Detection of complex echoes in noise by an echolocating dolphin

Abstract

Dolphins echolocate with short broadband acoustic signals that have good time resolution properties. Received echoes are often complex, with many resolvable highlights or components caused by reflection of the incident signal from external and internal boundaries of a target and from different propagational modes within a target. A series of experiments was performed to investigate how dolphins perceive complex echoes. Echoes were produced by a microprocessor-controlled electronic target simulator that captured each emitted click and retransmitted the signal back to the animal after an appropriate time delay. The use of this "phantom" target allowed for precise control of the number of highlights, the time separation between highlights, and the relative amplitudes of highlights in the simulated echoes. An echolocating dolphin was trained to perform a target detection task in the presence of masking noise using these phantom echoes. The properties of simulated echoes were systematically varied, and corresponding shifts in the dolphin's detection threshold were observed, allowing for inferences of how the dolphin perceived echoes. The dolphin performed like an energy detector with an integration time of approximately 264 microseconds.

Clicking for calamari: toothed whales can echolocate squid Loligo pealeii

“…Echosounders are used routinely to survey for squid of commercial interest (Jefferts et al 1987, Goss et al 2001), but it has been debated whether toothed whales are able to detect squid (Fristrup & Harbison 2002) using echolocation clicks of shorter duration and broader bandwidth (Au 1993. We report the first TS measurements of a cephalopod using a toothed whale click and receiver model akin to that of the toothed whale auditory system.…”

“…Squid-eating toothed whale species produce a variety of echolocation clicks of durations from 20 to 300 μs in the frequency range from 15 to 130 kHz (Au 1993, Madsen et al 2005b. One hypothesis to account for the differences in duration of The order of removal of the pen, the eyes, and the beak was alternated between squid echolocation clicks is that species limited in peak power may compensate by producing longer clicks with a higher energy flux density for the same peak pressure (Au et al 1999.…”

“…Little is known about the ranges at which toothed whales detect prey in the wild (Au et al 2004, but the extensive studies on trained delphinids in target detection experiments may offer a useful framework to estimate the range at which a squid of known TS can be detected. Au and coworkers have shown that a trained bottlenose dolphin can detect a 7.6 cm steel sphere at ranges of more than 100 m in noisy shallow water (Au 1993). Madsen et al (2004) combined these data from trained delphinids with click parameters derived from wild false killer whales and Risso's dolphins to estimate the ranges at which different prey targets may be detected by foraging toothed whale predators.…”

“…Available data from auditory experiments with toothed whales show that they have very short integration times (around 264 μs) when detecting echolocation clicks (Au et al 1988). Following Au (1993), we therefore computed the energybased target strength (TS E ) as the ratio between the energy flux density of the incident click and the energy flux density of the returning echo with reference to 1 m from the target in a 264 μs window to give a more relevant measure for assessing the performance of toothed whale biosonar systems: (2) where E r is the back-calculated energy of return at 1 m and E i is the incident energy calculated as: (3) (4) where p is the instantaneous pressure, the integration time τ is 264 μs, and T is the duration of the incident click (in this case always < 264 μs).…”

Frequency and Energy Difference Detection of Dolphin Biosonar Signals Using a Decomposition Algorithm

“…The results of Au et al [4] indicate that dolphins may have developed a unique way to process complex broadband transient sonar (acoustic) echoes for target discrimination. Traditionally, discrimination hypotheses have been developed by examining how dolphins may be using temporal or spectral cues in received echo signals for target discriminations.…”