Abstract
In this work we report an illusion of proximity of a sound source created by a sonic crystal placed between the source and a listener. This effect seems, at first, paradoxical to naïve listeners since the sonic crystal is an obstacle formed by almost densely packed cylindrical scatterers. Even when the singular acoustical properties of these periodic composite materials have been studied extensively (including band gaps, deaf bands, negative refraction, and birrefringence), the possible perceptual effects remain unexplored. The illusion reported here is studied through acoustical measurements and a psychophysical experiment. The results of the acoustical measurements showed that, for a certain frequency range and region in space where the focusing phenomenon takes place, the sonic crystal induces substantial increases in binaural intensity, direct-to-reverberant energy ratio and interaural cross-correlation values, all cues involved in the auditory perception of distance. Consistently, the results of the psychophysical experiment revealed that the presence of the sonic crystal between the sound source and the listener produces a significant reduction of the perceived relative distance to the sound source.
Highlights
The task of reconstructing the auditory space from the pressure waves arriving at the eardrums is one of the most challenging operations that our brains can perform
A simple way to understand the principle behind this negative refraction focusing phenomenon is to give it a geometric acoustics interpretation: sound rays emanating from the source are inverted along the tangent direction when entering into the sonic crystal, and inverted back when leaving
The results reported here show that the presence of a sonic crystal between an acoustic source and a listener creates an illusion of proximity, and that this occurs for a certain range of frequencies corresponding to the focusing phenomenon
Summary
The task of reconstructing the auditory space from the pressure waves arriving at the eardrums is one of the most challenging operations that our brains can perform. Under certain circumstances, our brains can be tricked, and the localization and spatial features of the sources can be manipulated without changing the actual sources of sound. An echo from a distant wall, a reflection from a curved surface or the formation of a “creeping wave” along a wall (as happens in the whispering gallery effect [4, 5]), dramatically alters our spatial perception of the sound sources. In this last case, the speech from a distant talker might be heard as being emitted from a nearby location or even from inside our heads
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