The effect of noise exposure on the details of distortion product otoacoustic emissions in humans.

The effect of noise exposure on amplitude and phase of distortion product otoacoustic emissions (DPOAEs) was examined by five different paradigms: across a wideband of frequency, microstructure, input/output function, primary frequency ratio tuning curve, and group delay. The aim was to investigate the vulnerability of these different features to moderate levels of noise exposure. Nine subjects were exposed to third-octave-band noise. The DPOAE amplitude was reduced frequency specifically with the greatest reduction approximately half an octave above the frequency of the noise. The degree of amplitude reduction was greatest at low stimulus levels. There were no observed effects on the shape of the primary ratio tuning curve. A weak tendency to a decrease was seen in group delays. Distinct microstructure was seen in the amplitude against frequency of five out of seven subjects. The maximum to minimum ratio of the microstructure decreased, and the whole pattern shifted toward lower frequencies after noise exposure. Evidence of multiple internal reflection or interference was seen in the periodicity of the microstructure. Using a simple model of the microstructure based on multiple reflections, the noise-induced changes were reevaluated. A reduction in maximum to minimum microstructure ratio could be interpreted as a decrease in the internal reflection coefficient. The implications of these observations for the interpretation of the DPOAE measurements are considered.