Bats, which echolocate using broadband calls, are believed to employ the passive acoustic filtering properties of the head and pinnae to provide spectral cues which encode 3-D target angle. Microchiropteran species whose calls consist of a single, constant frequency harmonic (i.e., some species in the families Rhinolophidae and Hipposideridae) may create additional acoustic localization cues via vigorous pinna movements. In this work, two types of echolocation cues generated by moving a pair of receivers aboard a model sensor head are investigated. In the first case, it is supposed that a common 3-D echolocation principle employed by all bats is the creation of alternative viewing perspectives, and that constant frequency (CF) echolocators use pinna movement rather than morphology to alter the acoustic axes of their perceptual systems. Alternatively, it is possible rhinolophids and hipposiderids move their ears to create dynamic cues--in the form of frequency and amplitude modulations--which vary systematically with target elevation. Here the use of binaural and monaural timing cues derived from amplitude modulated echo envelopes are investigated. In this case, pinna mobility provides an echolocator with a mechanism for creating dramatic temporal cues for directional sensing which, unlike interaural timing differences, do not degrade with head size.

No abstract

No abstract

Using only two receivers, animals are able to determine the location of a target in three dimensions. Echolocating bats, which employ broadband calls, have access to spectral cues that may be used, in conjunction with interaural disparities of intensity (IIDs) and/or time (ITDs), to encode a 3-D target angle. This work addresses the question: what cues might a tone-emitting echolocator employ to determine target azimuth and elevation? To this end, a 6-degree-of-freedom robotic sensor head was built to investigate the direction cues that might be generated by the systematic pinnae scanning movements used by many species of tone emitting bats. The first strategy investigated was the use of receiver motion to rotate the SONAR horizon through discrete IID sampling positions so that a 3-D target angle can be resolved across IID readings taken at two or more receiver positions. Next, the change in amplitude measured by continuously scanning pinnae was used to create temporal cues, which vary systematically with the target angle. In this scheme, angular resolution depends upon scan speed (ms) rather than interaural separation (microsecond). Finally, the use of Doppler shifts to encode the target angle through the cosine law was investigated.

No abstract