Abstract
BACKGROUND: The definition of normal hearing according to the DIN standard refers to the air conduction threshold of tympanometry. However, middle ear function is not specifically considered in this definition. The present study aimed to prospectively collect audiometric data for normal hearing that are based not only on tone audiometry but also supplemented by objective data on middle ear function. METHODS: The examinations were carried out in 84 ears of test subjects aged 18-25 years with normal hearing according to tone audiometry. Objective middle ear diagnostics were performed using classic tympanometry, broadband absorbance tympanometry, and stapedius reflex threshold measurement. A distortion product otoacoustic emission (DPOAE) measurement was also carried out. RESULTS: Compliance of more than 2 mmho was found in 7 of the 84 ears examined, which are considered separately in the following. The resonance frequency of these 7 ears was below the first quartile of the comparison group (759 Hz), and the absorbance of this group was above the third quartile of the absorbance of the group with regular compliance for frequencies up to 750 Hz. In the group with increased compliance, in more cases than in the comparison group (14% vs. 5%) were no DPOAEs detectable at at least one frequency. In 6 of the 7 ears with increased compliance, stapedius reflex thresholds were found within the whisker range of the group with regular compliance at all measured frequencies. CONCLUSION: In the DIN standard for classification of normal hearing, the definition of normal hearing is based solely on tone audiometric parameterization. The reference values measured in this study with regard to compliance of the eardrum show clear inter-individual variance. Supplementing the assessment of middle ear function, such as by measuring compliance, resonance frequency, and/or absorbance curves, could provide additional information to confirm normal hearing.