Visualization of the onset of distortion-product otoacoustic emissions, and measurement of their latency

This paper describes a method for visualization of the onset of distortion-product otoacoustic emission (DPOAE) waveforms in the time domain. The DPOAE waveforms are obtained using ensemble averaging of samples of microphone output. A rectangular sample window is used, and the primary tones are turned on within the sample window. The phases of the primary tones (f₁ and f₂) are varied systematically between samples in such a way that the primary tones, and all DPOAEs (e.g., 2f₂-f₁, 3f₁-2f₂, 2f₁), except the DPOAE of interest (e.g., 2f₁-f₂), are cancelled in the ensemble average. Visualization of the DPOAE onset allows measurement of the onset latency (OSL) of the DPOAE. These direct measurements of OSL are compared to phase- gradient latencies (PGLs) in the same ears determined by measuring the phase change of the DPOAE as a function of DPOAE frequency. The direct measures of OSL vary from >10 to <1 ms, decrease with increasing frequency and increasing stimulus level, and are shorter in rabbits than humans. The direct measures of OSL are, in general, quantitatively similar to PGL estimates, but there are exceptions. Visualization of DPOAE onset also allows quantification of DPOAE rise times, and reveals phase and amplitude changes of the DPOAE that occur several milliseconds after onset in rabbits and humans. It is proposed that the phase and amplitude changes result from vector summation of multiple components of the DPOAE signal, each with a different latency.