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 prod uct otoacoustic emissions (DPOAEs) was examined by five different para digms: across a wideband of frequency, microstructure, input/output fu nction, primary frequency ratio tuning curve, and group delay. The aim was to investigate the vulnerability of these different features to m oderate levels of noise exposure. Nine subjects were exposed to third- octave-band noise. The DPOAE amplitude was reduced frequency specifica lly with the greatest reduction approximately half an octave above the frequency of the noise. The degree of amplitude reduction was greates t 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 s een in group delays. Distinct microstructure was seen in the amplitude against frequency of five out of seven subjects. The maximum to minim um ratio of the microstructure decreased, and the whole pattern shifte d 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 m ultiple reflections, the noise-induced changes were reevaluated. A red uction in maximum to minimum microstructure ratio could be interpreted as a decrease in the internal reflection coefficient. The implication s of these observations for the interpretation of the DPOAE measuremen ts are considered. (C) 1996 Acoustical Society of America.