Segregation of ambiguous pulse-echo streams and suppression of clutter masking in FM bat sonar by anticorrelation signal processing

Abstract

Big brown bats often fly in conditions where the density and spatial extent of clutter requires a high rate of pulse emissions. Echoes from one broadcast still are arriving when the next broadcast is sent out, creating ambiguity about matching echoes to corresponding broadcasts. Biosonar sounds are widely beamed and impinge on the entire surrounding scene. Numerous clutter echoes typically are received from different directions at similar times. The multitude of overlapping echoes and the occurrence of pulse-to-echo ambiguity compromises the bat’s ability to peer into the upcoming path and determine whether it is free of collision hazards. Bats have to associate echoes with their corresponding broadcasts to prevent ambiguity, and off-side clutter echoes have to be segregated from on-axis echoes that inform the bat about its immediate forward path. In general, auditory streaming to resolve elements of an auditory scene depends on differences in pitch and temporal pattern. Bats use a combination of temporal and spectral pitch to assign echoes to “target” and “clutter” categories within the scene, which prevents clutter masking, and they associate incoming echoes with the corresponding broadcast by treating the mismatch of echoes with the wrong broadcast as a type of clutter. [Supported by ONR.]