blast noise level. The method used here to predict peak level has not been validated for low frequency blast noise. Existing experimental data can provide only a partial validation against blast noise from rifle shots. Improved confidence in these predictions 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 for charge 7 shots. The predicted performance of the earmuff/earplug combination is much better than the single protectors due to the additional blocking of low frequency noise. For all other protectors the peak SPL in the ear canal is in the 145–155 dB range, which is significantly higher than the limit of 140 dB stipulated in the HSE Act. DTA Report 406 13 SPL(peak) ComTac XPI-FB + EAR SEL(A) 138 107 ProTac II 148 H10A 114 149 Combat Arms - Closed 114 145 115 Tactical XP 152 EP7 Closed Cap 149 SportTac 118 151 ComTac XPI-FB 135 116 119 155 140 145 150 dB 155 160 105 120 110 115 120 125 dB Figure 9: Simulated performance of selected hearing protectors. All are Peltor branded protectors except for the EP7 which is manufactured by Surefire. The error bars are obtained from the round-to-round variations in noise level. The protectors are ranked based on the A-weighted sound exposure 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 sets a limit on the number of shots the crew can be exposed to in one day. The estimated maximum, based on charge 7 data and assuming identical pressure waveforms, is given in Table 4. The maximum numbers that satisfy the more permissive NATO RSG-029 limit of 98 dB for LAeq,8h are a factor of 20 higher, a consequence of the increase of the daily limit by 13 dB (see Appendix B). For comparison, these numbers are also given in the final column. The RSG-029 recommendations are further discussed in Section 7. The HSE limit of 85 dB leads to very low numbers of shots that may be fired during the course of a day. If these limits are adopted they may have an impact on training outcomes. The HSE limit also seems overly restrictive in light of subjective experience. During the Exercise Brimstone measurements the author used the H10A earmuffs, and was positioned where the blast noise levels were at least as high as those experienced by the gun crew. The maximum number of shots allowed in one day for the H10A was estimated at 33 (see Table 4). In the course of the measurements the author was exposed to about 25 shots, including 14 at charge 7. At this exposure level, the author did not discern any hearing discomfort or temporary loss 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 Act RSG-029 Shots/Day Shots/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 protectors. 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. This can be mitigated to some extent with the use of active earmuffs that preferentially amplify low level sounds, to ensure they are audible through earplugs, while rejecting high level impulse noise. 7 THE NATO RSG-029 RECOMMENDATIONS ON IMPULSE NOISE EXPOSURE Research in NATO into the effects of impulse noise on hearing began in 1979 with the establishment of the Research Study Group RSG-6 titled “On the Effects of Impulse Noise” [15]. The group came to the conclusion that exposure limit criteria in use at the time were probably overprotective for large calibre weapons, and recommended that new blast data should be collected. The US representatives to RSG-6 proposed new experiments, known as the Blast Overpressure Project (BOP), designed to measure the temporary threshold shift of test subjects exposed to blasts with varying duration and intensity. By 1994 the US BOP had been completed and additional impulse noise data had also been collected in France and Germany. A new NATO Research Study Group, RSG-029, was formed to reconsider the effects of impulse noise in human 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 consensus 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 integrating sound level meters, including those in use by the NZDF, are capable of directly measuring the SEL. On the basis of the available data the RSG divided impulse sources into two categories, 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 0.9–3 ms characteristic of blasts from heavy calibre weapons and explosives. The Aduration is defined as the length of the initial positive phase of a blast pressure wave. For both sources there is a critical exposure level that should not be exceeded for a single impulse: for short duration impulses the limit is 116 dB SEL(A); for long duration impulses the limit is 135 dB SEL(A). For multiple blasts, the daily noise exposure LAeq,8h should not exceed 80 dB for short impulses, and 98 dB for long duration blasts. The daily exposure limit of 80 dB on LAeq,8h for short duration impulses is more restrictive than the current New Zealand limit of 85 dB. However, the 98 dB limit on long duration blast noise is significantly more permissive - the number of identical blasts that can be safely tolerated is a factor of 20 greater with a 98 dB limit on LAeq,8h than an 85 dB limit. For convenience, the NATO RSG-029 recommended impulse noise exposure limits are summarized here in Table 5 [15, 1.7.4]. Source Small arms Blasts A-duration (msec) Single impulse limit on SEL(A) Daily limit on LAeq,8h 0.2–0.3 116 80 0.9–3 135 98 Table 5: Recommended impulse noise exposure limits given by NATO RSG-029 in 2003 [15]. The “Blasts” category includes muzzle blast from heavy calibre weapons and blast noise from explosive charges. Given the unique nature of impulse noise it is worth considering whether a new standard, specifically for muzzle blast and explosive noise sources, could be implemented for the NZDF. The recommendations of the RSG-029 are an appropriate place to begin, since they are based on the limited data that is available regarding physiological responses to impulse noise. Moreover, the metric that was proposed - the A-weighted SEL - can be measured with modern integrating sound level meters. 4 The 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 Light Gun by the Defence Technology Agency in June 2015 at Exercise Brimstone. The purpose of the assessment was to determine the sound pressure levels near the gun crew and estimate the effectiveness of different types of hearing protection. The acoustic pressure levels were recorded using a Bruel & Kjaer 4941 high pressure microphone. The microphone was placed approximately three metres back from the muzzle at zero barrel elevation, one metre from the centreline of the 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, while charge 7 shots were measured at low elevation only. The second two serials were carried out with the gun under a camouflage net. Impulse noise testing of hearing protectors is not yet routine. Instead, a procedure based on manufacturer supplied octave band attenuations was used to predict noise levels in the ear canal for a number of candidate hearing protectors. For each protector the octave band attenuations were cubic spline interpolated onto a regular frequency grid. This curve was used to generate a transfer function that transforms the free-field acoustic pressure into the pressure in the ear canal. Measurements of the phase response of hearing protectors are not generally available, but numerical experiments indicate the maximum effect of the unknown phase shift to be 1–2 dB. No blast noise data is available for model validation using longer duration impulse noise (0.9–3 ms). However, the method was partially validated using attenuation data obtained from 5.56 mm rifle shots and an artificial head. Based on the measurements and DTA’s hearing protector model the main findings and conclusions regarding the L119 sound pressure levels are: 1. The peak sound pressure level (SPL) was 173–176 dB at charge 6 and 175–176 dB at charge 7. These pressure levels are very high and hearing protection is essential; 2. There is a double peak in the acoustic pressure of the charge 7 shots that may 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 shots in open air increasing to 144–146 dB SEL(A) for charge 7 shots under the camouflage net. The higher acoustic energy levels of the latter shots may be due to reverberation caused by the camouflage net; 4. The computer simulation model indicates that single hearing protectors can reduce the peak SPL to 145–155 dB, which is not compliant with the Health and 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, double protection may hamper communication of the gun crew; 6. For double hearing protection the maximum daily exposure is 160 charge 7 shots under the HSE Act limit of 85 dB on LAeq,8h ; 7. The NATO RSG-029 study group on impulse noise hazard found that long duration impulse noise (0.9–3 ms) is less damaging and recommended a daily exposure limit of 98 dB on LAeq,8h . This equates to 20 times as many shots per day as an 85 dB limit. However, RSG-029 also recommended a reduced limit of 80 dB for small arms fire, which is more restrictive than the HSE Act. Subjective experience supports the more permissive RSG-029 recommendations for the L119; 8. The NATO RSG-029 study group did not recommend a limit on peak SPL for impulse noise. Instead, they proposed limits on the SEL(A) for single impulses of 135 dB SEL(A) for long duration blasts and 116 dB SEL(A) for small 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 a 10 ms time window. There is reverberation following the initial blast wave for the shots under the camouflage net (centre and bottom), particularly for the charge 7 shots. The initial blast wave for charge 7 is split into two peaks; this was already shown for the first shot in serial 3 in Fig. 6. The double peak in the charge 7 shots 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 first six 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 includes essentially all the energy in a shot. The window begins 5 ms before the peak impulse level. The waveform is scaled so that the acoustic pressure in kilopascals is 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 the energy. The waveforms shown in Fig. 11 were taken at high elevation angles in open air (see Fig. 5, Top). These waveforms clearly contain the features of the ideal Friedlander wave, i.e. a rapid rise to the impulse peak, then a slower decay followed by a partial vacuum. Slightly before 20 ms there is what appears to be a reflection of the initial part of the shock wave. Since this arrives about 10 ms after 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 were at low barrel elevations and were fired from beneath camouflage netting (Fig. 5, Centre). These waveforms have a substantial amount of reverberation not present in the open air shots in serial 1. The reverberation in these recordings is probably due to partial reflections from the camouflage net. The pressure waveforms for the charge 7 shots (Figs. 13 and 14) also have a noisy tail, which again is likely due to reverberation inside camouflage netting (Fig. 5, Bottom). Note also the substantial increase in the acoustic energy below 100 Hz in the spectrum; this is probably associated with the reverberation. Inevitably, there are reflections from the ground and nearby obstacles which tend to interfere with the recording of the direct path muzzle blast. It is also important to understand the effects of the environment on the acoustic pressure waveform. However, in the interests of recording a clean pressure signal it would be preferable for future shots to be recorded in open air. DTA Report 406 19 Charge 6, Mean Elevation 1174 mils Pressure (kPa) 15 10 5 0 -5 -10 5 10 20 25 30 35 40 45 40 45 40 45 Charge 6, Mean Elevation 413 mils 15 Pressure (kPa) 15 10 5 0 -5 -10 5 10 20 25 30 35 Charge 7, Mean Elevation 297 mils 15 Pressure (kPa) 15 10 5 0 -5 -10 5 10 15 20 25 30 35 time (millisecs) Figure 10: A close-up on the initial part of the recorded acoustic pressure waveforms. Top: serial 1 (three shots); Centre: serial 2 (three shots); Bottom: serial 3 (eight shots). DTA Report 406 20 Acoustic Pressure Charge 6, 1159 mils 10 5 0 -5 30 Amplitude (Pa-sec) Pressure (kPa) 15 -10 10 102 104 102 104 30 Charge 6, 1179 mils 10 5 0 -5 Amplitude (Pa-sec) Pressure (kPa) 20 0 15 -10 20 10 0 30 Charge 6, 1185 mils 10 5 0 -5 -10 Amplitude (Pa-sec) 15 Pressure (kPa) Spectrum 20 10 0 20 40 60 80 time (millisecs) 102 frequency (Hz) 104 Figure 11: The acoustic pressure and spectrum for the shots in serial 1. DTA Report 406 21 Acoustic Pressure Charge 6, 378 mils 10 5 0 -5 30 Amplitude (Pa-sec) Pressure (kPa) 15 -10 10 102 104 102 104 30 Charge 6, 369 mils 10 5 0 -5 Amplitude (Pa-sec) Pressure (kPa) 20 0 15 -10 20 10 0 30 Charge 6, 491 mils 10 5 0 -5 -10 Amplitude (Pa-sec) 15 Pressure (kPa) Spectrum 20 10 0 20 40 60 80 time (millisecs) 102 frequency (Hz) 104 Figure 12: The acoustic pressure and spectrum for the shots in serial 2. DTA Report 406 22 Acoustic Pressure Charge 7, 315 mils 10 5 0 -5 30 Amplitude (Pa-sec) Pressure (kPa) 15 -10 10 102 104 102 104 30 Charge 7, 309 mils 10 5 0 -5 Amplitude (Pa-sec) Pressure (kPa) 20 0 15 -10 20 10 0 30 Charge 7, 295 mils 10 5 0 -5 -10 Amplitude (Pa-sec) 15 Pressure (kPa) Spectrum 20 10 0 20 40 60 80 time (millisecs) 102 frequency (Hz) 104 Figure 13: The acoustic pressure and spectrum for the first three shots in serial 3. DTA Report 406 23 Acoustic Pressure Charge 7, 288 mils 10 5 0 -5 30 Amplitude (Pa-sec) Pressure (kPa) 15 -10 10 102 104 102 104 30 Charge 7, 291 mils 10 5 0 -5 Amplitude (Pa-sec) Pressure (kPa) 20 0 15 -10 20 10 0 30 Charge 7, 299 mils 10 5 0 -5 -10 Amplitude (Pa-sec) 15 Pressure (kPa) Spectrum 20 10 0 20 40 60 80 time (millisecs) 102 frequency (Hz) 104 Figure 14: The acoustic pressure and spectrum for the second three shots in serial 3. DTA Report 406 24 Appendix B B.1 SOUND LEVEL METRICS Sound exposure level The sound exposure level (written SEL or LE ) is the constant sound level that has the same amount of energy in one second as a transient noise event. The SEL is a measure of the total energy in a sound. It is defined by [16] Z T 2 p SEL = LE = 10 log10 2 dt 0 p0 where p is the acoustic pressure and p0 is the reference pressure. When an A-weighting is applied to the pressure the sound exposure level is denoted by SEL(A) or LAE . B.2 Equivalent continuous sound level The quantity LAeq is known as the A-weighted equivalent continuous sound level and is defined by [16] Z T 2 1 pA LAeq = 10 log10 dt T 0 p20 where pA is the A-weighted acoustic pressure and p0 = 20 µPa is the reference pressure. This is the RMS sound level with the measurement duration used as the averaging time. B.3 Eight hour equivalent continuous sound level It is common for health and safety legislation to prescribe limitations on the total sound energy received over a working day, rather than the RMS sound level. For this purpose the A-weighted 8-hour equivalent continuous sound level was introduced. It is defined by Z T 2 pA 1 dt LAeq,8h = 10 log10 28800 0 p20 = SEL(A) − 44.6 where T is the duration of the noise in seconds and pA is the A-weighted acoustic pressure. It is related to the equivalent continuous sound level by T LAeq,8h = LAeq + 10 log10 . 28800 The LAeq,8h is also known as the daily personal noise exposure and may also be written 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 1 LN Aeq,8h = LAeq,8h + 10 log10 N = SEL1 (A) − 44.6 + 10 log10 N 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 by Murphy, 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 is equivalent to a limit on the integral of the squared A-weighted acoustic pressure of 3600 Pa2 s. The daily noise exposure level is related to the integral of the squared A-weighted pressure by Z T p2A (t) dt = 1.152 × 10−5 × 10LAeq,8h /10 . 0 Using this relation we can re-express level limits on the LAeq,8h metric into limits on the squared pressure integral. Values for some common noise exposure limits are given in Table 6. LAeq,8h (dB) Pa2 s Pa2 h 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 Pa2 s and Pa2 h. DTA Report 406 26 Appendix C THE CLASS RATING SYSTEM In the Class method hearing protectors are now assigned to one of five hearing protector classes according to their acoustic performance. The Class rating is closely related to the SLC80 which is defined by X [B(f )−A(f )]/10 SLC80 = 100 − 10 log10 10 f where the sum is over all octave band frequencies f and A = A(f ) is the assumed protective value at octave band frequency f . The values B are certain specified band levels and are given in Table 7. f (Hz) 125 250 500 1000 B (dB) 71 81 89 93 f (Hz) 2000 4000 8000 B (dB) 95 93 86 Table 7: Specified band levels B for the SLC80 calculation. A personal hearing protector should be selected on the basis of the LAeq,8h to which a worker is exposed during a working day. The classes are defined in Table 8 [18, p46]. SLC80 10–13 14–17 18–21 22–25 ≥ 26 Class 1 2 3 4 5 LAeq,8h < 90 90–95 95–100 100–105 105–110 Table 8: The relationship between the SLC80 and the Class rating. The selection 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 internal report, 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 Assessment, Measurement and Control. Oxford: Elsevier Butterworth-Heinemann, 2004. [4] E. H. Berger, “Preferred methods for measuring hearing protector attenuation,” 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 Accurately Predicts Pressure Levels in the Ear Based on Standard Performance Metrics,” Technical Report ARL-TR-6562, Army Research Laboratory, 2013. [6] R. Mlynski and E. Kozlowski, “Determining attenuation of impulse noise with an electrical equivalent of a hearing protection device,” International Journal of Occupational Safety and Ergonomics, vol. 19, no. 1, pp. 127–141, 2013. [7] E. Berger, “EARLog 14 - Protection from Infrasonic and Ultrasonic Noise Exposure,” 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 impulse peak 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 sound measurement,” Tech. Rep. PU 341/99, DTIC Document, 1999. [10] A. Dancer, K. Buck, P. Hamery, and G. Parmentier, “Hearing protection in the 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 Working Group 57, NAS-NRC committee on Hearing, Bioacoustics and Biomechanics, 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 the auditory hazard of impulse noise,” USAARL Report 86-7, 1986. DTA Report 406 28 [14] E. H. Berger, ed., The Noise Manual. American Industrial Hygiene Association, 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 Practice. CRC Press, fourth ed., 2009. [17] W. Murphy, A. Khan, and P. Shaw, “An Analysis of the Blast Overpressure Study 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, Department of Labour, 2002. DTA Report 406 29 INITIAL DISTRIBUTION No. of Copies NEW ZEALAND Director, DTA 2 Defence Library, HQ NZDF 1 Legal Deposit Office, National Library of New Zealand 2 plus a pdf WO Myron Prendergast pdf AUSTRALIA DSTO Research Library, Edinburgh 1 CANADA DRDKIM, Ottawa 1 UNITED KINGDOM DSTL Knowledge Services, Porton Down 1 DEFENCE TECHNOLOGY AGENCY Devonport Naval Base. T +64 (0)9 445 5902 Private Bag 32901. F +64 (0)9 445 5890 Devonport, Auckland www.dta.mil.nz New Zealand 0744