title: Low body mass index and jaw movement are protective of hearing in users of personal listening devices.
abstract: OBJECTIVE
  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.
  
  
  METHODS
  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.
authors.affiliation: Department of Otolaryngology, Cheng Hsin General Hospital, Taipei, Taiwan.
authors.forename: Lieber Po-Hung
authors.initials: LP
authors.lastname: Li
authors.forename: Ann Yi Chiun
authors.initials: AY
authors.lastname: Chuang
authors.forename: Catherine
authors.initials: C
authors.lastname: McMahon
authors.forename: Tao-Hsin
authors.initials: TH
authors.lastname: Tung
authors.forename: Joshua Kuang-Chao
authors.initials: JK
authors.lastname: Chen
mesh_terms: D000161:Acoustic Stimulation / Q000295:instrumentation*; D000293:Adolescent; D000328:Adult; D001307:Auditory Perception / Q000502:physiology*; D015992:Body Mass Index; D004424:Ear Canal / Q000502:physiology*; D005260:Female; D006309:Hearing / Q000502:physiology*; D034381:Hearing Loss / Q000503:physiopathology*; D006320:Hearing Tests; D006801:Humans; D007568:Jaw / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D055815:Young Adult
keywords: Personal listening devices; body mass index; hearing loss