ASA 127th Meeting M.I.T. 1994 June 6-10

4aPP6. Discrimination of time-asymmetric-modulated noise.

Michael A. Akeroyd

Roy D. Patterson

MRC Appl. Psychol. Unit, 15 Chaucer Rd., Cambridge CB2 2EF, UK

Modulated noises with asymmetric temporal envelopes have identical long-term power spectra, yet sound very different. A ``ramped'' noise---constructed by amplitude modulating a wideband noise with a repeating exponential rise function---sounds like a repeating noise, whereas a ``damped'' noise---constructed with the same modulation function, but time-reversed---sounds like a snare drum struck by brushes. A damped noise has none of the usual timbre associated with a wideband noise. In experiment 1, listeners reliably discriminated a damped noise from a ramped noise, for modulation half-lives from 2 to 64 ms at modulation frequencies of 10 and 20 Hz. At higher frequencies (40 and 80 Hz) performance deteriorated. The maximum noise spectrum level was 52 dB SPL. The experiment was replicated at 32 and 12 dB SPL, and gave essentially the same results. It is proposed that the sounds' timbre was used by listeners as the decision statistic. [Work supported by an MRC Research Studentship to the first author and by a DRA grant.]