ArticlePDF Available

Exploration of flat hearing protector attenuation and sound detection in noise

Authors:

Abstract

Flat-response devices are a class of hearing protectors with nearly uniform attenuation across frequency. These devices can protect the individual wearer while maintaining the spectral balance of the surrounding sounds. This is typically achieved by reducing the muffling effect of conventional hearing protectors which provide larger attenuation at higher than lower frequencies, especially with earmuffs. Flat hearing protectors are often recommended when good speech communication or sound perception is essential, especially for wearers with high-frequency hearing loss, to maintain audibility at all frequencies. However, while flat-response devices are described in some acoustical standards, the tolerance limits for the definition of flatness are largely unspecified and relatively little is known on the exact conditions when such devices can be beneficial. The purpose of this study is to gain insight into the interaction between the spectrum of the noise, the shape of the attenuation-frequency response, and the hearing loss configuration on detection thresholds using a psychoacoustic model of sound detection in noise.
Hearing Research Laboratory
Christian Giguère
Audiology/SLP Program
University of Ottawa, Canada
Elliott H. Berger
Division Scientist
3M, Indianapolis, U.S.A.
Exploration of Flat Hearing Protector
Attenuation and Sound Detection in Noise
168th Meeting ASA, Indianapolis, 27-31 October 2014
Context
Flat-response HPDs have nearly-uniform attenuation
across frequency.
They better preserve the spectral balance of sounds and
are often recommended when good speech
communication or sound perception is essential,
especially for users with high-frequency hearing loss.
Few studies are available and relatively little is known
regarding when such devices can be beneficial (Casali et
al. 2010 for a review).
Flat/uniform-attenuation HPDs are described in some
standards (e.g. EN 458; CSA Z94.2), but the definition of
“flatness” is not specified.
Context
One exception is the German criterion for railway and
road traffic workers for good “SIGNAL AUDIBILITY (GENERAL),
SPEECH INTELLIGIBILITY AND PERCEPTION OF INFORMATIVE
OPERATING SOUND(Liedkte, 2002, 2009)
Based on Zwicker’s model, the critical slope is 3.6 dB/oct.
Fulfill(NO)
(slope≥3.6dB/oct)
Fulfill(YES)
(slope<3.6dB/oct)
Methods
Purpose:
Study interaction between the noise spectrum, shape of
the HPD function and hearing loss on detection thresholds.
Design:
Two noise spectra from NIOSH 100 database
Two hearing configurations (normal and hearing loss)
Four HPD attenuation sets
Effect of slope at a fixed protected level
Effect of protected level at a fixed slope
Etymotic®Musicians Earplugs™ and 3M™ HiFi™ earplugs
German guidelines (Fulfill NO and YES)
Outcome:
Predicted detection thresholds using a psychoacoustic
model of warning sound perception in noise (Zheng et al.,
2007) based on Glasberg-Moore’s model.
Noises
NIOSH 100 Database:
NIOSH(9,8): avg. of all spectra with large LC-LA= 8-9 dB
NIOSH(3,2): avg. of all spectra with typical LC-LA= 2-3 dB
Hearing Configurations
Two hypothetical users:
Normal Hearing (NH)
Hearing loss (HLoss): Male, 55 yrs,
35 yrs @ 90 dBA, 10th percentile
Audiogram Auditoryfilters
1600Hz 3200Hz
NH
HLoss
ISO1999
HPD1
HPD2
Data Analysis
UNP
HPD1
HPD2
Threshold elevation IN NOISE (dB SPL)
HPD1
HPD2
Detection Thresholds (dB SPL) HEARINGLOSS
HPD2
HPD1
UNP
inNoise
NORMALHEARING
inQuiet
Results
Dataset 1: Selected slopes at a fixed protected level (75 dBA)
NIOSH(9,8) NIOSH(3,2)
8 dB/oct6 dB/oct0 dB/oct 4dB/oct2 dB/oct
Results
Dataset 1: Effect of slope at a fixed protected level (75 dBA)
NIOSH(9,8) NIOSH(3,2)
8 dB/oct6 dB/oct0 dB/oct 4dB/oct2 dB/oct
HEARINGLOSS
Upward spread of maskingAbsolute hearing threshold
Results
Dataset 2: Selected protected levels at a fixed slope (4 dB/oct)
NIOSH(9,8) NIOSH(3,2)
85 dBA
80 dBA75 dBA70 dBA60 dBA 65 dBA
Results
Dataset 2: Effect of protected level at fixed slope (4 dB/oct)
NIOSH(9,8) NIOSH(3,2)
HEARINGLOSS
Upward spread of maskingAbsolute hearing threshold
85 dBA
80 dBA75 dBA70 dBA60 dBA 65 dBA
Results
Dataset 3: Etymotic® Musicians Earplugs™
and 3M™ HiFi™ (ER20) earplugs
ER9 0.1
Mean
Slope
ER15 0.3
ER20 1.4
ER25 1.3
NIOSH
(9,8)
NIOSH
(3,2)
dB/oct
84
81
77
71
dBA
85
82
77
72
dBA
Results
NIOSH(9,8) NIOSH(3,2)
HEARINGLOSS
Upward spread of maskingAbsolute hearing threshold
ER25
ER20ER15ER9
Dataset 3: Etymotic® Musicians Earplugs™
and 3M™ HiFi™ (ER20) earplugs
Results
Mean
Slope
NO 4.7
YES 2.3
NIOSH
(9,8)
NIOSH
(3,2)
dB/oct
73
67
dBA
70
68
dBA
Dataset 4: German guidelines (Liedkte 2002, 2009)
Fulfill
Criterion
Results
NIOSH(9,8) NIOSH(3,2)
HEARINGLOSS
Upward spread of maskingAbsolute hearing threshold
Fulfill criterion YES NO
Dataset 4: German guidelines (Liedkte 2002, 2009)
Conclusions
Complex interaction between noise spectrum, hearing
status, HPD function and protected level on sound detection.
For normal hearing, maximum effect is 2 dB even under
“extreme” conditions.
For hearing loss, sound detection above 2500 Hz becomes
progressively more sensitive to the slope of the HPD function
and the protected level achieved.
HPD slope not exceeding 2-3 dB/oct at a protected level not
lower than 75-80 dBA needed to control effects to less than
5 dB up to 5000 Hz for moderately-severe hearing loss.
Further work to focus on quantifying benefits of flat HPDs in
more complex tasks involving speech communication, music
perception/playing, and recognition and interpretation of
important sounds to attend to in one’s environment.
... Recently, the computational approach has been used to study the impact of uniform and non-uniform hearing protectors on sound detection thresholds in noise (Giguère & Berger, 2014. It was found that hearing protectors are expected to show little effect relative to unprotected listening for NH users over a wide range of conditions, including attenuation slopes up to 8 dB/octave or protected levels as low as 60 dBA. ...
... Worker W 3 was much more sensitive to the hearing protection function and showed decrements in scores for slopes above 2 dB/octave; consequently, at a slope of 4 dB/octave (Figure 3), this worker showed deterioration relative to unprotected listening at all protected levels studied. These findings mirror those from another recent computational study (Giguère & Berger, 2014, but that one on sound detection in noise. Those predictions demonstrated little effect of slope and protected level for NH listeners, whereas for HI listeners detection thresholds above 2000-3000 Hz were shown to be sensitive to both parameters, with effects increasing as a function of the severity of the hearing loss. ...
Article
Objective: To investigate the effects of hearing protection on speech recognition in noise. Design: Computational study using a speech recognition model that was previously empirically validated. Study sample: Recognition scores were calculated in unprotected and protected conditions for four sets of hearing protector attenuation functions in two different noises, for three simulated hearing profiles illustrative of those anticipated in the noisy workplace. Results: For a normal-hearing profile, recognition scores were not sensitive to the slope of the attenuation function and the overall amount of noise reduction, but protected conditions provided a small but consistent 7-12% benefit compared to unprotected listening. For profiles simulating hearing loss, recognition scores were much more sensitive to the attenuation function. Substantial drops of 30% or more were found compared to unprotected listening in some conditions of steep attenuation slopes and large noise reductions. Attenuation functions modelled from real hearing protectors with nearly-flat attenuation yielded a benefit compared to unprotected listening for all hearing profiles studied. These findings were true in both noises. Conclusions: Limiting the slope of the hearing protector attenuation function and/or the overall amount of noise reduction is useful and warranted for workers with hearing loss to prevent adverse effects on speech recognition.
... Simulations were also carried out with two additional datasets of HPD functions, as reported in [10]. To investigate the effects of purposely-flat real products, the mean attenuation values of the Etymotic ® 0XVLFLDQV (DUSOXJVOE DQG WKH 0OE +L)LOE (DUSOXJ ZHUH FRQVLGHUHG 7KH GHYLFHV KDYH mean slopes varying from -0.3 to 1.4 dB/octave in the range from 125 to 4000 Hz and, when applied to NIOSH (9,8) and NIOSH (3,2), yield protected values in the range 71-85 dBA. ...
Conference Paper
Full-text available
Few studies document the exact conditions when flat/uniform hearing protectors can be beneficial in the noisy workplace. This modeling study reports on the interaction between the user’s hearing loss profile and the shape and amount of the attenuation function on sound detection thresholds in noise. For normal-hearing users, detection thresholds are found to be hardly affected by use of hearing protectors, even in extreme conditions of low-frequency noise and steeply sloping attenuation functions. With aging and noise-induced hearing loss, sound detection above about 2000 Hz becomes progressively more sensitive to the slope of the attenuation function as well as to the overall protected level achieved. Shallower slopes may be warranted for users with hearing loss to limit the upward spread of masking in low-frequency noise, while controlling the total amount of attenuation at high frequencies prevents excessive elevation of absolute thresholds. Decisions regarding hearing protector selection also entail consideration of the principal auditory tasks that are anticipated and the important sounds to which a worker may need to attend.
Article
Full-text available
A psychoacoustic model is presented to facilitate the installation of acoustic warning devices in noisy settings, reflecting a major upgrade of a former tool, Detectsound. The model can be used to estimate the optimal level and spectrum of acoustic warning signals based on the noise field in the workplace, the hearing status of workers, and the attenuation provided by hearing protectors. The new version can be applied to a wider range of situations. Analyses can now be conducted to meet the functional requirements for a specific worker or to suit the needs for a group of co-workers sharing a work area. Computation of optimal warning signals can also be made from estimated hearing parameters based on the worker age, gender, and level and duration of noise exposure. The results of a laboratory validation study showed that the mean error in estimating detection thresholds for normal hearing individuals is typically within +/-1 dB with a standard deviation of less than 2.5 dB in white noise or continuous noise fields. The model tends to yield slightly overestimated warning signal detection thresholds in fluctuating noises. Proper application of the tool also requires consideration of the variability in estimating noise levels, hearing status, and hearing protector attenuation under field conditions to ensure that acoustic warning signals are sufficiently loud and well adjusted in practice.
Article
Augmentations or enhancements to conventional HPDs, that is, those which attenuate noise strictly through static, passive means, are generally delineated into passive (non-electronic) and active (powered electronic) designs. While powered electronic augmentations are reviewed in Casali 1 (a parallel paper elsewhere in this issue), passive augmentations are represented by mechanical networks to achieve flat-by-frequency attenuation; level-dependent leakage pathways that house acoustically-variable occluders to yield minimal attenuation during quiet periods but sharply increasing attenuation upon intense noise bursts (such as gunfire); quarter-wave resonance ducts to bolster attenuation of specific frequencies; selectable cartridges or valves that enable passive attenuation to be adjusted for specific exposure needs; and dynamically adjustable-fit devices that provide adjustment features to enable personalized fit to the user as well as some degree of attenuation control. Intended benefits of passive augmented HPDs (akin to those of active devices as well) include (1) more natural hearing for the user, (2) improved speech communications and signal detection, (3) reduced noise-induced annoyance, (4) improved military tactics, stealth maintenance and gunfire protection, and (5) provision of protection that is tailored for the user's needs, noise exposure, and/or job requirements. This paper provides a technical overview of passive augmented HPDs that were available or have been prototyped circa early-2010. In cases where no empirical research results on the passive augmentations and their performance were available in the research literature, this review relied on patents, corporate literature, and/or the author's experience. For certain augmentations, a limited amount of empirical, operational performance research was available and it is covered herein. Finally, in view that at the juncture of this article the United States (U.S.) Environmental Protection Agency (EPA) was in the process of promulgating a comprehensive new federal law to govern the testing and labeling of hearing protectors of various types, those elements of the proposed law that pertain only to specific passive augmentation technologies are mentioned herein, 2 along with references to relevant standards on hearing protector attenuation testing.
Article
This study was undertaken in order to document, in a group of subjects affected by a noise-induced hearing loss, the relation between the loss of auditory sensitivity and the loss of frequency selectivity at mid-frequencies, namely 1000 and 3000 Hz. Auditory filter shapes were estimated using the notched noise method. Twelve notch widths were tested, six symmetrical and six asymmetrical with respect to the signal frequency; the spectral level of the noise was set at 50 dB SPL. Data were collected with 22 noise-exposed workers having different degrees of hearing loss. The findings indicate that above a certain degree of hearing loss, which seems to be around 30 dB HL, frequency selectivity tends to decrease linearly with increase in loss of sensitivity. Even when the degree of hearing loss is similar in origin and in magnitude, there is a wide variation among subjects in auditory filter bandwidth. Based on the data collected in this study, it is not possible to adequately predict the auditory filter bandwidth of an individual from hearing threshold levels.
Hearing protection devices The Noise Manual, Fifth Edition
  • E H Berger
E.H. Berger: Hearing protection devices. Chapter 10 In: E.H. Berger, L.H. Royster, J.D. Royster, D.P. Driscoll and M. Layne (eds.). The Noise Manual, Fifth Edition, American Industrial Hygiene Association, Fairfax VA, 2003, 379-453.
2-14: Hearing protection devices - Performance, selection, care, and use. Canadian Standards Association
  • Csa Z
CSA Z94.2-14: Hearing protection devices - Performance, selection, care, and use. Canadian Standards Association, 2014.
Specifying a general criterion for hearing protectors with the aim of ensuring good acoustic perception
  • M Liedkte
M. Liedkte: Specifying a general criterion for hearing protectors with the aim of ensuring good acoustic perception. International Journal of Occupational Safety and Ergonomics 15 (2009) 163-174.
68: Methods of Estimating Effective A-Weighted Sound Pressure Levels When Hearing Protectors are Worn
  • Asa S
ANSI/ASA S12.68: Methods of Estimating Effective A-Weighted Sound Pressure Levels When Hearing Protectors are Worn. Acoustical Society of America, 2007 (R2012).
Investigation of Hearing Protector Attenuation Function on Sound Detection and Speech Recognition in Noise. 40 th Annual Conference of the National Hearing Conservation Association
  • C Giguère
  • E H Berger
C. Giguère, E.H. Berger: Investigation of Hearing Protector Attenuation Function on Sound Detection and Speech Recognition in Noise. 40 th Annual Conference of the National Hearing Conservation Association, New Orleans LA, 19-21 February 2015.