type of personal music player was evaluated by multiple linear regression (SPSS 11.5). Independent Student’s t- tests were used for analysis of the data by gender and comparison of hearing thresholds in the left and right ears (SPSS 11.5). Statistical significance was defined by p values less than 0.05. RESULTS Of the 490 selected subjects, 167 (34.1%) were males and 323 (65.8%) were females. Their age distribution is summarized in Table 1. Of the 490 students, 94 (19.2%) were 13 years-old, 71 (14.5%) were 14, 79 (16.1%) were 15, 52 (10.6%) were 16, 95 (19.4%) were 17, and 99 (20.2%) were 18 years old. Daily use of personal music players ranged from 1 to 3 hours in 229 students (46.7%). In addition, 164 students (33.5%) used these players for less than 1 hour per day, 62 (12.7%) used them 3 to 5 hours per day, 7 (1.4%) used them for more than 5 hours per day (1.4%), and 28 (5.7%) did not use personal music players (Table 2). The overall Table 1. Distribution of the Subjects according to Gender and Age Characteristics Gender Myung Gu Kim, et al. personal music players, and they are widely used for listen- ing to music and radio programs, and for studying foreign language programs, and it was thought that NIHL could occur in Korean adolescents. We, therefore, assessed the use of personal music player among middle and high school students and the relationship between hearing threshold and usage pattern of personal music players. MATERIALS AND METHODS Four hundred and ninety middle and high school students were randomly selected and recruited from hospitals in four regions (2 sites in Seoul city, Gyeongi-do and Masan city) of Korea between June 1 and July 31, 2007. Students were excluded if they had serous otitis media or perforation of the tympanic membrane, as determined by otoscopic examination; if they had hearing loss due to other previous diseases; tinnitus; previous or current history of otologic disorders; previous exposure to loud sounds produced by toys or guns; or previous exposure to loud noises in concert halls. After explaining the objectives of this study, we obtained written informed consent from each student, and the study protocol was approved by our Institutional Review Board. Each student was asked to fill out a questionnaire, which included questions regarding daily use of personal music players, overall usage period, listening method and types of personal music players, along with questions regarding the gender and age of each student. The cumulative usage period was calculated by multiplying daily usage time by usage period. Each subject underwent a pure tone audiometry examina- tion (Virtual -320, USA), which was performed at a hearing level testing unit in a university hospital equipped with facilities recommended by the American National Standard Institute (ANSI S3.1-1999).9 For hearing measurements, a mixed method was used in order to minimize the effect of temporal threshold shifts and the learning effect. The signal intensity started at 30 dB and increased in 20 dB steps until the subject responded. The signal intensity was then increased in 5 dB steps until the subject responded. For each signal intensity, the procedure was repeated until the subject responded two of three times. Hearing was measured using a randomized presentation order for tonal frequencies. Hearing thresholds were expressed as the trisec- tional average hearing level using thresholds at the 500 Hz, 1 kHz and 2 kHz. Hearing thresholds were also measured at 4 kHz in all subjects. For the statistical analysis of collected data, the hearing level threshold according to age, daily usage time, usage period, cumulative usage period, listening method, and 772 Yonsei Med J http://www.eymj.org Volume 50 Number 6 December 2009 No (%) Male Female 13 14 15 16 17 18 167 (34.1) 323 (65.8) 94 (19.2) 71 (14.5) 79 (16.1) 52 (10.6) 95 (19.4) 99 (20.2) 490 (100) Age (yrs) Total Table 2. Distribution of the Subjects according to Daily Use Time and Used Period of Personal Stereo System Status No (%) Daily use time (hr) 0 28 (5.7) 0-1 164 (33.5) 1-3 229 (46.7) 3-5 62 (12.7) > 5 7 (1.4) Total use period (yrs) 0 28 (5.7) 0-1 108 (22.0) 1-3 237 (48.4) 3-5 93 (19.0) > 5 24 (4.9) Total 490 (100)