Echolocation: Measurement of Pitch versus Distance for Sounds Reflected from a Flat Surface

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Abstract
When sound containing many frequencies is reflected from a flat surface, an observer in the field of both the incident and reflected sounds hears a broad tone with an associated pitch that varies inversely with distance from the surface. Measurements made in three experiments have led to an explanation of the phenomenon. First, reference tones were sounded and positions located where their pitches matched those of a sound field produced with thermal noise. Again with thermal noise as a source, the field was carefully analyzed with a sound spectrograph. With an array of discrete frequencies used as the source in the third experiment, the regular array of standing waves thus produced gave rise not only to a pitch change at the barrier, but also to similar pitch changes at points in front of it. The pitch that is heard is a subjective tone produced by the ear from the sound pattern in front of the reflector. This pattern is caused by interference between the incident and reflected sounds. Although the phenomenon can be produced with a very narrow band of frequencies, it cannot be produced with a single pure tone. It is most easily noticed with frequencies ranging from 200 to 2000 cps and does not depend on frequencies above 10 000 cps.

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