Download The effects of single and double hearing protection on the

The effects of single and double hearing protection on the
localization and segregation of spatially-separated speech signals
(L)
Douglas S. Brungart
Air Force Research Laboratory, WPAFB, Ohio
Alexander J. Kordik
Sytronics, Dayton, Ohio
Brian D. Simpson
Air Force Research Laboratory, WPAFB, Ohio
共Received 25 December 2003; revised 21 May 2004; accepted 9 July 2004兲
Recent results have shown that auditory localization in the horizontal plane is dramatically worse for
listeners wearing double hearing protection 共earplugs and earmuffs兲 than it is for listeners wearing
single hearing protection 共earplugs or earmuffs alone兲. This suggests that double hearing protection
might also impair the spatial unmasking that normally occurs when two simultaneous talkers are
spatially separated in azimuth 共the so-called ‘‘cocktail party’’ effect兲. In this experiment, normal
hearing listeners wearing no hearing protection, single hearing protection 共earmuffs兲, or double
hearing protection were asked to perform a speech intelligibility task that required them to segregate
two simultaneous talkers who were either presented from the same loudspeaker or spatially
separated by 90° in azimuth. The listeners were also asked to determine the location of the target
talker in each trial. The results show that the listeners were unable to reliably determine the location
of the target talker when they wore double hearing protection, but that they were still able to benefit
from the spatial separation of the competing talkers. This suggests that the use of double hearing
protection causes spatially separated sound sources to be heard at locations that are inaccurate but
still distinct enough to enhance the segregation of speech.
关DOI: 10.1121/1.1786812兴
PACS numbers: 43.50.Hg, 43.66.Vt, 43.66.Qp 关AR兴
I. INTRODUCTION
Hearing protection devices play an essential role in preventing long-term occupational hearing loss, but they can
also impair the localization cues that listeners use to process
and analyze surrounding sound sources. This is especially
true in environments with noise levels that exceed 100 dBA,
where double hearing protection 共a combination of earplugs
and earmuffs兲 should be worn to avoid the possibility of
permanent hearing loss 共NIOSH, 1998兲. Recent results from
our laboratory have shown that listeners wearing double
hearing protection are much worse at sound localization than
those wearing earplugs alone or earmuffs alone. Earplugs
and earmuffs have long been known to impair the spectral
cues used for the localization of brief sounds in the horizontal plane 共Abel and Hay, 1996; Abel and Armstrong, 1993;
Vause and Grantham, 1999; Bolia, D’Angelo, Mishler, and
Morris, 2001兲, but most studies have shown that listeners
wearing single hearing protection can adequately localize
sound sources in azimuth when they are on long enough to
facilitate the use of exploratory head movements 共Noble,
Murray, and Waugh, 1990兲. However, two recent studies in
our laboratory have shown that listeners wearing double
hearing protection are reduced to near chance localization
performance in the horizontal plane even with continous
sound sources that allow them to make unrestricted head
movements 共Brungart, Kordik, Simpson, and McKinley,
2003; Simpson, Bolia, McKinley, and Brungart, 2002兲.
J. Acoust. Soc. Am. 116 (4), Pt. 1, October 2004
Pages: 1897–1900
One possible explanation for the dramatic decrease in
localization performance that occurs when double hearing
protection is worn is that the interaural localization cues that
are usually present in audio signals that enter the ears
through the ear canals are being contaminated by bone- and
tissue-conducted sounds that enter the listener’s ears directly
through the surface of the head. This kind of bone conduction is already known to place a limit on the amount of
attenuation that can be achieved with conventional earplug
and earmuff hearing protection devices 共Zwislocki, 1957;
Berger, Kieper, and Gauger, 2003兲. Once these devices attenuate the direct sound entering the ear canal below the
level of the sound reaching the cochlea through bone or tissue conduction, additional attenuation no longer has any effect on the perceived level of the sound or on the long-term
damage it can cause to the listener’s hearing. Researchers
who have estimated the bone-conduction limit by measuring
sound detection thresholds with increasingly effective earplug and earmuff combinations have found that the boneconduction limit on conventional hearing protection ranges
from roughly 39 dB of attenuation at 2 kHz to roughly 55 dB
of attenuation at 500 Hz 共Berger, 1983; Berger et al., 2003兲.
Although bone-conduction pathways have traditionally
been examined in the context of the limitations they impose
on the effective attenuation of hearing protection devices, it
is likely that these flanking pathways also influence how well
listeners are able to localize sound sources while wearing
hearing protection. Sound localization in the horizontal plane
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depends primarily on direction-dependent changes in interaural time and intensity differences caused by the diffraction
of air-conducted soundwaves around the listener’s head and
torso. These interaural difference cues would be badly corrupted in a bone-conducted sound, which would propagate
directly through the head at a much higher rate than a normal
air conducted signal. Consequently, it may not be surprising
that a listener wearing double hearing protection that reaches
the bone-conduction attenuation limit at frequencies at or
above 1 kHz would perform much worse in a sound localization task than a listener wearing single hearing protection
that does not approach the bone-conduction limit at any frequency. Indeed, similar reductions in localization ability
have been reported for listeners with severe conductive hearing losses who experience the same kind of increase in the
level of bone-conducted sound relative to the level of airconducted sound as listeners wearing double hearing protection 共Zurek, 1986; Noble, Byrne, and LePage, 1994兲.
Although a reduction in sound localization ability is
clearly one important consequence of the use of double hearing protection, it may not be the only consequence. The degradation in sound localization that occurs when double hearing protection is worn may also impact the processing of
complex auditory scenes with multiple simultaneous sound
sources. In normal listening environments, the ability to segregate and process multiple simultaneous sound sources is
greatly enhanced when those sound sources are spatially
separated. The classic example of this is the dramatic increase in speech intelligibility that occurs when a target
speech signal is spatially separated from a masking speech
signal, often referred to as the ‘‘cocktail party’’ effect
共Cherry, 1953兲. To the extent that double hearing protection
disrupts the interaural localization cues that listeners use to
segregate spatially separated sounds, one might expect the
benefits of spatial separation inherent in the cocktail party
effect to be greatly reduced in listeners wearing double hearing protection. Indeed, Zurek 共1986兲 has speculated that a
similar reduction in spatial unmasking should occur in listeners with severe conductive hearing losses. If this kind of
reduction in complex sound processing ability does occur in
listeners wearing double hearing protection, then steps
should be taken to alert the users of double hearing protection to this problem and to eliminate situations where a lack
of spatial segregation ability could pose a safety hazard in
high noise environments. The purpose of this study was to
examine the effect that double hearing protection has on the
segregation of multiple simultaneous speech sources.
II. METHODS
A. Subjects
Six volunteer subjects, five males and one female, were
paid to participate in the experiment. All had normal hearing
共⬍15 dB HL from 500 Hz to 8 kHz兲, and their ages ranged
from 21 to 24 years.
B. Hearing protection devices
Three hearing protection 共HP兲 conditions were tested in
the experiment: No HP, where no hearing protection device
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was used; Single HP, where the subjects wore earmuffs
共Tasco Soundshield兲; and Double HP, where the subjects
wore fully inserted foam earplugs 共E•A•R Classic ®兲 under
earmuffs 共TASCO Soundshield兲. In conditions where HP devices were used, they were fitted under the supervision of an
experimenter. Prior to this study, the real ear attenuation at
threshold method was used to determine the attenuation characteristics of the Single HP and Double HP hearing protection devices for each of the six participants in this experiment. These attenuation results, which are reported in detail
elsewhere 共Brungart et al., 2003兲, indicate that the combination of earplugs and earmuffs used in the Double HP condition of this experiment reach the bone-conduction limit of
attenuation for frequencies at or above 1 kHz, while the earmuffs used in the Single HP condition produce less attenuation than the bone conduction limit at all frequencies.
C. Apparatus
The experiment was conducted in the Air Force Research Laboratory’s Auditory Localization Facility 共ALF兲, a
large anechoic chamber housing an aluminum-frame geodesic sphere with small loudspeakers mounted at each of its
272 vertices 共Brungart et al., 2003兲. These 272 loudspeakers
were not capable of producing sufficiently intense auditory
stimuli for the Double HP condition of this experiment. Consequently, the ALF was modified by mounting two large
powered loudspeakers with twin 8-in. woofers 共Barbetta
Sona兲 at ⫾45° azimuth in the horizontal plane 1.4 m away
from the subject, who was positioned on a platform in the
center of the sphere. These powered loudspeakers were used
to generate all the stimuli used in the experiment. In order to
allow the subjects to use a computer mouse to make their
responses, a 14 in. color cathode-ray tube was also mounted
outside the sphere between the two loudspeaker locations
directly in front of the subject.
D. Threshold measurement
At the start of each block of trials, the subjects were
fitted with the appropriate level of hearing protection and
asked to stand in the middle of the sphere facing the midpoint between the two loudspeakers. Then they participated
in a two-down, one-up adaptive threshold procedure 共Levitt,
1971兲 to estimate their hearing threshold level with that hearing protection device. In each trial of this two-interval
forced-choice procedure, light emitting diodes located over
the left and right buttons of a two-button response box were
used to visually cue two consecutive 250-ms observation intervals, separated by a pause of 100 ms. A 250-ms burst of
pink noise1 was presented in one of these two observation
intervals, and the subject’s task was to determine which interval contained the noise burst. Two consecutive correct responses resulted in a 3 dB decrease in the noise level, and
each incorrect response resulted in a 3 dB increase in the
noise level. This procedure was repeated until ten reversals
occurred, and the last five reversals were averaged to estimate the detection threshold for that particular subject with
that hearing protection device. The mean detection thresholds measured with this procedure were roughly ⫺3 dB
Brungart et al.: Letters to the Editor
FIG. 1. The left panel shows performance in the left-right discrimination
task, where the subjects had to identify which speaker the target phrase
originated from. The right panel shows the percentage of correct color and
number identifications in each condition of the experiment. The error bars
represent the 95% confidence intervals calculated from all the raw data at
each data point.
sound pressure level 共SPL兲 in the No HP condition, 22 dB
SPL in the Single HP condition, and 37 dB SPL in the
Double HP condition.2
E. Procedure
Once the threshold measurement was complete, data
collection started in the speech intelligibility test. This test
was based on the coordinate response measure 共CRM兲, a
call-sign-based intelligibility test consisting of target and
masking phrases of the form ‘‘Ready 共call sign兲 go to 共color兲
共number兲 now.’’ The phrases themselves were drawn from
the publicly-available CRM corpus 共Bolia, Nelson, Ericson,
and Simpson, 2000兲, which contains CRM phrases for all
combinations of eight call signs 共‘‘Arrow,’’ ‘‘Baron,’’ ‘‘Charlie,’’ ‘‘Eagle,’’ ‘‘Hopper,’’ ‘‘Laker,’’ ‘‘Ringo,’’ ‘‘Tiger’’兲, four
colors 共‘‘blue,’’ ‘‘green,’’ ‘‘red,’’ ‘‘white’’兲, and eight numbers 共1– 8兲 spoken by four male and four female talkers. In
this experiment, the stimulus always consisted of two simultaneous CRM phrases, a target phrase randomly selected
from all the phrases containing the call sign ‘‘Baron,’’ and a
masking phrase randomly selected from all the phrases spoken by a different same-sex talker with a different call sign,
color, and number than the target phrase. These phrases were
adjusted to set their rms power levels 30 dB higher than the
hearing threshold measured at the start of the block, and
presented either from the same randomly selected loudspeaker 共‘‘nonspatial trials’’兲 or from different randomly selected loudspeakers 共‘‘spatial trials’’兲. In each case, the subject’s task was to first identify the color and number
combination contained in the phrase containing ‘‘Baron,’’
and then to determine whether the target phrase came from
the left or right loudspeaker. The subjects were permitted to
move their heads during this procedure, but they were not
specifically encouraged to do so. Each block consisted of 15
spatial and 15 nonspatial trials, and each subject participated
in four blocks of trials with each of the three hearing protection levels, with the order of the blocks randomized across
the different subjects.
III. RESULTS
The results of the experiment are shown in Fig. 1. The
left panel of the figure shows performance in the left-right
J. Acoust. Soc. Am., Vol. 116, No. 4, Pt. 1, October 2004
discrimination task of the experiment. These results show
that left-right discrimination was near 100% correct in both
the No HP and Single HP conditions, but that it fell to
roughly 60% correct in the Double HP condition.3 A twofactor within-subjects analysis of variance 共ANOVA兲 on the
arcsine-transformed data from each individual subject confirmed that this drop in performance was significant (F (2,10)
⫽27.95, p⬍0.0001). This finding is consistent with our earlier experiment that showed that listeners wearing double
hearing protection are unable to reliably localize sound
sources in the horizontal plane 共Brungart et al., 2003兲. Note
that spatial separation of the talkers had no effect on leftright discrimination in any of the hearing protection conditions. Thus, it appears that the listeners were unable to reliably determine the location of the target talker in the Double
HP condition even when both speech signals originated from
the same loudspeaker and there was no need to use the target
call sign to determine the location of the target phrase.
The right panel of Fig. 1 shows performance in the
color-number identification task of the experiment. In this
task, spatial separation significantly improved performance
in every hearing protection condition tested 共two-factor
within subjects ANOVA on the individual arcsinetransformed scores兲 (F (1,5) ⫽211.63, p⬍0.0001). As in the
left-right discrimination task, there was a significant decrease
in performance in the Double HP condition (F (2,10) ⫽23.19,
p⬍0.0002). There was also a significant 共15 percentage
point兲 decrease in the magnitude of the spatial advantage in
the Double HP condition (F (2,10) ⫽5.60, p⬍0.05). Nevertheless, despite the poor location discrimination data in the
Double HP condition, the intelligibility advantages of spatial
separation in that condition were substantial 共69% percent
correct identifications spatial versus 38% non-spatial兲.
On the surface, it seems somewhat odd that listeners
could obtain such a large intelligibility advantage from spatial separation in a condition where they could not reliably
distinguish between two spatial locations that were separated
by 90° in azimuth. Indeed, the 60% correct discrimination
scores achieved in the left-right discrimination task, while
technically better than chance, were lower than what is generally considered to be the discrimination threshold for the
minimum audible angular change in the location of a sound
source, or MAA 共Mills, 1958兲. The most likely explanation
for the spatial unmasking in the Double HP condition is that,
although listeners in that condition could not accurately determine the actual locations of the talkers, they could tell that
the talkers were originating from different locations, and this
helped them to segregate the two speech signals into different streams and improve their score on the speech intelligibility test.
IV. SUMMARY AND CONCLUSIONS
This paper has presented the results of an experiment
examining whether the decreases in localization ability that
occur when listeners wear double hearing protection also impair their ability to segregate spatially separated speech signals. As in previous studies, the results of this experiment
have shown that listeners who can reliably localize sound
sources while wearing single hearing protection cannot do so
Brungart et al.: Letters to the Editor
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when they are wearing double hearing protection. However,
despite this decrease in localization ability, listeners wearing
double hearing protection still seem to gain substantial, albeit
somewhat reduced, intelligibility benefit from the spatial
separation of the competing talkers. Thus it seems that the
degraded localization cues that prevent listeners from hearing sound sources in the correct locations do not necessarily
make all sound sources appear to originate from the same
location. This suggests that listeners wearing double hearing
protection may not be as impaired in their ability to process
complex auditory scenes as their poor localization scores
would suggest. It also offers some hope that listeners who
have been thoroughly trained while wearing double hearing
protection 共or those who are constantly exposed to boneconducted sound due to conductive hearing loss兲 may eventually be able to learn to improve their localization ability.
Previous research has shown that listeners who receive training can learn to adapt to a variety of different disruptions in
the cues they would normally use to localize sounds. For
example, Hofman and his colleagues 共1998兲 conducted an
experiment where listeners’ normal localization cues were
disrupted by inserting plastic molds into their left and right
ears. The results of this experiment showed that listeners
could, over a several day period, learn to localize relatively
well with these disrupted cues. Furthermore, once they
adapted to these cues, they were then able to localize with or
without the earmolds with no addional periods of adaptation.
Shinn-Cunningham, Durlach, and Held 共1998兲 reported
similar results for localization cues that were systematically
remapped in the horizontal plane. Thus, to the extent that
double hearing protection produces disrupted location cues
that change systematically with source location, one might
expect listeners to eventually be able to learn to use these
disrupted cues to localize sounds. Further research is now
needed to determine the extent, if any, to which listeners are
also able to adapt to the disrupted localization cues that occur with double HP.
ACKNOWLEDGMENT
This work was sponsored in part by AFOSR Grant No.
01-HE-01-COR.
This noise was generated by using a DSP processor 共Tucker-Davis RP2兲 to
rectangularly gate the output of an analog pink noise generator 共GenRad兲.
Note that, although the noise was both pink and broadband prior to transmission to the loudspeakers, no attempts were made to correct for the
loudspeaker and amplifier responses to ensure that this signal was pink at
the location of the center of the head.
1
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J. Acoust. Soc. Am., Vol. 116, No. 4, Pt. 1, October 2004
2
The standard deviations of these threshold measurements were approximately 5 dB in the No HP condition, 7 dB in the Single HP condition, and
13 dB in the Double HP condition.
3
Across the individual subjects in the Double HP condition, one listener
performed relatively well at the left-right identification task 共⬇80% correct兲, one performed relatively poorly 共⬇30% correct兲, and the others fell
between 50 and 70% correct responses. These differences are consistent
with the relatively large individual differences found in our earlier study on
localization with double hearing protection 共Brungart et al., 2003兲.
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Brungart et al.: Letters to the Editor