Auditory filter shapes at low frequencies

Abstract

Auditory filter shapes were measured in normally hearing subjects for center frequencies (fc) of 100, 200, 400, and 800 Hz using the notched-noise method [R. D. Patterson and I. Nimmo-Smith, J. Acoust. Soc. Am. 67, 229–245 (1980)]. Two noise bands, each 0.4fc wide, were used; they were placed both symmetrically and asymmetrically about the signal frequency, to allow the measurement of filter asymmetry. The overall noise level was always 80 dB SPL Stimuli were delivered monaurally using Sennheiser HD424 earphones. Except for fc = 100 Hz, the auditory filters were asymmetric, the upper skirt being steeper than the lower skirt. At 100 Hz the filters were more symmetric, or showed an asymmetry in the opposite direction. This can probably be attributed to the reduced effectiveness of the lower noise band due to the transmission characteristics of the outer and middle ear. The equivalent rectangular bandwidths of the filters had average values of approximately 29, 42, 74, and 160 Hz for values of fc of 100, 200, 400, and 800 Hz, respectively. The signal-to-masker ratio at the output of the filter required to achieve threshold tended to increase with decreasing fc.