Bottlenose dolphin (Tursiops truncatus) echolocation beams are typically characterized as symmetrical -3 dB beamwidths; however, the functional width of the beam during target detection has not been explored. Angular target detection thresholds of an echolocating dolphin were examined to more fully describe the functional characteristics of the echolocation beam. The dolphin performed an echolocation detection task with its head held in a fixed orientation. Targets were placed 9 m in front of the dolphin [0 degrees position (P(0))] and systematically moved right or left until target detection reached chance probability. A 24-element hydrophone array placed 1 m in front of the dolphin was used to measure vertical and horizontal echolocation beamwidths. Detection thresholds were 26 degrees left and 21 degrees right of P(0) for a sphere target and 19 degrees left and 13 degrees right of P(0) for a cylinder target. Estimates of maximum horizontal and vertical beamwidths ranged up to 40 degrees and 29 degrees , respectively, and exhibited large variability. The dolphin nominally steered the maximum response axis of the echolocation beam up to 18 degrees in the horizontal, 12 degrees in the upward vertical, and 4 degrees in the downward vertical. These results suggest that the dolphin can steer and modify the width of the echolocation beam.

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.

The transmitting beam patterns of echolocation signals emitted by an Atlantic bottlenose dolphin Tursiops truncatus were measured in the vertical and horizontal planes with an array of seven hydrophones. Particular emphasis was placed on accurately verifying the animal's position on a bite-plate/tail-rest stationing device using underwater video monitoring equipment. The major axis of the vertical beam was directed at an angle of 5 degrees above the plane defined by the animal's lips. This angle was 15 degrees lower than previously measured. The vertical beam measurements indicate that the major axis of the transmitting beam is aligned with the major axis of the receiving beam. The horizontal beam was directed forward. The directivity index of 26.5 dB calculated from the beam pattern measured in both planes agreed well with previous calculation of 25.4 dB.

Masked underwater pure-tone thresholds were obtained for an Atlantic bottlenose dolphin using an up–down staircase method of stimulus presentation and a go/no-go response paradigm. Two types of masking noise were used: a broadband noise and variable bandwidth noise with sharp low- and high-frequency cutoffs. The animal’s critical ratio was measured at frequencies of 30, 60, 90, 100, 110, 120, and 140 kHz. For frequencies of 100 kHz and below, the critical ratios were similar to those measured by Johnson [J. Acoust. Soc. Am. 44, 965–967 (1968)]. The dolphin’s critical bandwidth at frequencies of 30, 60, and 120 kHz was measured with the variable bandwidth noise. The critical bandwidth was 10.4 dB (11 times) wider than the critical ratio at 30 kHz, 8.2 dB (6.6 times) wider at 60 kHz, and 3.5 dB (2.2 times) wider at 120 kHz.

No abstract

This paper uses advanced time-frequency signal analysis techniques to generate new models for bio-inspired sonar signals. The inspiration comes from the analysis of bottlenose dolphin clicks. These pulses are very short duration, between 50 and 80 micros, but for certain examples we can delineate a double down-chirp structure using fractional Fourier methods. The majority of clicks have energy distributed between two main frequency bands with the higher frequencies delayed in time by 5-20 micros. Signal syntheses using a multiple chirp model based on these observations are able to reproduce much of the spectral variation seen in earlier studies on natural dolphin echolocation pulses. Six synthetic signals are generated and used to drive the dolphin based sonar (DBS) developed through the Biosonar Program office at the SPAWAR Systems Center, San Diego, CA. Analyses of the detailed echo structure for these pulses ensonifying two solid copper spherical targets indicate differences in discriminatory potential between the signals. It is suggested that target discrimination could be improved through the transmission of a signal packet in which the chirp structure is varied between pulses. Evidence that dolphins may use such a strategy themselves comes from observations of variations in the transmissions of dolphins carrying out target detection and identification tasks.

Dolphins demonstrate an adaptive control over echolocation click production, but little is known of the manner or degree with which control is exercised. Echolocation clicks (N approximately 30,000) were collected from an Atlantic bottlenose dolphin (Tursiops truncatus) performing object discrimination tasks in order to investigate differential click production. Seven categories of clicks were identified using the spectral conformation and relative position of -3 and -10 dB peaks. A counterpropagation network utilizing 16 inputs, 50 hidden units, and 8 output units was trained to classify clicks using the same spectral variables. The network classified novel clicks with 92% success. Additional echolocation clicks (N > 24,000) from two other dolphins were submitted to the network for classification. Classified echolocation clicks were analyzed for animal specific differences, changes in predominant click type within click trains, and task-related specificity. Differences in animal and task performance may influence click type and click train length.

Aerial and underwater audiograms for two young female northern fur seals (Callorhinus ursinus) and one young female California sea lion (Zalophus californianus) were obtained with the same procedure and apparatus. Callorhinus hears over a larger frequency range and is more sensitive to airborne sounds than Zalophus or any other pinniped thus far tested in the frequency range of 500 Hz to 32 kHz. Sensitivity of Callorhinus to waterborne pure tones, ranging from 2 to 28 kHz, is equal or superior to all other pinnipeds tested in this same frequency range. Like Zalophus, the upper frequency limit for underwater hearing (as defined by Masterton et al. 1969) in Callorhinus is about one‐half octave lower than the three phocid species thus far tested. Callorhinus' upper frequency limit in air is about 36 kHz and under water it is about 40 kHz. Comparison of air and water audiograms shows Callorhinus is no exception to previous behavioral findings demonstrating that the „pinniped ear” is more suitable for hearing in water than in air. Similar to Zalophus and Phoca vitulina, Callorhinus shows an anomalous hearing loss at 4 kHz in air. The basis for this insensitivity to airborne sounds at 4kHz and not at lower or higher frequencies is presumably caused by specialized middle ear mechanisms matching impedance for waterborne sounds. Critical ratio curves for Callorhinus are similarly shaped to ones obtained for humans but are shifted upwards in frequency. Compared to all other marine mammals thus far evaluated, the critical ratios for Callorhinus are the smallest yet reported.