NTT Basic Res. Labs., 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-01, Japan
Roy D. Patterson
MRC Applied Psychology Unit, Cambridge CB2 2EF, UK
A theory of asymmetric intensity enhancement around acoustic transients is presented. Experiments with ramped and damped tones [R. D. Patterson, J. Acoust. Soc. Am. 93, 2293 (A) (1993)] have shown that reversing an asymmetric envelope changes the timbre of the sound. A ``delta-gamma'' filter is introduced to produce asymmetric intensity enhancement around transients to explain the change in perception. The filter is the derivative of a gamma function, (cursive beta)G[sub m](f,t)/(cursive beta)t. Sound envelope changes are then detected by the sign of (cursive beta)G[sub m](f,t)/(cursive beta)t*|s(t)|, for sound envelope |s(t)|. If it is assumed that the onset of a sound is more important biologically than what comes shortly thereafter, then this onset information can be enhanced by the filter at the expense of suppressing the following part during the negative value of the delta-gamma filter. It is shown that this filter can be implemented as a difference between the original and delayed envelopes of the output of the gammatone filter. The compression, suppression, and envelope estimation processes in the auditory periphery do not affect the sign of the delta-gamma filter. The theory can explain the experimental results obtained with damped and ramped tones.