Echolocation of Extended Surfaces

Abstract

Relatively large surfaces return echo complexes not only to flying bats but to birds that emit flight calls. The spreading loss during the return of a ground echo to a flying bird is only 6 dB, because the total path length has doubled. Over the range from 1 – 6 kHz the ground reflection loss (GRL), or ratio of echo intensity from an actual surface below that to be expected from a perfect reflector, varies even for relatively smooth ground with grass and low vegetation from less than 5 to more than 30 dB, tending to increase with frequency. Due to complicated interference patterns between reflected sound waves from different parts of a large and usually irregular surface, GRL may differ by 10–20 dB from the average value. By utilizing only the peaks, a flying animal could hear components of ground echoes at much greater altitudes than would be predicted from average values. Since these interference patterns are frequency dependent, broad-band signals such as frequency sweeps should be audible to higher altitudes than constant frequencies, as well as containing potentially useful information about the ground or water surface.