On the effects of a subharmonic masker on the loudness of a pure tone

Loudness matching functions between a 1-kHz tone in quiet and in the presence of a 70 dB(SPL) 0.5-kHz subharmonic masker were studied to determine the alterations in loudness due to intensity shifts at the stimulus frequency arising from the generation of the second harmonic of the masker. Such effects are understood to be due to a quadratic nonlinearity occurring in the inner ear transduction process. In the experiment, masked thresholds were measured at 30° phase angle intervals of the stimulus. Matching functions were obtained for phase conditions corresponding to minimum and maximum thresholds. Both the shape of the matching functions and their relative displacement from each other were consistent with the predictions of the vector summation model [Clack et al., J. Acoust. Soc. Am. 52, 536–541 (1972); Schubert, J. Acoust. Soc. Am. 45, 790–791 (1969)]. Assuming that the threshold shifts were due to constructive and destructive interference between the 1-kHz tone and the second harmonic of the masker, a single effective matching function was constructed by means of the vector summation model. This calculation demonstrated the ability of loudness matching to probe nonlinear cochlear mechanisms. Matching functions generated from several proposed loudness functions were compared with the data.