Download Blast Noise of the L119 Light Gun

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
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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