| A series of simulated auditory motion experiments was conducted to achieve three goals: (1) to measure human's sensitivity to changes of displacement, velocity, and acceleration, (2) to determine the acoustic cues used by listeners to make these discriminations, and (3) to determine whether velocity and acceleration are inferred from displacement. The acoustic waveforms of a low- and high-velocity sound source were reconstructed over headphones using principles of sound radiation and propagation from a moving source and diffraction theory from a spherical model of the head. Stimuli were synthesized to include three acoustic cues: the Doppler effect, intensity, and interaural time difference (ITD). An adaptive two-interval, forced-choice procedure was used to measure thresholds for detection of a change in the displacement, velocity, and acceleration of the source passing directly in front of the listener in a linear trajectory from left to right. Differential velocity and acceleration thresholds differed among listeners but were not related simply to the thresholds for displacement. A second set of experiments was conducted to determine if the differences in thresholds are due to differences in the cues used to make these discriminations. Stimuli were synthesized as before except the three dynamic cues were independently perturbed from trial to trial so that correlations with listener responses could be computed to estimate the relative weight given to each acoustical cue. There were clear individual differences. In general, for the discrimination of displacement, listeners' responses were most highly correlated with intensity or interaural time difference. For the discrimination of velocity, and to a lesser extent, acceleration, responses were most highly correlated with the Doppler effect. This consistent pattern, which suggests discriminations of velocity and acceleration might differ from the discrimination of displacement, was found for sound sources with low-pass, high-pass, or randomized spectra. |
