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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 1986 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. BIBLIOGRAPHY 1. Levey S, Levey T, Fligor BJ. Noise exposure estimates of urban MP3 player users. J Speech Lang Hear Res 2011;54:263–277. Laryngoscope 123: August 2013 2. Vogel I, Brug J, Hosli EJ, van der Ploeg CP, Raat H. 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