Evoked acoustic emissions from the human ear. II. Normative data in young adults and influence of posture.

Click-evoked acoustic emissions were recorded in 10 normally hearing young adults and evaluated by methods previously described. Five of the subjects were tested on both ears. A clear response could be traced down to or below the psychoacoustic threshold in all ears. However, the response pattern differed significantly from one ear to another, yielding an intra- and intersubject variability of the same order of magnitude. The key parameter of the evoked emission seems to be the group latency, since both peak-to-peak amplitude and frequency content are defined only at a specific latency. Two methods were applied to establish group latencies objectively and the two latency measures were almost identical. The individual input-output functions, based on peak-to-peak amplitude values as well as RMS values, exhibited nonlinearity. The latency versus frequency relationship was ambiguous. Retests of five ears at 4-5 week intervals demonstrated a high stability of the response pattern from the individual ear. Five subjects were tested in the sitting as well as in the recumbent position and the response pattern were unaffected by posture.