introduction - National Hearing Conservation Association

Impulse Peak Insertion Loss for Hearing Protection Devices Tested with an Acoustic Shock Tube
.
William J. Murphy1, Pamela S. Graydon1, Matthew Strobel1,2, Katrina Freeland1,3
1National
INTRODUCTION
Several hearing protection devices were tested for impulse peak insertion loss (IPIL)
using an acoustic shock tube according to the ANSI S12.42-2010 standard.
The
protector types included earmuffs with electronic level-limiting circuitry, standard
earplugs and earplugs with an orifice or filter that provided increasing attenuation with
impulse level. Measurements were conducted using two GRAS 45 CB test fixtures and
nominal peak impulse levels of 132, 150 and 168 dB. This poster will report the results
from tests of sixteen products tested at the NIOSH Impulse Noise Testing Laboratory.
Nonlinear earplug IPILs ranged between about 10-15 dB for 132-dB and 25-35 dB for the
168-dB impulses. Earmuff IPILs exhibited a similar range of performance. The passive
foam earplugs provided upwards of 40-55 dB IPIL.
Institute for Occupational Safety and Health, 2New York Institute of Technology, 3Gonzaga University
RESULTS
RESULTS
Earplugs in Open Condition:
One earmuff combination and two electronic earmuffs were tested in the passive
condition, with the electronics turned off. The earplug combination was the Peltor Tactical
Pro earmuff and Etymotic Research ETYPlug. The other earmuffs were the Peltor Tactical
Pro earmuff and the Peltor SoundTrap earmuff. The Passive conditions exhibited excellent
agreement between the fixtures. For the Peltor Tactical Pro and the combination of the
TacticalPro and ETYPlug at 150 dB, the WMU fixture yielded lower IPIL results than the
NIOSH fixture. Similarly, the Peltor SoundTrap earmuff was lower for the 130 dB impulses.
Measurements were conducted for open earplugs. The 3M Combat Arms earplug (CAE) version 2 and
Version 4, the Ear Blast Busters with Hocks Noise Brake, the Surefire EP-3 and EP-4 and the Etymōtic
Research ETYPlugs were evaluated. Although the ETYPlugs are not strictly an earplug with a nonlinear
valve, the acoustic filter acts in a nonlinear manner and provides more attenuation at the highest impulse
levels.
First the results are compared between fixtures with a scatter plot of the IPIL values from each fixture.
The red diagonal line indicates equality between the two fixtures. For the Open valve conditions, the two
fixtures exhibit excellent agreement. The standard deviations all intersect the diagonal line. Each
protector is depicted with different symbols and different colors.
Second, the IPIL values measured with the NIOSH and WMU fixtures are shown for each protector and
impulse level. The NIOSH IPIL values are the three bars on the left and the WMU IPIL are the three bars on
the right of each group of six bars. The standard deviations are indicated at the top of each bar as a line.
Generally we see that the open ear conditions yielded about 10 dB IPIL at 132 dB peak SPL for the CAE
version 2 and 4, Hocks and SureFire products. The IPILs for the same products were about 15 to 18 dB at
150 dB and about 30 to 35 dB at 168 dB peak impulse level. At the 168 dB impulse level, the ETYPlug had
the lowest IPIL of about 26 dB. The other protectors had IPIL values between 30 and 35 dB.
Figure 4. Comparisons of the IPIL values for the NIOSH and WMU fixtures for the Passive Earmuff Conditions. Error bars
represent 1 standard deviation. Each group of bars shows the NIOSH fixture at 132, 150 and 168 and the WMU fixture at 132
150 and 168 dB in that order.
Finally in Figure 4, the electronic earmuffs tested with the electronics turned off are
displayed. The Peltor Tactical Pro and Etymōtic Research ETYPlugs were tested twice. In
the earlier test, the ear canal had a slight tear that compromise the seal of the ETYPlugs.
Figure 1. Arrangement of two GRAS 45 CB fixtures in front of the NIOSH acoustic shock tube. Test levels achieved
with the shock tube were 132, 150 and 168 dB peak SPL as measured with the GRAS 67SB probe microphone.
METHODS
A variety of nonlinear hearing protection devices (HPDs) were tested in the NIOSH
Impulse Laboratory. Earmuffs and earplugs were selected based upon the products that
were included in the Army’s AHAAH Hearing Protection Module. A variety of muffs and
plugs with the nonlinear features (valves, filters or electronic sound restoration) were
purchased for testing.
The acoustic shock tube was used to generate impulses at 132, 150 and 168 dB peak
sound pressure level and the HPDs were fitted on the GRAS 45 CB acoustic manikins
shown in Figure 1 owned by NIOSH (left) and Western Michigan University (right). A
GRAS 67SB blast probe sampled the impulse waveform.
Six models of nonlinear earplugs, five models of electronic earmuffs and four models of
passive earplugs were selected for testing. Five samples of each model were purchased
for testing. The samples were fitted on each manikin, once for each test shot and were
cycled across the fixtures and levels. The blast probe was used to establish the average
transfer functions for 132, 150 and 168 dB impulse levels between the blast probe and
the unoccluded fixture,
Figure 2. Comparisons of the IPIL values for the NIOSH and WMU fixtures for the Open Earplug Conditions. Error bars represent 1 standard deviation.
Each group of bars shows the NIOSH fixture at 132, 150 and 168 and the WMU fixture at 132 150 and 168 dB in that order.
Earplugs In Closed Condition:
The 3M Combat Arms, SureFire EP3 and EP4 earplugs were tested with the valves closed. In addition The
3M UltraFit, and SureFire EP5 earplugs were tested. Other foam earplugs have been tested, but due to a
tear in the silicon of the GRAS 45 CB fixture in the pinnae and/or ear canals, the data are not presented. For
these earplugs, the comparison of the two fixtures exhibit agreement to within a few decibels. For
instance the UltraFit earplug provided IPIL of about 38 to 45 dB.. For the 3M Combat Arms version 4, the
IPILs from NIOSH fixture (left 3 bars) were about 2 dB less than those of the WMU fixture. The 3M Combat
Arms version 2 IPIL values were higher for the 150 dB impulses on the WMU fixture. Generally, the NIOSH
fixture exhibited IPIL values that were 1 to 2 dB less than the WMU fixture. These differences can be
attributed to the seal of the pinna portion with the ear canal because the right and left ears exhibited small
but consistent differences, that are not seen on the earmuff data.
.
SUMMARY / RECOMMENDATIONS
 Hearing protector tests with acoustic fixtures are sensitive to the seal of the protector
with the manikin head and ear canal. Minor tears in the material of the GRAS 45 CB
fixture can significantly degrade the measured IPIL values by as much as 8 decibels.
 The position of the acoustic test fixtures in the sound field of the impulse source (e.g.
shock tube, rifle or explosive charge) should be carefully measured and the arrival
time of the impulses at the manikin ears used to check the rotation of the test fixture.
The transfer function was used to estimate the unoccluded impulse level for each
occluded impulse,
.
 Attenuations for the open filter conditions exhibit remarkable similarity for the
SureFire and Combat Arms earplugs. The ETYPlug has greater IPIL for the low level
but more than 5 dB less IPIL than the other earplugs.
The estimated unoccluded waveform was then used to determine the Impulse Peak
Insertion Loss by calculating the dB ratio of the peaks in the occluded and unoccluded
conditions,
Figure 3. Comparisons of the IPIL values for the NIOSH and WMU fixtures for the Active Earmuff Conditions. Error bars
represent 1 standard deviation. Each group of bars shows the NIOSH fixture at 132, 150 and 168 and the WMU fixture at 132
150 and 168 dB in that order.
.
Figure 3. Comparisons of the IPIL values for the NIOSH and WMU fixtures for the Closed Earplug Conditions. Error bars represent 1 standard
deviation. Each group of bars shows the NIOSH fixture at 132, 150 and 168 and the WMU fixture at 132 150 and 168 dB in that order.
*The findings and conclusions in this poster are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health