Download Most Comfortable Listening Level and Speech Attenuation by

INTERNATIONAL JOURNAL OF
OCCUPATIONAL SAFETY AND ERGONOMICS 1995, VOL. 1, NO. 2, 153-159
Most Comfortable Listening Level and Speech
Attenuation by Hearing Protectors
Tomasz Letowski
Donna M. Magistro
Amy C. Ritter
Pennsylvania State University
H earing protectors atte n u ate both th e background noise and th e useful sounds e m b e d ­
ded in noise such as th e sounds o f speech and w arn in g signals. An effective hearing
p ro tecto r is o ne th a t attenu ates background noise w h ile leaving su fficien t en erg y of
speech and w arn in g signals to reach th e ear of th e w orker. A t present, h ow ever, th ere
are no established criteria fo r assessing effective ch an g e in sp eech -to-n o ise ratio
caused by hearing protection devices (HPDs). O ne such criterion could be a ch an g e in
m o st c o m fo rta b le (listening) level (M CL) fo r speech caused by th e presence of HPDs. In
this stu d y th e H P D -related shift in M C L fo r speech presented in q uiet w a s m easu red and
co m p ared w ith tw o m easures of noise attenu atio n: N oise Reduction Rating (NRR) and
h ig h -m e d iu m -lo w (H -M -L ). T h e results indicate th at the M C L shift m a y be a sensitive
m easure o f speech attenu atio n by HPDs, w hich to g e th e r w ith th e ap p ro p ria te H - M - L
m a y describe technical p ro perties of HPDs.
hearing protection
m ost c o m fo rta b le level
speech perception
1. INTRODUCTION
An im portant issue in hearing conservation is to determ ine the effects of hearing protection
on the w earers’ ability to comm unicate in noise, listen to operating machinery, and respond to
warning signals. Hearing protection devices (H PD s) attenuate both the background noise and
the signals that need to be heard at the workplace. Therefore, it is reasonable to assume that
when speech and noise have similar spectral properties, both of them should be attenuated to
the same degree. That means that the speech-to-noise (S/N) ratio outside and inside the
protector should be the same. This does not mean, however, that speech recognition in noise
will rem ain unaffected by HPD s because it depends on both the S/N ratio and the absolute
level of the noise (Pekkarinen, Viljanen, Salmivalli, & Suonpaa, 1990).
In many practical situations, the effect of HPD s on speech perception in noise is compli­
cated by the fact that speech and noise have different spectral properties. In such cases the
presence of H PD s may affect the S/N ratio in the ear of the wearer. This effect can be either
positive or negative depending on the spectral properties of speech and noise as well as on the
attenuation characteristics of the HPD.
The primary m ethod of assessing noise attenuation of H PDs is the real-ear attenuation at
threshold (R EA T) m ethod (Am erican National Standard Institute [ANSI], 1984; International
Organization for Standardization [ISO], 1990a). In this m ethod, the thresholds of hearing, or
one-third octave bands of noise centered at 125,250,500,1,000,2,000,4,000, and 8,000 Hz, are
m easured with and without HPDs. Noise attenuation of an H PD at a given frequency is calcu­
lated as the shift in the threshold of hearing caused by the presence of the HPD. In the U nited
States, commercially available HPD s have to have assigned a noise reduction rating (N R R )
Correspondence and requests for reprints should be sent to Tom asz Letow ski, D epartm ent o f Com m unication
Disorders, Pennsylvania State University, U niversity Park, PA 16802.
153
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T. LETOWSKI, D. M. MAGISTRO, AND A. C. RITTER
value indicating its weighted average noise attenuation in the 125- to 4,000-Hz range (Environ­
m ental Protection Agency, 1979; Occupational Safety and H ealth A dm inistration, 1981). Within
countries belonging to the ISO, two synthetic rating m ethods are recom mended: the signal-tonoise ratio (SNR) m ethod and the high-m edium -low (H -M -L ) m ethod (ISO, 1990b). A ccord­
ing to Lundin (1992), the H -M -L measures are better predictors of the A-weighted sound
pressure levels under the H PD than the N R R or the SNR measures. The H -M -L m easure
estim ates noise reduction by a given HPD for three classes of noises:
1.
2.
3.
Class H characterized by Lc - LA = - 2 dB,
Class M characterized by Lc - LA = 2 dB,
Class L characterized by Lc - LA = 10 dB,
where L c and LA are C-weighted and A-weighted levels of noise, respectively. In other words,
the H, M, and L values represent the mean attenuation of an H PD at high, medium, and low
frequencies.
In order to predict the effect of an H PD on speech perception in noise, it is necessary to
know the spectral characteristics of the actual speech and noise signals as well as the fre­
quency-dependent attenuation offered by the HPD. These data could be subsequently used in
calculating an index of speech attenuation provided by the HPD. Some problem s with using
this m ethod are its complexity and time-consuming character.
A faster and equally accurate approach might be to determ ine a most com fortable level
(M CL) for speech in noise with and without HPD s and to consider the difference between
these values as a m easure of the effective speech attenuation (ESA) provided by a given HPD.
A further simplification of this methodology could be the assessment of MCLs for speech in
quiet both with and without HPDs. The difference between both MCLs could be subsequent
used as a comprehensive m easure of speech attenuation by individual HPDs. This measure,
together with the H -M -L values or other comprehensive measures of noise attenuation, could
be used as an indicator of how speech com m unication may be affected by a given HPD.
The purpose of this study was to determ ine the sensitivity and reliability of the shift in
MCLs for speech in quiet caused by the presence of an HPD. This inform ation is needed for
assessing the feasibility of ESA becoming a criterion for the assessment of speech attenuation
by HPDs.
2. METHOD
2.1 Subjects
Two matching groups of subjects (G roup A and G roup B) participated in the study; each group
consisted of 10 subjects (22-34 years old) recruited from the Penn State student population.
All subjects were otologically normal, had normal bilateral thresholds of hearing (H L < dB
HL), and had no recent history of ear pathology.
2.2 Hearing Protectors
Three hearing protectors were com pared in the study: (a) a formable foam earplug
(FFE); (b) a three-flanged insert hearing protector (TFP); and (c) a hearing protection
earm uff (M UFF). All protectors were fitted under the experim enters’ control. The experimenter-assisted fit was selected over the naive subject fit to ensure high repeatability of
insertions and to com pare collected data with the REA T data obtained under similar con­
ditions by an independent certified laboratory.
D uring fitting, an FFE earplug was compressed into a narrow cylindrical shape and inserted
into the ear until flush with entrance to the ear canal. Similarly, a TFP was inserted flush with
the entrance of the ear canal. That is, the plug stem was the only item exposed from the ear
canal. Fitting the M U FF was done by positioning the muffs over the ears so that the entire
SPEECH ATTENUATION
155
Attenuation (dB)
Frequency (Hz)
Figure 1. Noise attenuation measured for FFE, TFP and MUFF HPDs using REAT procedure
(ANSI, 1984).
external ears were surrounded by the muff cushions. The muffs were held in place by a
standard headband fitted over the top of the subject’s head. The force exerted by the earmuffs
on the subject’s head was about 13.7 N. The R EA T characteristics of the three H PD s are shown
in Figure 1.
2.3 Instrumentation
All tests were conducted in an audiom etric test booth with an am bient noise level suitable for
ears open testing (ANSI, 1991). The subject was seated in the center of the booth with two
m atched custom-built loudspeaker systems placed symmetrically 1 m away from the subject’s
head. Both loudspeakers operated simultaneously and were adjusted to produce identical
sound levels at the subject’s location with no subject present. The subject was asked not to
move during testing and to hold his or her head in the same position. A continuous speech
signal (an Auditec recording of a story read by a male talker) was used as the test signal. The
speech signal was played from a Fisher CRW-50 cassette recorder through a Beltone 2000
audiom eter and a M cIntosh power amplifier and delivered to the loudspeakers. The long-term
average spectrum (LTAS) of the speech signal used in this study is shown in Figure 2. The Lc —
L a value was 4 dB.
2.4 Procedure
The m ost com fortable level (MCL) tests were conducted nine times with H PD s (H PD ) and nine
times without H PD s (NOP). A single experim ental series consisting of six test conditions consti­
tuted an experim ental series: N O P -H P D -N O P -H P D -N O P -H P D where H PD and N O P mean
conditions with and without HPDs, respectively. The series was presented once for each of the
H PD s resulting in 18 M CL determ inations. Subjects in G roup A participated in the testing of
F FE H PD s whereas, subjects in G roup B participated in the testing of TFP and M U FF HPDs. In
each test condition, M CL determ ination began at 10 dB H L (without hearing protectors) and 30
dB H L (with hearing protectors); re: pure tone average threshold. The speech signal was in­
creased in 2-dB steps until the subject indicated that the signal level was com fortable and the
spoken story easy to follow. The M CL determ ination was repeated three times, and the average
M CL was defined as the m edian value. The effective speech attenuation (ESA ) level provided
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T. LETOWSKI, D. Nl. MAGISTRO, AND A. C. RITTER
Relative Level (dB)
Figure 2.
Long-term average spectrum of th e continuous speech signal used in th e study.
by a given H PD was calculated as the difference between the MCL level obtained with the H PD
and the M CL level obtained with no hearing protector in a preceding trial.
3. RESULTS
The average M CL level obtained for 10 subjects listening to speech w ithout H PDs was 37 dB
H L (SD = 5.3 dB) in G roup A and 37 dB HL (SD = 4.7 dB) in G roup B. These values
correspond well to the 40 dB H L that is used conventionally in audiological clinics as the
presentation level in speech recognition testing. The average MCL levels m easured with FFE,
TFP, and M UFF HPD s were 67.0 (SD = 6.0 dB), 56.9 (SD = 7.8 dB), and 62.3 (SD = 9.1 dB)
dB HL, respectively. The average ESA levels (dB) obtained in each series of tests and for all
three H PD s are listed in Table 1. A repeated-m easures analysis of variance (ANOVA) on two
variables, hearing protector (H PD ) and repetition (series) with one nesting variable (group),
showed significant differences F(2,18) = 11.21, p < .01, in the ESA levels among H PD s but
no series or group effect at 0.05 level. Therefore, all ESA data have been collapsed and
averaged across all three repetitions. The collapsed data are presented in Table 2. Table 3
TABLE 1. Means and Standard Deviations of Effective
Speech A ttenuation Levels (dB) for Three Hearing
Protection Devices (HPDs) Measured in Three
Consecutive Series
Series 2
Series 1
HPD
FFE
TFP
MUFF
Series 3
M
SD
M
SD
M
SD
29.7
20.4
25.4
6.5
4.6
6.2
30.3
20.2
25.6
6.8
5.1
6.2
30.3
19.2
25.0
6.9
5.1
6.9
Note. FFE = form able foam earlplug; TFP = three-flanged insert
hearing protector; MUFF = hearing protection earmuff.
SPEECH ATTENUATION
157
TABLE 2. Means and Standard Deviations
of Effective Speech A ttenuation Levels
(dB) for Three Hearing Protection Devices
(HPDs) Tested in th e Study
Effective Speech Attenuation
HPD
FFE
TFP
MUFF
M
SD
30.1
19.9
25.3
6.5
4.7
6.3
Note. FFE = form able foam earlplug; TFP =
three-flanged insert hearing protector; MUFF =
hearing protection earmuff.
TABLE 3. Noise Reduction Rating (NRR),
L (Low), M (M edium ) and H (High)
A ttenuation Values (in dB) for Three
Hearing Protection Devices (HPDs) Tested
in th e Study.
HPD
FFE
TFP
MUFF
NRR
H
M
L
29
26
25
30
29
33
24
26
26
22
24
16
Note. FFE = form able foam earlplug; TFP =
three-flanged insert hearing protector; MUFF =
hearing protection earmuff.
contains the N R R and L -M -H values (dB) calculated from R EA T data provided by an
independent certified laboratory. The L -M -H values were calculated according to ISO (1990b)
using a 2 X standard deviation correction factor to yield similar probability of protection as
the N R R coefficient (EPA, 1979; ISO, 1990b, OSHA, 1981).
4. DISCUSSION
Several authors who com pared speech perception with and without H PD s reported better
speech recognition and detection of warning signals with H PDs in noise levels exceeding 80-85
dB (A ) (Kryter, 1946; Pekkarinen et al., 1990; Wilkins & M artin, 1979). Pollack (1957) and
Michael (1983) reported little effect of HPD s on speech perception in noise up to 100 dB(A)
levels. Similarly, Letowski and Me Gee (1993) observed little effects of H PD s on the detection
of warble-tone signals in 100 dB (A ) noise. There are also reports indicating negative effects of
H PDs on speech perception at low S/N ratios (S/N < 0 dB) occurring at high noise levels
(Baum an & M arston, 1986; Chung & Gannon, 1979). The different effect of H PD s observed in
the reported studies can, to some extent, be related to the differences in speech and noise
spectra used in those studies. In addition, HPDs seem to have negative effects on speech
perception in noise by subjects suffering from sensorineural hearing loss (Pekkarinen et al.,
1990). Inspection of the data presented in Tables 2 and 3 indicates no clear relationship
between the H -M -L values and the ESA values obtained in this study. However, an intriguing
finding of this study was a close relationship between N R R and ESA values obtained in the
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T. LETOWSKI, D. M. MAGISTRO, AND A. C. RITTER
case of FFE and M UFF HPDs. This may indicate that N R R , which is a poor predictor of
A-weighted sound pressure level under the HPD, may actually be a good predictor of ESA by
the HPD.
In contrast to the F FE and M UFF HPDs, the ESA value m easured for the TFP was
considerably less than the H -L -M and N R R value reported for this plug. This plug comes in
three sizes intended for small, medium, and large ear canals. D espite the availability of all three
sizes of the plug, 2 of our subjects were apparently misfitted because their ESA values were
much lower than the rest of the group. It seemed that these subjects had ear sizes that fell
betw een the sizes provided by the m anufacturer, or were otherwise misfitted. Even a very
skilled person cannot always determ ine ear sizes (Smith, Bordon, Patterson, Mozo, & Camp,
1980). This may be a partial explanation of the observed discrepancy between ESA and the
attenuation values.
W hen the 2 misfitted subjects were excluded from the analysis, the ESA value for the
three-flanged plug increased slightly to 22 dB (SD = 4.1 dB), but it was still less than its N R R
value. Therefore, it may be hypothesized that the observed low value of attenuation may be at
least partially due to the specific relation between the frequency response of the plug (see
Figure 1), on the one hand, and the spectrum of the speech signal (see Figure 2), on the other.
This would m ean that ESA and N R R values are not always in agreem ent. This issue requires
further studies.
Because the Lc - LA value for the speech signal spectrum used in this study was 4 dB, the
m ean value of the medium and low attenuations listed in Table 3 should be a good estim ate of
the noise attenuation provided by an HPD. These mean values are 23,25, and 21 for FFE, TFP,
and MUFF, respectively. C om pared to the corresponding ESA values, they show - 7 , 3 (value
for 8 subjects), and - 4 dB change in the theoretical S/N ratio due to the wearing of HPDs,
respectively. These are significant differences encouraging further studies of the ESA index. It
is, however, necessary to stress the fact that both the sample of subjects and the sample of
H PD s were relatively small in this pilot study. Therefore, these data should be treated only as
preliminary.
5. CONCLUSIONS
All three HPD s utilized in this study differed substantially in their ESA values, indicating the
good sensitivity of this index. Intrasubject variability was small and insignificant, whereas
intersubject variability was com parable to that observed in the R EA T studies. B oth larger and
smaller speech attenuation, in comparison to noise attenuation by HPDs, was observed and
encourages further investigation. These studies should include a greater variety of HPDs,
larger groups of subjects, and an assessment of the H PD -based M CL shift for speech in noise.
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