MEASUREMENT OF DISTORTION-PRODUCT PHASE IN THE EAR CANAL OF THE CAT

Amplitudes of odd order distortion products (DPs) that are detected in animal ear canals have been used to probe cochlear health, to search for cochlear amplification, and to measure aspects of cochlear mechani cal frequency response. Like the DP amplitude, DP phase is also an imp ortant measure of the cochlear mechanical response. Reported here are measurements of DP phase in the ear canal of the cat. The phase data s how frequency-dependent time delays. One of these delays is a function of f(2), the frequency of the higher-frequency primary. Hence the DP phase phi(d) is of the form phi(d)= phi(0) + omega(d) tau where omega( d) is the DP angular frequency and tau is a fixed time delay. Our resu lts show that phi(d) is independent of input level a(2) as long as the ratio a(2)/a(1) less than or equal to 2, where a(2) and a(1) are the amplitudes of the input tones. As a(2)/a(1) becomes greater than two, the fixed time delays increase for DPs whose frequencies are less than the frequencies of the input tones. When both levels are varied toget her the delay increases as the levels decrease. There can be phase cha nges as large as pi. associated with deep nulls in the DP magnitude fo r the two lower-frequency DPs. Features of the nulls may be modeled as suming that there is partial reflection of the DP wave from the DP pla ce. The assumption of energy reemitted from the DP place also explains amplitude-ratio-dependent time delays and 2 pi-level-dependent bifurc ations in phase. The DP phase shows different dependencies for f(2)<1 kHz compared to f(2)>2 kHz. (C) 1997 Acoustical Society of America.