Frank Grasso
David C. Mountain
Dept. of Biomed. Eng., Boston Univ., 44 Cummington St., Boston, MA 02215
Certain natural acoustic transients produce broad and relatively featureless spectra. Spectrographic analysis has revealed that rustling leaves, breaking sticks, and flowing water, which are likely to have shaped the evolution of the mammalian auditory system, are examples of this class of transients. Both types of signals are composed of the superposition of smaller transients, but breaking sticks produce short duration components which are separated by brief silences while the rustle of leaves and flowing water consist of frequent, overlapping components. These differences in the temporal characteristics form a perceptual basis for the discrimination of these signals. It is found that these transients are easily classified by human listeners provided that a sample of adequate length is given. The results imply that classification of these acoustic transients must be based on temporal features instead of spectral features. A second implication is that the spectrum of the signal at the tympanic membrane will be shaped more by filtering due to the path of propagation from the source to the listener than the nature of the acoustic source. [Research supported by ONR.]