Auditory filter nonlinearity across frequency using simultaneous notched-noise masking.

Psychoacoustic masking experiments have been widely used to investigate cochlear function in human listeners. Here we use simultaneous notched-noise masking experiments in normal hearing listeners to characterize the changes in auditory filter shape with stimulus level over the frequency range 0.25-6 kHz. At each frequency a range of fixed signal levels (30-70 dB SPL) and fixed masker levels (20-50 dB SPL spectrum level) are used in order to obtain accurate descriptions of the filter shapes in individual listeners. The notched-noise data for individual listeners are fitted with two filter shape models: a rounded exponential (roex) shape in which the filter skirt changes as a linear function of probe-tone level and the other, in which the gain of the tip filter relative to the filter tail changes as a function of signal level [Glasberg and Moore, J. Acoust. Soc. Am. 108, 2318-2328 (2000)]. The parameters for these fitted models are then described with a simple set of equations that quantify the changes in auditory filter shape across level and frequency. Both these models fitted the data equally well and both demonstrated increasing tip-tail gain as frequency increased.