Download A TTS Study

#27: Validating the Effective Attenuation of Passive Hearing Protectors: A TTS Study
Melissa A. Theis1, Hilary L. Gallagher2, & Richard L. McKinley2
[email protected], [email protected], [email protected]
1Oak
Ridge Institute for Science and Technology, 2Air Force Research Laboratory
Supported in part by the Air Force Office of Scientific Research (AFOSR)
METHODS
Measuring noise attenuation of hearing protectors in a laboratory
environment has consistently yielded higher attenuation values
than what is achieved in the field. There has been widespread
acceptance in the scientific community that the “gold standard” of
measuring hearing protection attenuation, ANSI S12.6 Methods for
Measuring the Real-Ear Attenuation of Hearing Protectors (REAT),
overestimates the level of attenuation of the protector based on the
difference between laboratory and real-world results. The human
temporary threshold shift (TTS) was measured in this study to
account for the total (air, tissue, and bone conduction pathways)
effect of noise exposure on the auditory system. The purpose of
this study was to measure the effective attenuation of passive
hearing protectors in terms of TTS for individual human subjects
with and without hearing protection and compare this response to
ANSI S12.68 estimated attenuation results. Results demonstrated
that S12.68 attenuation values, when compared to the measured
effective attenuation values using TTS, underestimated the level of
noise attenuation for passive hearing protectors.
Hypothesis: An open ear noise exposure and an equivalent occluded ear noise
exposure (under a hearing protector) will produce similar TTS2
i.e. effective noise attenuation of a hearing protector will match the predicted
ANSI S12.68 attenuation using the octave-band method.
RESULTS & DISCUSSION
Individual subject data analysis
Independent variables:
• Passive hearing protector, noise exposure (duration)
y = ‐2.33x
+ 15.49x ‐
13.16
2nd2 order
polynomial
y= -2.33x2 + 15.49x – 13.16
15
12
Open
9
Earplug
6
132 dB Ambient
3
Dependent variable:
• TTS2
0
0 2%
12.5
25
50
75
100
Noise Dose (%)
Experimental Procedures:
13 volunteer subjects (5 male, 8 female)
• Normal audiometric thresholds
Open and occluded ear noise exposures
(12.5% - 100% daily allowable dose)
• 94 dBA noise level
• 7.5 – 60 minute duration
• 2nd order polynomials were calculated from
94 dBA Open and 4 kHz Occluded TTS2 forMax TTS Response
Earplugs
18
TTS (dB)
ABSTRACT
the open ear responses to determine the
effective noise dose for occluded ear
exposures.
Using quadratic formula:
y = -2.33x2 + 15.49x - 13.16
y (TTS2 , occluded ear) = 1.5 dB
x = 2% effective noise dose
ANSI S12.68 Predicted:
Effective Dose based on TTS:
100%
2%
Noise dose comparisons per subject for each device
Comparing the Predicted to the Measured Noise Dose using TTS ‐
2 Circumaural
Noise Dose (%)
120
S12.68 Predicted Dose
Measured Effective Dose
100
80
60
40
• Graphs illustrate the differences found
between the ANSI S12.68 predicted noise dose
using the octave band method and the
measured effective noise dose for all 3 devices
20
0
PreDPOAE
(DoD Instruction 6055.12)
85 dBA – 8 hours – 3 dB per doubling exchange rate
•ANSI S12.68 describes the octave band method, the “gold
standard” for estimating effective A-weighted sound pressure levels
under hearing protectors
•The human temporary threshold shift two minutes post noise
exposure (TTS2 ) was used to accurately assess the true effect of
noise on the auditory system because it is the only method that can
account for all of the potential sound transmission pathways.
Noise
Exposure
Post-Exposure TTS2
Measurement
(4000 Hz)
PostDPOAE
120
Ambient Noise Level Computation (Occluded)
Frequency (Hz)
Noise Spectrum and Level, under HPD
A‐weighted Correction
Measured Attenuation
Noise Spectrum and Level, ambient
125
85.5
‐16
22.5
124
250
85.5
‐8.6
28.5
123
500
85.5
‐3.2
31.8
121
1000
85.5
0
41.5
127
2000
85.5
1.2
34.2
119
4000
85.5
1
40.7
125
8000
85.5 94.0
‐1.1
45.0
132 134.6
Average ambient noise levels:
Circumaural: 127 dB, Headset: 105 dB, Earplug: 130 dB
S12.68 Predicted Dose
Measured Effective Dose
100
80
•For all subjects, across devices, the octave
band method overestimated the effective noise
dose
i.e., underestimated the effective noise
attenuation
60
40
•To convert difference of effective noise dose to
a difference in decibels:
20
0
Subject ID
Comparing the Predicted to the Measured Noise Dose using TTS ‐
2 Earplug
120
S12.68 Predicted Dose
Measured Effective Dose
100
80
60
40
20
0
dB = 10*log10 (predicted dose/effective dose)
Average difference in dB for
all subjects
Circumaural = 9 dB
Headset = 10 dB
Earplugs = 11 dB
Subject ID
•The objective of this experiment was to measure the TTS to
determine the effective noise attenuation of three passive hearing
protective devices: a circumaural earmuff, an acoustically
transparent headset, and earplugs.
CONCLUSIONS AND RECOMMENDATIONS
Subject tracking hearing threshold level
according to ANSI S12.6
Circumaural: David Clark H10-13XL Headset: Sennheiser HD 457 Earplugs: Adaptive Technologies, Inc.
Subject ID
Comparing the Predicted to the Measured Noise Dose using TTS ‐
2 Headset
Noise Dose (%)
•Military personnel working in high noise environments can be
exposed to noise levels up to 150 dB.
• DoD Hearing Conservation Programs set safe noise exposure
limits to reduce risk of noise induced hearing loss (NIHL).
Attenuation
Measurement
Noise Dose (%)
INTRODUCTION
Pre-Exposure
Threshold Measurement
(4000 Hz)
Subject wearing circumaural earmuff
during noise exposure
•On average, the ANSI S12.68 Octave Band Method underestimated the effective noise
attenuation the subjects received (therefore significantly overestimating the effective dose
as estimated using TTS2)
•Additional subjects and HPDs should be examined to further substantiate these findings
•Potential explanations for the unexpected human response to occluded noise
presentations:
•
Changes in acoustic impedance seen by tympanic membrane
•
Changes in the sound transmission via bone and tissue conduction
•
Psychological / perceived protection level