Download Low body mass index and jaw movement are protective of hearing

The Laryngoscope
C 2013 The American Laryngological,
V
Rhinological and Otological Society, Inc.
Low Body Mass Index and Jaw Movement Are Protective of
Hearing in Users of Personal Listening Devices
Lieber Po-Hung Li, MD, PhD; Ann Yi Chiun Chuang, MD; Catherine McMahon, PhD;
Tao-Hsin Tung, PhD; Joshua Kuang-Chao Chen, MD
Objectives/Hypothesis: Sound pressure level delivered through personal listening devices (PLDs) and reaching the ear
drum might be affected by body size and jaw movements. This study aimed to investigate whether jaw movement and/or
smaller body mass index (BMI) resulted in decrease of sound pressure level within the ear canals of PLD users via an earbud
earphone.
Study Design: Case series.
Methods: Forty-five normal-hearing subjects (16 males; mean age, 23.3 years) participated in this study. A probe-microphone system was used to measure sound pressure level in the external ear canal with music delivered from a media player
via an earbud earphone. Test materials consisted of two 20-second excerpts from a heavy metal music piece. Subjects were
instructed to adjust the volume of the media player to conform to three conditions for sound pressure measurement:
comfortable, loud, and maximum. Measurements were then repeated while subjects mimicked chewing action under the same
listening conditions.
Results: Sound pressure levels were significantly lower when measured with jaw movement than without jaw movement (P < .05). Sound pressure levels monitored with/without jaw movement were generally lower in subjects with a
BMI<23 than those with a BMI 23 (P < .05).
Conclusions: Jaw movement and low BMI (<23) reduced the overall sound level of PLDs at the ear canal. Sound pressure levels detected in the external ear canal of our subjects using earbud earphones were significantly lower under conditions of jaw movement/BMI <23. Our research invites further studies on a larger group of PLD users to correlate these variables with hearing threshold shifts over time.
Key Words: Personal listening devices, body mass index, hearing loss.
Level of Evidence: 4.
Laryngoscope, 123:1983–1987, 2013
INTRODUCTION
Personal listening devices (PLDs) have become
increasingly more popular in recent years. However, it is
of concern that PLD users appear to have a tendency to
exceed suggested maximums for sound exposure in
either intensity and/or duration,1 given the lack of perceived personal vulnerability of adolescents to hearing
loss resulting from loud sound.2 As such, concerns have
From the Department of Otolaryngology (L.P-H.L., J.K.-C.C.), and
Department of Medical Research and Education (T.-H.T.), Cheng Hsin
General Hospital, Taipei, Taiwan; Faculty of Medicine (L.P-H.L., J.K.-C.C.),
School of Medicine, National Yang-Ming University, Taipei, Taiwan;
Medical Department, Mackay Memorial Hospital (A.Y.C.C.), Taipei, Taiwan;
Centre for Language Sciences (C.M.), Macquarie University, Sydney,
Australia; Integrated Brain Research Laboratory, Department of Medical
Research and Education, (L.P-H.L.), Taipei Veterans General Hospital,
Taipei, Taiwan.
Editor’s Note: This Manuscript was accepted for publication
November 29, 2012.
The work was supported by Cheng Hsin General Hospital (9631)
of Taiwan.
The authors have no other funding, financial relationships, or
conflicts of interest to disclose.
Send correspondence to Joshua Kuang-Chao Chen, MD, Head,
Department of Otolaryngology, Cheng Hsin General Hospital, No. 45,
Cheng Hsin St., Pai-Tou, Taipei 112, Taiwan.
E-mail: [email protected]
DOI: 10.1002/lary.23955
Laryngoscope 123: August 2013
been raised that music-induced hearing loss, a subtype
of noise-induced hearing loss, might be evolving into a
public health problem, with significant increases in
noise-induced hearing loss identified in adolescent populations.3–6 Although efforts have been made to develop
strategies of hearing protection for PLD users,7,8 the
results have been inconclusive.
Earbuds seem to be the most common style of earphones used by PLD consumers, and provide far less
attenuation of external noises.9,10 The sound pressure
level trapped in the human external ear canal for music
output from an earbud earphone tends to be lower in
comparison to other types of earphones with better
attenuation abilities under equivalent listening conditions, because earbud earphones allow more sound
pressure level to find its way out of the ear canal.10,11
Consequently, individuals increase the volume of their
PLDs because they have more difficulty hearing the target signal through the competing noise. Therefore, using
an earbud is potentially more harmful to hearing than
other styles of headphones.
Previous research revealed that, along with jaw
movement, variations in sound intensity can be detected
in a concealed ear canal with a bone-conduction stimulus.12 According to our clinical observations, eating/gum
chewing seems to be one of the most common activities
Li et al.: Low BMI and Jaw Movement Helps PLD Users
1983
PLD users are likely to do while listening to music
through their earphones. Considering the potential deviation/attenuation in sound output levels, the action of
chewing might, in fact, be beneficial for earbud users in
reducing overall levels of noise. The fluctuation of sound
pressure level from PLDs via an earbud earphone (i.e.,
air-conduction stimuli) assessed in the human ear canal
accompanied with jaw movement, however, has not yet
been considered.
Obesity and/or a high body mass index (BMI [kg/
m2]) have long been noted as potential risk factors for
the development of hearing loss.13,14 The underlying
mechanism(s) might at least in part be attributed to a
possible vulnerability to pathophysiological changes in
the auditory pathway of subjects with obesity and/or a
high BMI.13,15 Considering this unavoidable susceptibility of the auditory system, protection strategies might
have a major effect on hearing conservation for PLD
users who are obese and/or have a high BMI. However,
the relationship between BMI and sound pressure level
in the ear canal of PLD users has not been addressed.
In the present study, 45 PLD users were recruited.
By means of a probe microphone system, the exact output
level (dB sound pressure level [SPL]) of music in the ear
canal of each subject through an earbud earphone connected to a PLD was measured. We aimed to investigate
whether or not jaw movement and low BMI resulted in an
average decrease of sound pressure level within the external ear canal while using PLDs via an earbud earphone.
MATERIALS AND METHODS
Subjects
Forty-five normal-hearing subjects (16 males and 29
females; 13–48 years old; mean age, 23.3 years) were studied.
Participants underwent pure-tone audiometry (PTA) to determine both air- and bone-conduction thresholds, using test
frequencies between 250 Hz and 8,000 Hz. All subjects had normal PTA results (thresholds 25 dB hearing loss for all
frequencies). No cerumen occlusion or middle ear disease was
identified. This study conformed to the Declaration of Helsinki.
Written informed consent was obtained from parents/subjects
with a protocol approved by the institutional ethics committee
of Cheng-Hsin General Hospital.
Experiment Paradigm
Experiments were conducted in an acoustically shielded
room. Music output came from the BenQ Joybee 220 media
player system (BenQ, Taipei, Taiwan). The Fonix FP40 probe
microphone system (Frye, Tigard, OR) was used to assess the
sound pressure level of music in the external ear canal, and the
microphone tip was placed within 5 mm of the eardrum. An earbud earphone connected to the media player was then plugged
in. Test materials were composed of two 20-second excerpts from
the heavy metal music piece “Faint,” performed by Chester Bennington of Linkin Park, at 0:30–0:50 (introduction) and 2:10–
2:30 (crescendo), respectively. Subjects were instructed to adjust
the volume of the media player to conform with three conditions
for the measurement of sound pressure level: the volume they
were comfortable with (comfortable volume), the volume they felt
was loud (loud volume), and the maximum volume available for
the media player (maximum volume). Measurements were then
Laryngoscope 123: August 2013
1984
repeated while subjects were told to mimic the action of gum
chewing under the same listening conditions.
Statistical Analysis
Statistical analysis was performed using the software SAS
8.1 (SAS Institute Inc., Cary, NC). Differences in sound pressure level between the two conditions of “without jaw
movement” and “jaw movement” were analyzed using a paired t
test. Differences of sound pressure level in terms of BMI were
evaluated by dividing subjects into two groups: BMI 23 and
BMI <23. Possible interactions between factors such as gender,
jaw movement, and BMI were assessed by using analysis of variance (ANOVA). The threshold for statistical significance was
set at P < .05.
RESULTS
All subjects accomplished the tests without difficulties. No hearing complaints were noted after the tests.
For the “introduction” excerpt of the music, the sound
pressure levels were significantly lower in the condition of
“jaw movement” (mean, 79.5 and 98.2 dB SPL for loud
and maximum volume, respectively) than that of “without
jaw movement” (mean 5 80.1 and 99.5 dB SPL for loud
and maximum volume, P 5 0.01 and P < 0.001, respectively; Table I). The sound pressure levels for the
“crescendo” excerpt of the music were also significantly
lower in the condition of “jaw movement” (mean, 72.9 and
105.5 dB SPL for comfortable and maximum volume,
respectively) than that of “without jaw movement” (mean,
73.9 and 106.0 dB SPL for comfortable and maximum volume, P < 0.001 and P 5 0.02, respectively; Table I).
Sound pressure levels monitored in terms of three
listening volumes with or without jaw movement were
generally lower in subjects with a BMI < 23 than those
with a BMI ⭌ 23 (Table II). For the “introduction” excerpt
of the music, the differences in sound pressure levels
were significant for the condition of “loud volume with
jaw movement” (mean, 78.2 dB SPL for subjects with a
BMI < 23; mean, 84.2 dB SPL for subjects with a BMI 23, P 5 .047), and the condition of “comfortable volume
without jaw movement” (mean 5 66.8 dB SPL for subjects with a BMI < 23, mean 5 68.7 dB SPL for subjects
with a BMI ⭌ 23, P 5 0.035; Table II). For the
“crescendo” excerpt of the music, the differences in sound
pressure levels were significant for the condition of “loud
volume with jaw movement” (mean, 85.8 dB SPL for subjects with a BMI < 23; mean, 92.7 dB SPL for subjects
with a BMI ⭌ 23, P 5 0.035; Table II), and the condition
of “loud volume without jaw movement” (mean 5 85.9 dB
SPL for subjects with a BMI < 23, mean 5 93.2 dB SPL
for subjects with a BMI ⭌ 23, p 5 0.006; Table II).
No interactions were revealed between factors when
ANOVA was done by gender 3 jaw movement 3 BMI, or
jaw movement 3 BMI stratified according to gender (Table III).
DISCUSSION
Jaw Movement Decreased Sound Pressure Level
One major and novel finding in the current study is
that jaw movement reduced the sound pressure level
Li et al.: Low BMI and Jaw Movement Helps PLD Users
TABLE I.
Sound Pressure Levels Detected With Earbud Earphones in the External Ear Canal of Personal Listening Device Users
Under Various Conditions of Listening Volumes Accompanied by Jaw Movement or No Jaw Movement.
Introduction
Comfortable
Crescendo
Loud
Maximum
Comfortable
Loud
Maximum
All participants, sound pressure level (n 5 45)
Jaw movement
No jaw movement
67.26 7.6
67.2 6 6.1
79.5 6 7.6
80.1 6 7.7
98.2 6 3.4
99.5 6 3.5
72.9 6 6.0
73.9 6 6.2
87.3 6 8.5
87.6 6 8.3
105.5 6 3.2
106.0 6 3.3
P
.98
.009*
<.001*
<.001*
.53
.02*
Males, sound pressure level (n 5 16)
Jaw movement
67.0 6 5.1
80.7 6 9.1
97.8 6 3.1
72.4 6 5.7
89.3 6 8.6
105.5 6 4.5
No jaw movement
80.8 6 8.6
98.4 6 3.1
73.1 6 6.0
90.0 6 8.4
105.0 6 4.4
P
.52
Females, sound pressure level (n 5 29)
.80
.11
.02*
.16
.08
Jaw movement
67.4 6 8.7
78.8 6 6.7
98.5 6 3.6
73.3 6 6.2
86.2 6 8.4
105.5 6 2.2
No jaw movement
P
67.2 6 6.6
.86
79.7 6 7.3
<.001*
100.1 6 3.7
<.001*
74.4 6 6.4
<.001*
86.2 6 8.1
.95
106.6 6 2.3
<.001*
67.3 6 5.2
Sound pressure levels are mean 6 standard deviation in dB SPL. Introduction and Crescendo are two 20-second excerpts from the heavy metal music
piece titled “Faint,” performed by Chester Bennington of Linkin Park at 0:30 to 0:50 and 2:10 to 2:30, respectively. Comfortable indicates comfortable volume,
the volume subjects were comfortable with. Loud indicates loud volume, the volume at which subjects felt it was loud. Maximum indicates maximum volume,
the maximum volume available for the media player. Jaw movement and no jaw movement indicate sound pressure level detected with or without jaw movement, respectively.
*Threshold for statistical significance using paired t tests was set at P < .05.
detected in the external ear canal when subjects listened
to music from PLDs (Table I). It is possible that the middle ear muscles might contract during jaw movement,
despite the consensual middle ear reflex that could also
be elicited by the presented music. This contraction
might cause less energy transmission in the forward
direction, resulting in higher ear canal levels, because
more energy is reflected back out. However, the mean intensity measured along with jaw movement was
significantly attenuated by 0.5 to 1.3 dB SPL for volumes at various listening conditions, in comparison to
that without jaw movement, although the size of the dif-
ference seemed to be small. It is necessary to clarify the
clinical relevance of the statistical significance in terms
of the relatively large within-condition standard
deviations in the present study. The standard deviations
reflected the interindividual variation on the result of intensity measurement, and the magnitude was actually
in line with previous research.10 A significant betweencondition difference of 1.6 dB, which was audible/detectable by normal hearing subjects, furthermore, could lead
to a 45% extension of permissible listening duration (i.e.,
14.66–21.21 minutes; Table I, introduction/maximum)
before reaching the 100% daily noise dose, according to
TABLE II.
Comparisons of Sound Pressure Levels Between Subjects With a BMI <23 and Those 23 Detected With Earbud Earphones
in the External Ear Canal Under Various Conditions of Listening Volumes With or Without Jaw Movement.
Introduction
Comfortable
Crescendo
Loud
Maximum
Comfortable
Loud
Maximum
Accompanied by jaw movement
BMI <23
BMI 23
66.9 6 6.9
68.5 6 9.9
78.2 6 6.5
84.2 6 9.5
98.4 6 3.6
97.8 6 2.8
72.4 6 5.5
74.9 6 7.6
85.8 6 7.4
92.7 6 10.5
105.2 6 2.4
106.6 6 5.0
P
.35
.047*
.59
.37
.04*
.10
Without jaw movement
BMI <23
66.8 6 4.8
78.9 6 6.7
99.7 6 3.7
73.4 6 5.7
85.9 6 6.9
105.7 6 2.9
BMI 23
68.7 6 9.6
84.5 6 9.6
98.8 6 2.7
75.8 6 7.8
93.2 6 10.8
106.8 6 4.4
P
.035*
.10
.57
.36
.006*
.33
Sound pressure levels are mean 6 standard deviation in dB SPL. Introduction and Crescendo are two 20-second excerpts from the heavy metal music
piece titled “Faint,” performed by Chester Bennington of Linkin Park at 0:30 to 0:50 and 2:10 to 2:30, respectively. Comfortable indicates comfortable volume,
the volume subjects were comfortable with. Loud indicates loud volume, the volume at which subjects felt it was loud. Maximum indicates maximum volume,
the maximum volume available for the media player. Jaw movement and no jaw movement indicate sound pressure level detected with or without jaw movement, respectively.
*Threshold for statistical significance using paired t tests was set at P < .05.
BMI 5 body mass index.
Laryngoscope 123: August 2013
Li et al.: Low BMI and Jaw Movement Helps PLD Users
1985
TABLE III.
Possible Interactions Among Gender, BMI, and Jaw Movement Under Various Settings of Listening
Volume Assessed by Using Two-Way ANOVA.
Introduction
Crescendo
Comfortable
Loud
Maximum
Comfortable
Loud
Maximum
Gender 3 BMI 3 jaw movement
.91
.91
.88
.88
.97
.74
BMI 3 jaw movement (female)
BMI 3 jaw movement (male)
.90
.96
.99
.89
.99
.82
.96
.88
.93
.98
.99
.76
Data are expressed as P values.
Introduction and Crescendo are two 20-second excerpts from the heavy metal music piece titled “Faint,” performed by Chester Bennington of Linkin
Park at 0:30 to 0:50 and 2:10 to 2:30, respectively. Comfortable indicates comfortable volume, the volume subjects were comfortable with. Loud indicates
loud volume, the volume at which subjects felt it was loud. Maximum indicates maximum volume, the maximum volume available for the media player. Jaw
movement and no jaw movement indicate sound pressure level detected with or without jaw movement, respectively.
Threshold for statistical significance using two-way analysis of variance (ANOVA) was set at P < .05. Gender 3 body mass index (BMI) 3 jaw movement
indicates possible interactions between gender, BMI, and jaw movement assessed by two-way ANOVA. BMI 3 jaw movement (female) and BMI 3 jaw movement (male) indicate possible interactions between BMI and jaw movement assessed by two-way ANOVA stratified according to gender, respectively.
the equation provided by National Institute for Occupational Safety and Health, Occupational Safety and
Health Administration, and American Academy of Audiology: T (min) 5 480/2(L-85)/3 (where L is the level of
exposure in dB SPL).10
To the best of our knowledge, the present research
is the first study ever reporting on the finding of a
decreased level for music output from a PLD through an
earbud earphone (i.e., air-conduction stimuli) resulting
from jaw movement in real ear assessment.
The mechanisms underlying the decline of the sound
pressure level due to jaw movement monitored in the
external ear canal of PLD users on listening to music via
an earbud earphone remains to be explored. One possibility is the leakage of acoustic energy owing to the change
in configuration of the ear canal caused by jaw movement.
Jaw movement has been shown to cause significant alterations in different dimensions (especially in the anterior/
posterior direction) of the external ear canal in humans.16
Furthermore, movement of the skull and mandible both
contribute to these changes.12 The degree of ear canal deformity provided by this movement can be up to 25%.16 It
was thus reasonable to infer that the changes bring about
some room between the ear canal and ear plug from time
to time, which in turn could lead to an intermittent
release of sound level output and therefore decreased
sound pressure level during jaw movement.
Another, perhaps more plausible, explanation is
that jaw movement might increase the ear canal space
due to the movement of the tympanic membrane created
by tensor tympani and/or tensor veli palatini muscles
contraction. The tensor tympani and tensor veli palatini
have long been noted to cause tympanic membrane
deflections observed during the acoustic reflex and the
opening of the eustachian tube, respectively.17,18 In
response to various external stimuli including jaw movement, the contraction of the above-mentioned muscles
could initiate a significant medial displacement of the
tympanic membrane.19,20 Because the scale of sound
pressure level is inversely correlated with the magnitude
of space volume,21 the “expanded” capacity of the external ear canal resulting from the inward movement of
the tympanic membrane induced by jaw movement could
Laryngoscope 123: August 2013
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consequently lead to the attenuation of sound pressure
level as observed in the present study.
PLD Users With Higher BMI
Another interesting finding in our study is that the
sound pressure levels monitored within the external ear
canal were generally lower in PLD users with a smaller
BMI when they listened to music through earbud earphones with a volume set by themselves (Table II and
III). The mean intensity measured with respect to various listening conditions was significantly lessened by 1.9
to 7.2 dB SPL in subjects with a BMI <23 than those
with a BMI 23, irrespective of whether jaw movement
was present or not. A significant between-condition difference of 7.3 dB led to a 4.4-times extension of
permissible listening duration (i.e., 72.18–389.88
minutes; Table II, Crescendo-Loud) before reaching the
100% daily noise dose.10 Some might argue that subjects
with a higher BMI choose higher listening levels probably due to having a higher hearing threshold (albeit
within the normal range) in the present study, because
previous research revealed that men tended to choose 5
to 6 dB SPL higher than women when both set the
music to a “loud” level.9 Male subjects in the group of
BMI 23 in our study, however, were fewer than female
subjects. The current study thus revealed that PLD
users with a higher BMI tended to listen to music under
the circumstances of a higher sound level via an earbud
earphone.
Previous research has shown that subjects with
obesity and/or a high BMI were at a higher risk for the
development of hearing loss.13,14 An elevated degree of
susceptibility to structural changes in central nervous
system (e.g., cerebral atrophy of temporal lobe) has long
been noted in those subjects.15 Medical factors, such as
cardiovascular disease, probably exacerbated by the condition of obesity/high BMI might add to the likelihood
for impairment of functions associated with temporal
cortex, including hearing.13 In combination with the
aforementioned vulnerability to structural/functional
changes, it is rational to conjecture that the propensity
to enjoy music with a higher output level as was verified
Li et al.: Low BMI and Jaw Movement Helps PLD Users
in the present study could furthermore lead to an
increased incidence of hearing loss by damaging peripheral receptors (i.e., organ of Corti/hair cells) in PLD
users with a BMI 23. Our finding thus implied that a
low BMI was protective for the hearing of PLD users on
listening to music via an earbud earphone in terms of
long-term music exposure.
There are some limitations of the study. The measurements with jaw movement did not address the
dynamic condition of the SPL (e.g., peak value in dB),
because the movement would be expected to produce
fluctuations in the ear canal. Further studies with a
measurement in quiet on jaw movement are needed to
reveal whether the movement of the mandible condyle
produced noise of its own, whether this noise contributed
to the aforementioned fluctuations, and whether the
magnitude of the noise depended on the dimension of
the jaw opening or not.
CONCLUSION
Jaw movement and low BMI seemed to be protective
for the hearing of PLD users. The sound pressure levels
with respect to various listening volumes detected in the
external ear canal of our subjects using earbud earphones
were significantly lower under the conditions of jaw movement/BMI <23. The aforementioned findings suggested
that certain physical characteristics of the listener might
mediate the sound pressure level in the ear canal. Our
research invites further studies on a larger group of PLD
users to correlate variables (such as jaw movement and
BMI) and hearing threshold shifts over time. A longitudinal study is also needed to prospectively show whether
such attenuation effect in terms of reduced sound pressure
level in external ear canal concealed by earbud earphones
helps to lessen the risk of hearing changes in PLD users.
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