ASA/CAA '05 Meeting, Vancouver, BC

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Hearing Silent Shapes: Identifying the Shape of a Sound-Occluding Surface

Lawrence Rosenblum -
Ryan Robart
University of California, Riverside
Riverside, CA 92521

Popular version of paper 5aPP2
Presented Friday morning, May 20, 2005
Joint ASA/CAA Meeting, Vancouver, BC

Most research on auditory perception has examined how we hear things that make sound. However, our hearing is also sensitive to properties of 'silent' objects that reflect and obstruct sound. This sensitivity is evidenced by the vast resources devoted to acoustic engineering of concert halls as well as the simulation of acoustic spaces by recording engineers. More formally, there is experimental evidence that humans can detect and behave with regard to objects that reflect or occlude sound (e.g., Rice, 1967; Rosenblum & Gordon, 2004). Impressively, human hearing is sensitive enough to determine the rough shape, size, and material of sound-reflecting objects (Rice, 1967).

Our current experiments examine whether human listeners can determine the rough shape of sound-occluding objects. Our results show that listeners can identify the shape of sound-occluding objects at better than chance levels, with some listeners displaying near perfect performance.

In our first experiment, eighteen University of California, Riverside undergraduates were first shown the three shapes they would be attempting to identify. The three shapes were a triangle, disc, and square cut from 3.81cm thick sound-insulating foam board and covered with black tape (see figure).

Each of the shapes had a surface area of 7575 cm2. The participants were also shown an array of eight horn-style loudspeakers which the shapes would be occluding. Participants were told that during the experiment, the loudspeakers would be emanating a white noise sound and that the shapes would be positioned to obstruct this sound. They were instructed that on each trial, they were to identify which shape was positioned in front of the loudspeakers based on how the shape obstructed the sound. Participants were then blindfolded and were presented a total of 60 randomized trials (20 for each shape) each of which lasted 10 seconds.

Results showed that on average, the square, circle, and triangle were correctly identified 38.89%, 41.11%, and 50.83% of the time, respectively. Statistically, each of the shapes was recognized at levels significantly greater than chance (which is 33.3%), and the triangle was recognized more accurately than either the square or circle.

A second experiment was conducted to determine the importance of sound intensity in making these judgments. It could be that listeners in the first experiment used differences in overall acoustic intensity to distinguish the shapes. To examine this possibility, the loudspeaker array was set up to emanate two levels of white noise.

Nineteen undergraduates were recruited to identify the three sound-occluding shapes for a total of 90 randomized trials (each of the three shapes was heard occluding each of the two white noise intensities 15 times). All other aspects of this experiment were the same as for the first experiment.

Results revealed that the mean percentage correct identification scores for the square, circle, and triangle were 52.95, 48.63, and 52.95 for the quieter white noise intensity, and 41.84, 48.63, and 61.37 for the louder intensity, respectively. Statistically, each of the shapes was recognized at levels significantly greater than chance for both the quieter and louder white noise intensities. Interestingly, two of the participants in this experiment showed extraordinary performance on the task, displaying mean judgments in the 90% range.

The fact that identification accuracy was not depressed when participants could no longer rely on a single loudspeaker intensity (as they did in the first experiment) suggests that our listeners were using acoustic information other than overall intensity for their shape judgments. Future research will be directed at determining the salient acoustic information for determining the shape of sound-obstructing objects. The outcome of this research should lead to a better understanding of how human hearing is sensitive to the sounds structured by silent objects. This understanding should help inform acoustic engineering as well as lead to better designs of sensory aids and rehabilitation programs for visually impaired individuals.

Gordon, M.S. & Rosenblum L.D. (2004), Perception of sound-obstructing surfaces using body-scaled judgments. Ecological Psychology 16 (2), 87-113. Rice, C. E. (1967). Human echo perception. Science, 155, 656-664.

This research was supported by a University of California Intramural grant awarded to the second author.

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