Time-frequency and advanced frequency estimation techniques for the investigation of bat echolocation calls.

In this paper, techniques for time-frequency analysis and investigation of bat echolocation calls are studied. Particularly, enhanced resolution techniques are developed and/or used in this specific context for the first time. When compared to traditional time-frequency representation methods, the proposed techniques are more capable of showing previously unseen features in the structure of bat echolocation calls. It should be emphasized that although the study is focused on bat echolocation recordings, the results are more general and applicable to many other types of signal.

The allocation of energy to echolocation pulses produced by soprano pipistrelles (Pipistrellus pygmaeus) during the wingbeat cycle.

Soprano pipistrelles exhibit considerable plasticity in both the structure and rate of echolocation call production. In search phase in the laboratory, calls are produced either as a single pulse per wingbeat cycle, or as double pulses. The amplitude of double pulses is reduced compared to the preceding single pulse. The energy flux density of either pulse of a double pulse per wingbeat was lower than a single pulse per wingbeat, and is achieved by a combination of reduction in both duration and amplitude. The combined energy of the double pulses is not significantly greater than the single pulse which precedes it. The decision to produce a double pulse may be an indication of the bat requiring additional information from the target. The production of double pulses per wingbeat may serve to achieve a higher rate of information flow for no significant increase in energetic expenditure when a possible target is detected. The proposal that echolocation during flight comes for free as a by-product of the intimate coupling among wingbeat, respiration, and echolocation is discussed.

Echolocation signal structure in the Megachiropteran bat Rousettus aegyptiacus Geoffroy 1810.

Rousettus aegyptiacus Geoffroy 1810 is a member of the only genus of Megachiropteran bats to use vocal echolocation, but the structure of its brief, click-like signal is poorly described. Although thought to have a simple echolocation system compared to that of Microchiroptera, R. aegyptiacus is capable of good obstacle avoidance using its impulse sonar. The energy content of the signal was at least an order of magnitude smaller than in Microchiropteran bats and dolphins (approximately 4 x 10(-8) J m(-2)). Measurement of the duration, amplitude and peak frequency demonstrate that the signals of this animal are broadly similar in structure and duration to those of dolphins. Gabor functions were used to model signals and to estimate signal parameters, and the quality of the Gabor function fit to the early part of the signal demonstrates that the echolocation signals of R. aegyptiacus match the minimum spectral spread for their duration and amplitude and are thus well matched to its best hearing sensitivity. However, the low energy content of the signals and short duration should make returning echoes difficult to detect. The performance of R. aegyptiacus in obstacle avoidance experiments using echolocation therefore remains something of a conundrum.