Measurements of Acoustic Impedance in Indians

Abstract

Impedance measures-tympanometry, acoustic impedance, and acoustic reflex threshold were determined on 136 Indian subjects (191 ears) using electro-acoustic bridge (Madsen ZO 70). The subjects included normal and pathological, varying in age from 6-56 years. Sex and age differences were observed with regard to static compliance and acoustic impedance. Any conclusion regarding the norms for acoustic impedance cannot be drawn as there is overlapping of pathogical and normal ears with regard to acoustic impedance. The normals obtained a wider range of 682-5840 acoustic ohms. Varying of air pressure in the ear canal had significant effect on acoustic reflex .threshold. The concept of impeda1tce was introduced in 1914 (Webster 1919). Its application to clinical audiology became evident from the day Metz (1946) published his classic monograph- •The Acoustic Impedance measured on Normal and Pathological ears'. Today, Acoustic Impedance measured at the tympanic membrane is "- - - not just another audiological test"-it constitutes-a whole new field of investigation with an inherent new methodology". (Zwislocki, 1965). Currently, acoustic impedance measurements constitutes the only means of direct examination of the middle ear function. They are performed at physiological vibration amplitudes and, therefore, give a direct estimate of the efficiency of the sound transmission. Except in acoustic reflex threshold tests, the sensorineural part of the auditory system is not involved. The acoustic impedance method is based on· a partial reflection do sound. It depends not only on the structure of the tympanic membrane but also on that of the middle ear components and cochlea. Through acoustic measurements on non~al and pathological ears and with the help of anatomy and acoustic theory, it is possible to analyze the middle ear function in detail and to correlate the acoustic charges measured at the tympanic membrane to middle ear pathologies. In a way impedance audiometry helps to look beyond the intact tympanic membrane and it tells about the sound transmitting characteristics of the middle ear. There appear to be numerous studies to establish norms for impedance measurements by using both Zwislocki's bridge and the electro-acoustic bridge. · Still there is scarcity of normative data and the available results show discrepancy for both the methods. Jerger et al (1972) in comparing normative data from six sources, three studies using Zwislocki's bridge and three studies using electro-acoustic bridge found variations. Normative data are very important for the classification of ears under test into normal