Insert Earphone Pressure Response in Real Ears and Couplers

Abstract

Pressures measured in five real ears and the standard 2-cm3 and Zwislocki couplers have shown that (1) Pressure in Zwislocki coupler is essentially identical to pressure in real ears (with no ear mold leaks) below 500 Hz implying an equivalent real ear volume of about 1.2 cm3. Pressure in 2-cm3 coupler is about 4 dB lower. (2) Between 500 Hz and 5 kHz, the real ear minus 2-cm3 coupler difference increases with frequency about 2.5 dB/oct to about +12 dB at 5 kHz. The real ear minus Zwislocki coupler difference, however, is no more than ±2 dB up to 5 kHz. (3) Above 5 kHz, there is evidence that pressure in real ears decreases with frequency relative to both couplers. Real ear pressure was measured using a probe-tube microphone (corrected to read ear drum pressure); five types of insert earphones (hearing aid receivers) were used. When dealing with insert earphones whose source impedance is very high relative to the input impedance of the coupler (this is usually the case), it is sufficient to measure the insert earphone pressure on the 2-cm3 coupler. Real ear drum pressure can then be determined by applying a correction curve (either graphically or by electronic filtering) to be presented. The Zwislocki coupler would then serve as a laboratory standard for comparison.