The creation of distortion products through the interaction of spontaneous otoacoustic emissions and externally generated tones.

Abstract

A spontaneous otoacoustic emission (SOAE) and external tones (XTs) were used as primaries f₂ and f₁, respectively (frequency of f₂ > f₁) to create 2f₁-f₂ distortion products (DPOAEs). The DPOAEs primarily reflect active processes in the cochlea because low- to medium-level primaries less than 60 dB SPL were employed. DPOAE characteristics were determined and compared to what is known from the literature about DPOAEs created using two XTs. SOAE frequency and amplitude were also described. One hundred twenty-two women were screened for the presence of SOAEs to obtain 10 subjects for the study. XTs were presented to the ear at SOAE/f₁ ratios between 1.08 and 1.22. Sound pressure levels were incremented in 5 dB steps and extended from levels equal to the initial SOAE amplitudes to levels at which the SOAEs and DPOAEs disappeared into the noise floor. Amplitude and frequency of the SOAE, XT, and DPOAE were recorded by placing a low-noise microphone in the ear canal and extracted using Fast Fourier Transform analysis. Results indicated that the DPOAE amplitudes and SOAE suppression characteristics were variable across subjects. Despite the variability, the following trends were noted: (1) at larger frequency ratios, DPOAE generation and SOAE suppression were associated with greater XT levels; (2) DPOAE growth functions were characterized by slopes less than 1 dB/dB, a maximum, rollover and disappearance into the noise floor with increasing XT levels; (3) maximum amplitude DPOAEs were observed at frequencies approximately one-half octave lower than the SOAE; (4) DPOAE thresholds were associated with little SOAE suppression whereas the maximum DPOAEs were associated with a greater amount of SOAE suppression; (5) the most common SOAE frequency shift, in the presence of XT stimulation, was a shift to a higher frequency just prior to total SOAE suppression. Results are interpreted using an electrical circuit model (modified from Yates, 1990) and pictorial representations of the SOAE, XT, and DPOAE vibrational patterns on the basilar membrane (modified from Yates, Geisler, Patuzzi, & Johnstone, 1989).