Search

The auditory filter shape of delphinid odontocetes was previously considered to be typically mammalian constant-quality in which filter bandwidths increase proportionally with frequency. Recent studies with porpoises demonstrate constant-bandwidth portions of the auditory filter. The critical ratios for a bottlenose dolphin were measured between 40 and 120 kHz by behaviorally determining the subject's ability to detect pure tones in the presence of white noise. Critical ratios as a function of frequency were constant, indicating the auditory filter acts as a constant-bandwidth system in this frequency range. Re-analysis of past studies supports these findings, and suggests the delphinid auditory system is best characterized as a constant-Q system below 40 kHz and a constant-bandwidth-like system between 40 kHz and 120 kHz before returning to a constant Q pattern at the highest frequencies.

High-frequency auditory filter shapes of an Atlantic bottlenose dolphin (Tursiops truncatus) were measured using a notched noise masking source centered on pure tone signals at frequencies of 40, 60, 80 and 100 kHz. A dolphin was trained to swim into a hoop station facing the noise/signal transducer located at a distance of 2 m. The dolphin's masked threshold was determined using an up-down staircase method as the width of the notched noise was randomly varied from 0, 0.2, 04, 0.6, and 0.8 times the test tone frequency. The masked threshold decreased as the width of the notched increased and less noise fell within the auditory filter associated with the test tone. The auditory filter shapes were approximated by fitting a roex (p,r(r)) function to the masked threshold results. A constant-Q value of 8.4 modeled the results within the frequency range of 40 to 100 kHz relatively well. However, between 60 and 100 kHz, the 3 dB bandwidth was relatively similar between 9.5 and 10 kHz, indicating a constant-bandwidth system in this frequency range The mean equivalent rectangular bandwidth calculated from the filter shape was approximately 16.0%, 17.0%, 13.6% and 11.3% of the tone frequencies of 40, 60, 80, and 100 kHz.

The focus of this study was to investigate how dolphins use acoustic features in returning echolocation signals to discriminate among objects. An echolocating dolphin performed a match-to-sample task with objects that varied in size, shape, material, and texture. After the task was completed, the features of the object echoes were measured (e.g., target strength, peak frequency). The dolphin’s error patterns were examined in conjunction with the between-object variation in acoustic features to identify the acoustic features that the dolphin used to discriminate among the objects. The present study explored two hypotheses regarding the way dolphins use acoustic information in echoes: (1) use of a single feature, or (2) use of a linear combination of multiple features. The results suggested that dolphins do not use a single feature across all object sets or a linear combination of six echo features. Five features appeared to be important to the dolphin on four or more sets: the echo spectrum shape, the pattern of changes in target strength and number of highlights as a function of object orientation, and peak and center frequency. These data suggest that dolphins use multiple features and integrate information across echoes from a range of object orientations.

The Atlantic bottlenose dolphin has a sonar detection range of 72 m for a 2.54‐cm solid sphere and 113 m for a 7.63‐cm water‐filled sphere in a noise‐limited environment. However, there are many natural environments in which the ambient noise is low and will not mask a dolphin auditory system. The dolphin’s detection range for the same 2.54‐ or 7.62‐cm sphere in a nonmasking environment should be considerably longer than 72 and 113 m, respectively. In order to perform a sonar experiment in an environment where the ambient noise is low, a pool must be extremely large, or an outdoor facility with low ambient noise conditions would need to be available. Another way of approaching this problem is to perform a passive listening experiment in which the signal presented to a dolphin simulates an echo from a specific target. We performed a listening experiment using both pure tones and simulated broadband echo from a 7.62‐cm water‐filled sphere. The detection threshold for narrow‐band tonal signals (audiogram) is compared with the detection threshold for a broadband echo signal in order to determine the relationship between an audiogram and sonar detection. The results obtained with the broadband echo also represent the sonar detection range of a dolphin in a nonmasking environment. [Work suported by ONR.]

Frequency selectivity capabilities of an Atlantic bottlenose dolphin were examined by calculating critical ratios from masked hearing data. Absolute sensitivity to pure tones from 40–140 kHz was measured as a base line, and masked sensitivity was determined for the same signals masked by three levels of white noise (52, 42, and 32 dB re:1 μ Pa2/Hz). Absolute and masked sensitivity were essentially constant between 40 and 120 kHz at each of the masking conditions. Sensitivity decreased approximately 100 dB per octave between 120 and 140 kHz. Critical ratios averaged across noise levels were constant between 40 and 120 kHz, averaging 26 dB. The critical ratio at 140 kHz was 42 dB. This flat trend in critical ratios between 40–120 kHz does not agree with the constant‐Q filter‐bank models used to account for earlier critical ratio and bandwidth measurements for the species [W. W. L. Au and P. W. B. Moore, J. Acoust. Soc. Am. 88, 1635–1638 (1990)], but comparison to other cetacean masked hearing work [Johnson et al., J. Acoust. Soc. Am. 85, 2651–2654 (1989)] indicates that the trend has been recorded before, but not explicitly reported.