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Modeling the interaction between the hearing protector attenuation function and the hearing loss profile on sound detection in noise

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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.
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Modeling the interaction between the hearing
protector attenuation function and the hearing
loss profile on sound detection in noise
Christian Giguère
Audiology/SLP Program, University of Ottawa, 451 Smyth Rd., Ottawa, Canada K1H 8M5.
Elliott H. Berger
Personal Safety Division, 3M, 7911 Zionsville Rd., Indianapolis, IN 46268, U.S.A.
Summary
Few studies document the exact conditions when flat/uniform hearing protectors can be beneficial
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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.
PACS no. 43.50.Hg, 43.66.Vt
1. Introduction
1
Hearing protector devices (HPDs) with flat or
nearly uniform attenuation across frequencies
preserve the spectral balance of workplace sounds
and are often recommended when, in addition to
protection, good signal audibility, speech
communication and auditory awareness are
essential [1]. Such HPDs may be especially
indicated for users with high-frequency hearing
loss to maintain audibility at all frequencies. Still,
few studies are available on the exact conditions
when the intended benefits of flat/uniform devices
would arise [2]. Furthermore, while flat/uniform
HPDs are described in several acoustical standards
and/or national documents [3-5], the definition of
³IODWQHVV´ LV often unspecified or given broad
tolerances.
In [5], a general criterion based on the
slope of the linear regression of the mean
attenuation values between 125 and 4000 Hz is
specified. Based on prior research with road traffic
and railway workers, HPDs with an attenuation
slope less than 3.6 dB/octave are deemed to fulfill
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speech intelligibility, and the perception of
operative sounds´ In a proposed revision to
standard EN458:2004 [4], flat devices are defined
more simply as HPDs with H minus L attenuation
values less than or equal to 9 dB. In addition to
flatness characteristics, the assumed protection
value of a HPD is also an important parameter to
considerer in some standards [3,4] in order to
reduce the risks of overprotection. HPDs selected
to achieve a protected level between 5 and 10 dBA
below the national regulation level are deemed to
achieve a good protection outcome.
It is unclear if such simple guidelines for
flatness and protection level achieved can find
general use given, among other factors, the wide
range of spectral characteristics of workplace
noises and the large spread of possible hearing
profiles in the workforce. The goal of this
modeling study is to gain more insight into the
complex interaction between the hearing protector
attenuation function, the noise spectrum, and the
hearing status of the worker.
Copyright© (2015) by EAA-NAG-ABA V , ISSN 2226-5147
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2.4. Simulation and data analysis
Pure-tone masked detection thresholds from 125 to
8000 Hz were computed in the two noises and for
the three hypothetical workers using the
algorithms specified in the psychoacoustic model
in [6]. This was carried out both for unprotected
listening and when protected according to the
attenuation functions from the two HPD datasets.
Unprotected detection thresholds were then
subtracted from protected detection thresholds to
highlight cases of threshold elevation from the use
of HPDs. Thus, the unprotected detection
thresholds served as a baseline to quantify the
effects of hearing protection for each individual
worker and noise. Such a threshold elevation can
occur due to an overly large loss of hearing
sensitivity from the combined effects of the
absolute hearing thresholds and the amount of
HPD attenuation at one or more frequencies
(referred to here as Case 1 elevation) and/or due to
an increase in the upward spread of masking from
the interaction between the noise spectrum, the
broadening of the auditory filters and the shape of
the HPD function (Case 2 elevation).
3. Results
Figure 4 shows the detection threshold elevation
(dB) arising from the attenuation functions of the
HPD Dataset 1, separately for the three workers.
The results are for NIOSH (9,8), the noise with a
large LC-LA. For Worker 1, the maximum
threshold elevation was less than 2 dB across the
entire range of frequencies and for all HPD
attenuation slopes simulated. The maximum effect,
occurring at 400 Hz, is related to a peculiarity in
the NIOSH (9,8) spectrum which shows a sudden
drop of noise energy near that frequency. This, in
turn, led to some upward spread of masking from
the lower frequency energy that passed more easily
through the HPDs with the largest slopes of
increasing attenuation with frequency (4-8
dB/octave).
For Worker 2, there were also minimal
effects over the entire range of frequencies, but
only up to a slope of about 4 dB/octave (Figure 4).
At steeper slopes of 6 and 8 dB/octave, the
detection thresholds progressively increased over
unprotected thresholds for frequencies above
3000-4000 Hz. This was almost equally the result
of Case 1 and Case 2 threshold elevation effects.
For Worker 3, only the flat attenuation function of
0 dB/octave produced minimal effects over the
entire range of frequencies. HPD Functions with
slopes of 2 dB/octave or greater progressively
incurred higher threshold elevations above 2000
Hz. Case 2 threshold elevation was the controlling
factor for the two largest slopes.
Results for the NIOSH (3,2) noise with the
HPD Dataset 1 (not shown) were similar to
NIOSH (9,8), except that the lower LC-LA value
and the less rich low frequency spectrum (Figure
1) produced somewhat less upward spread of
masking. There were no threshold elevation effects
at 400 Hz for any of the three workers with this
noise.
Figure 5 shows the detection threshold
elevation (dB) arising from the attenuation
functions of the HPD Dataset 2 applied to noise
Figure 2. HPD Dataset 1: Attenuation functions for
various slopes (dB/octave) at a fixed protected level of
75 dBA in NIOSH (9,8).
Figure 3. HPD Dataset 2: Attenuation functions for
various protected levels (dBA) at a fixed slope of 4
dB/octave in NIOSH (9,8).
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1970
4. Discussion and conclusions
This computational study highlights the complex
nature of the interaction between the hearing loss
profile of the user, the HPD attenuation function
on sound detection in noise, and to some extent the
noise spectrum.
For normal-hearing users, the maximum
effect was less than 2 dB even under the most
extreme condition investigated of a low-frequency
noise (large LC-LA value between 8-9 dB) and use
of a HPD with a very steep attenuation curve (8
dB/octave). Furthermore, sound detection in noise
appears quite insensitive to the overall protected
level achieved for normal-hearing users, down to
at least 60 dBA. These results indicate that, for
sound detection in noise, there may be little value
in placing restrictions on attenuation slope and
protected level achieved for this class of user.
Such a finding corroborates earlier reports that
auditory perception in noise is hardly affected by
use of HPDs for normal-hearing users [1].
By contrast, for users with hearing loss,
detection thresholds in some noise situations may
be affected by use of hearing protectors with
steeply sloped attenuation functions and/or
excessive attenuation. The negative effect may be
due to elevated absolute hearing thresholds arising
from a too-large HPD attenuation in the high
frequencies in conjunction with the hearing loss
(Case 1 elevation) and/or from an upward spread
of masking due to steep HPD attenuation functions
in the presence of either broadened auditory filters
or low-frequency noise (Case 2 elevation).
From Figures 4 and 5, the effects of HPDs
on sound detection in noise are found to be quite
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signal frequency. This finding presents a special
challenge when setting general guidelines for HPD
selection applicable to all users, as shown below.
Assuming the maximum detection threshold
elevation in noise to be not larger than 5 dB at
4000 Hz, to minimize adverse effects on the
speech frequency range for example, then a fairly
wide prescription consisting of an attenuation
slope of no more than 6-7 dB/octave and a
protected level not lower than 60-65 dBA may be
suitable for a user with mild hearing loss (W2).
Using the same criterion, a more stringent
prescription consisting of a slope not exceeding 3-
4 dB/octave and a protected level not less than 75
dBA may be required for a user with moderate-
severe high frequency hearing loss (W3). On the
other hand, if good sound detection is required at
frequencies greater than 4000 Hz, more stringent
requirements may be needed for HPD selection. It
is also important to note that workers W2 and W3
specified in this study are only two of a myriad of
possible case studies. In general, there are much
wider variations in hearing profiles across users
than variations in attenuation across available HPD
products, which greatly compounds to the problem
of setting general selection guidelines.
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
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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. Not surprisingly,
these products led to reduced threshold-elevation
effects that were comparable to the shallowest
slopes investigated in Figure 4. Even for the
worker with the most hearing loss (W3), the
maximum predicted threshold elevation was 5 dB
or less up to 5000 Hz for all products in both
NIOSH (9,8) and NIOSH (3,2) noises. Simulations
were also carried out using the average attenuation
curves reported in [5] for groups of real hearing
protectors fulfilling and not fulfilling the German
criterion of a mean slope of less than 3.6
dB/octave. Note that these curves were found to
possess mean slopes of about 2.3 dB/octave
(fulfill) and 4.7 dB/octave (not fulfill). For the
normal-hearing worker (W1), the maximum
predicted threshold elevation was less than 2 dB
up to 8000 Hz for both groups of protectors in
either noises, similar to results for Datasets 1 and 2
(Figures 4-5), and the difference between the two
groups was at most 1 dB. For worker 3, the
threshold elevation due to the upward spread of
masking (Case 2) was consistent with that
presented in Figure 4 for each group of protectors
at comparable slopes, and thus favored the group
fulfilling the criterion. However, the controlling
factor for the threshold elevation was the loss of
hearing sensitivity (Case 1) and, owing to the
nearly equal amounts of attenuation they provide
at mid-to-high frequencies, both groups of
protectors led to the same outcome above 3000 Hz.
In practice, decisions regarding HPD
selection for hearing-impaired users will entail
some knowledge about the principal auditory tasks
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to carry out in the given workplace. Proper
consideration of the characteristics of acoustic
signals to attend to is also important. With regards
to the perception and design of audible danger
signals, for example, ISO 7731:2003 [11] specifies
that ³in the case of persons wearing hearing
protection or having a hearing loss, sufficient
signal energy should be present in the frequency
range below 1500 Hz.´ Such a recommendation is
clearly in line with the results of this
computational study. Below 2000 Hz, sound
detection in noise was found to be quite insensitive
to the HPD attenuation function over a wide range
of noise conditions and types of users from normal
hearing to moderate-severe hearing losses due to
aging and noise (Figures 4 and 5). Thus, warning
signal perception may be little affected by HPDs
for a wide range of users when proper
consideration is first given to the design of
warning sounds in the workplace. On the other
hand, there are many instances of incidental
sounds over which we have little or no control,
such a malfunctioning machine or parts falling
from a conveyor belt [1]. In these cases, hearing-
impaired users may benefit from shallower
attenuation slopes and reduced amounts of
attenuation, especially in cases where such sounds
are likely to generate mostly high-frequency
acoustic energy.
Finally, it is important to note that the
present study assumed continuous noises in the 95-
96 dBA range. In practice, workplace noises may
fluctuate in level over the course of the day. In
quieter periods, the effect of HPDs may be more
pronounced. Also, due to medical conditions and
other reasons, some workers may have hearing loss
configurations that depart from the typical profiles
due to aging and noise-induced hearing loss
investigated in the present study. Sound detection
in noise is also only one of many possible auditory
dimensions where flat/uniform hearing protectors
may have an impact. A recent computational study
also showed that speech perception may also be
little affected by the HPD attenuation function
over a wide range of situations, except for users
with a substantial amount of hearing loss [12].
Still, flat/uniform hearing protectors preserve the
spectral balance of sounds and they may provide
substantial benefits in terms of user acceptance
resulting from improved sound quality and
auditory situational awareness such as the
recognition, interpretation, or in the case of
entertainment-sounds such as music, the
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environment. Further work is needed in this area.
References
[1] 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] J.G. Casali: Passive augmentations in hearing
protection technology circa 2010 including flat-
attenuatin, passive level-dependent, passive wave
resonance, passive adjustable attenuation, adjustable-
fit devices: Review of design, testing and research.
International Journal of Sound and Vibration 15
(2010) 187-195.
[3] CSA Z94.2-14: Hearing protection devices -
Performance, selection, care, and use. Canadian
Standards Association, 2014.
[4] EN 458: Hearing protectors - Recommendations for
selection, use, care and maintenance ± Guidance
document. European Standard, 2004.
[5] 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.
[6] Y. Zheng, C. Giguère, C. Laroche, C. Sabourin, A.
Gagné, M. Elyea: A psychoacoustic model for spec-
ifying the level and spectrum of acoustic warning
signals in the workplace. Journal of Occupational and
Environmental Hygiene 4 (2007) 87-98.
[7] ISO 1999: Acoustics ± Estimation of noise-induced
hearing loss. International Organization for
Standardization, 2013.
[8] C. Laroche, B. Josserand, H. Tran Quoc, R. Hétu, B.R.
Glasberg: Frequency selectivity in workers with noise-
induced hearing loss. Hearing Research 64 (1992) 61-
72.
[9] ANSI/ASA S12.68: Methods of Estimating Effective
A-Weighted Sound Pressure Levels When Hearing
Protectors are Worn. Acoustical Society of America,
2007 (R2012).
[10] C. Giguère, E.H. Berger: Exploration of flat hearing
protector attenuation and sound detection in noise.
168th Meeting of the Acoustical Society of America,
Indianapolis IN, 27-31 October 2014.
[11] ISO 7731: Ergonomics ² Danger signals for public
and work areas ² Auditory danger signals.
International Organization for Standardization, 2003.
[12] C. Giguère, E.H. Berger: Investigation of Hearing
Protector Attenuation Function on Sound Detection
and Speech Recognition in Noise. 40th Annual
Conference of the National Hearing Conservation
Association, New Orleans LA, 19-21 February 2015.
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... Si cela se produit, un signal qui était audible sans PICB ne le sera plus avec le PICB et on parle alors de surprotection. Suivant la terminologie proposée par Giguère & Berger (Giguère et al., 2015), on parle dans ce cas de dégradation de type 1. Ce sont principalement les auditeurs ayant des pertes auditives très importantes qui risqueraient de subir cette détérioration. Comme indiqué précédemment, pour de faibles pertes auditives (ou leur absence), les niveaux auxquels il pourrait y avoir une surprotection ne nécessitent pas l'utilisation de PICB (la parole par exemple). ...
... Il s'agirait finalement de l'effet inverse de celui qui permettrait une amélioration chez les normo-entendants. Suivant à nouveau la terminologie proposée par Giguère & Berger (Giguère et al., 2015), on parlera dans ce cas d'une dégradation de type 2. ...
... Une autre approche est proposée par Giguère & Berger (Giguère et al., 2015) pour évaluer l'impact du port de PICB sur la détectabilité. Plus précisément, les auteurs cherchent à évaluer l'interaction entre les profils d'atténuation de PICB et les atteintes auditives. ...
Thesis
Full-text available
Il est difficile de prévoir l’effet de protections auditives sur la capacité d’un salarié à détecter une alarme dans un bruit de fond. Parce que sa capacité de détection sera influencée par les spectres des bruits et alarmes, son statut auditif ainsi que la fonction d’atténuation du protecteur utilisé, il demeure compliqué de prévoir cet effet sans avoir recours à des tests subjectifs de perception. Ces tests sont compliqués à mettre en œuvre, en particulier à cause de la difficulté à recruter une population de participants qui représente la variété des statuts auditifs que l’on trouve dans le monde du travail. Dans le contexte de la sécurité au travail, les travaux présentés évaluent d’abord les effets de deux protecteurs auditifs en ayant recours à la méthode classique des tests subjectifs. Ensuite, on propose et évalue deux méthodes alternatives pour évaluer plus facilement et plus rapidement l’effet du port de protection auditive sur la détection dans le bruit, en tenant compte du statut auditif. La première méthode propose de modifier les signaux sonores utilisés lors des tests afin qu’ils soient perçus par des auditeurs normo-entendants comme s’ils étaient malentendants. Le temps de recrutement de réalisation des tests serait alors considérablement réduit. La seconde méthode consiste en un modèle développé pour prédire la détection dans le bruit selon le statut auditif. Les résultats de ces méthodes sont comparés à ceux obtenus par la méthode classique. Malgré certaines limitations, les deux méthodes présentent des résultats satisfaisants, ce qui encourage à perfectionner leur développement. En particulier, le modèle peut être utilisé en l’état pour facilement réaliser une estimation de l’effet du port de protecteur auditif sur l’audibilité des alarmes
... Also, the results revealed great variability in the percentages of correct responses, as well as in the available studies, indicating the interference of various variables. When wearing an HPD, the variation found in the different studies ranged from 6 to 37% [26,[43][44][45], whereas when not wearing an HPD, the variation in the percentage of correct responses ranged from 7 to 21% [43,45]. ...
... Also, the results revealed great variability in the percentages of correct responses, as well as in the available studies, indicating the interference of various variables. When wearing an HPD, the variation found in the different studies ranged from 6 to 37% [26,[43][44][45], whereas when not wearing an HPD, the variation in the percentage of correct responses ranged from 7 to 21% [43,45]. ...
Article
Purpose: To compare speech intelligibility in noise with and without hearing protection device (HPD). Materials and Methods: 51 workers were distributed into three groups: noise-induced hearing loss (NIHLG), normally hearing noise-exposed (NG), and normally hearing nonexposed to noise (CG). A free field system was used to emit monosyllables (65, 70, 75 dB) and pink noise in different signal-to-noise ratios (SNR) (0, -5, -10, -15). Results: In situations with HPD, all groups had a decrease in the percentage of correct responses with an increase in noise level. It was observed that the HPD had little effect on speech intelligibility in the NIHLG and NG. Considering the effect caused by the HPD on speech intelligibility, it was observed that the group that had the greatest loss was the CG in the SNR of -5, -10, and -15. Conclusion: Although the speech intelligibility is influenced by the hearing threshold, the noise level and SNR are crucial to a good speech intelligibility, either with or without an HPD. It is highlighted that the NG had worse results when compared with the CG, which may indicate changes in the auditory pathway resulting from continuous noise exposure, even in the absence of changes in the audiometric thresholds.
... Likewise, Giguère and Berger [35] show that the sound detection thresholds in noise for normal-hearing individuals is hardly affected by use of HPDs, even for a low-frequency ambient noise. These conclusions differ in the case of an individual with hearing loss (as discussed in section 3.2.4). ...
... According to the section 3.2.4, the effect of this variability could impact the evaluation of the acoustical comfort attributes, mostly for hearing-impaired individuals for which the situational awareness is more impacted by the HPD attenuation characteristics (global attenuation and attenuation slope) [34,35]. HPD attenuation should thus ideally be quantified in comfort studies, e.g. using FAES systems according to [77]. ...
Article
Structured abstract:Objective: This paper proposes a comprehensive literature review of past works addressing Hearing Protection Devices (HPDs) comfort with the aim of identifying the main sources of variability in comfort evaluation. Design: Literature review. Study samples: Documents were hand searched and Internet searched using 'PubMed', 'Web of Science', 'Google Scholar', 'ProQuest Dissertations and Theses Professional', 'Scopus' or 'Google' search engines. While comfort constructs and measurement methods are reviewed for both earplugs and earmuff HPD types, results and analyses are provided for earplugs only. Results: The literature shows that the multiple sources of the perceived comfort measurement variability are related to the complexity of the concept of comfort and to the various physical and psychosocial characteristics of the triad 'Environment/Person/Earplug' which differ from one study to the other. Conclusions: Considering the current state of knowledge and in order to decrease comfort measurements variability, it is advised to (i) use a multidimensional construct of comfort and derive a comfort index for each comfort dimensions, (ii) use exhaustive and valid questionnaires, (iii) quantify as much as triad characteristics as possible and use them as independent or control variables, and (iv) assess the quality of the earplug fitting and the attenuation efficiency.
... Likewise, Giguère and Berger [35] show that the sound detection thresholds in noise for normal-hearing individuals is hardly affected by use of HPDs, even for a low-frequency ambient noise. These conclusions differ in the case of an individual with hearing loss (as discussed in section 3.2.4). ...
... According to the section 3.2.4, the effect of this variability could impact the evaluation of the acoustical comfort attributes, mostly for hearing-impaired individuals for which the situational awareness is more impacted by the HPD attenuation characteristics (global attenuation and attenuation slope) [34,35]. HPD attenuation should thus ideally be quantified in comfort studies, e.g. using FAES systems according to [77]. ...
Article
Objective: This article presents a comprehensive literature review of past works addressing Hearing Protection Devices (HPD) comfort and to put them into perspective regarding a proposed holistic multidimensional construct of HPD comfort. Design: Literature review. Study samples: Documents were hand searched and Internet searched using “PubMed”, “Web of Science”, “Google Scholar”, “ProQuest Dissertations and Theses Professional”, “Scopus” or “Google” search engines. While comfort constructs and measurement methods are reviewed for both earplugs and earmuff types, results and analyses are provided for the earplug type only. Results: This article proposed a multidimensional construct of HPD comfort based on four dimensions: physical, functional, acoustical and psychological. Seen through the prism of the proposed holistic construct of HPD comfort, the main comfort attributes of earplugs have been identified for each comfort dimension. Conclusions: The observed lack of consensus on the definition of HPD comfort in the scientific community makes it difficult to prioritise the importance of comfort attributes yet necessary for future design of comfortable earplugs.
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The influence of wearing hearing protectors on the detection of seven railway warning signals in noise was evaluated by comparisons of the masked thresholds measured with and without hearing protectors, out of a total of 80 listeners. The results show that wearing hearing protection devices (HPDs) improves the audibility for normal hearing listeners whereas it tends to impede the audibility for hearing impaired listeners. Moreover, the impediments greatly depend on the warning signal acoustical characteristics. Statistical analyses were performed in order to propose a criterion for hearing impaired listeners that guarantees their security when wearing hearing protectors. If we do not consider one given high-pitched signal that is not suitable as a warning signal, the conclusion is that security is assured when the average absolute hearing threshold (average at 500, 1000 and 2000 Hz for the best ear) of the listeners remains lower than a hearing level of 30 dB.
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The effect of wearing hearing protectors on the audibility of warning signals has been evaluated for three specific railway-related jobs: track workers, train drivers and platform agents. Masked thresholds were measured in the laboratory, on railway agents having normal hearing, by using warning signals and background noises typical of each job. Out of the 36 situations tested in total, statistical analyses showed that wearing the earplugs improves the perception in 11 situations; deteriorates the perception in 10 situations and has no significant effect in 15 situations (as compared to no hearing protector). The deteriorations essentially concern the signals which have no (or not enough) energy in the low-frequency range (f<1500 Hz) when they have to be heard in background noises which dominate in the low frequency range. To prevent the deteriorations, these signals could be modified by adding some energy in the low-frequency range (f<1500 Hz).
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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.
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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.
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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.
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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.
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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
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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.
Specifying a general criterion for hearing protectors with the aim of ensuring good acoustic perception
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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.
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ISO 1999: Acoustics ± Estimation of noise-induced hearing loss. International Organization for Standardization, 2013.
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ISO 7731: Ergonomics ² Danger signals for public and work areas ² Auditory danger signals. International Organization for Standardization, 2003.
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.