Whitlow W. L. Au

Hearing by whales and dolphins

1 Hearing in Whales and Dolphins: An Overview.- 2 Cetacean Ears.- 3 In Search of Impulse Sound Sources in Odontocetes.- 4 Communication and Acoustic Behavior of Dolphins and Whales.- 5 Acoustics and Social Behavior of Wild Dolphins: Implications for a Sound Society.- 6 The Auditory Central Nervous System of Dolphins.- 7 Electrophysiological Measures of Auditory Processing in Odontocetes.- 8 Psychoacoustic Studies of Dolphin and Whale Hearing.- 9 Echolocation in Dolphins.- 10 Acoustic Models of Sound Production and Propagation.

Measurement of echolocation signals of the Atlantic bottlenose dolphin, Tursiops truncatus Montagu, in open waters

The echlocation signals of two Atlantic bottlenose dolphins, Tursiops truncatus, were measured while the animals were involved in a target‐detection experiment conducted in open waters. The time intervals between successive pulses in a pulse train were found to be highly variable, although the intervals were longer than the time needed for an acoustic signal to travel from the animals to the target and back. Sound pressure levels of the echoranging signals were measured for target ranges of 60, 70, 75, and 80 yds. The peak‐to‐peak click‐source level at 1 yd showed little variation with the target range; the average level was 120.4 dB re 1 μbar for one animal and 122.3 dB for the other. These open‐water sound pressure levels are at least 30 dB higher than any click‐source levels reported in the literature. Oscilloscope photographs and their Fourier transforms of these high‐amplitude clicks are presented. The typical clicks had average durations of 40 μsec, with peak energies between 120 and 130 kHz, much h...

The Acoustics of the Snapping Shrimp Synalpheus Parneomeris in Kaneohe Bay

Snapping shrimp are among the major sources of biological noise in shallow bays, harbors, and inlets, in temperate and tropical waters. Snapping shrimp sounds can severely limit the use of underwater acoustics by humans and may also interfere with the transmission and reception of sounds by other animals such as dolphins, whales, and pinnipeds. The shrimp produce sounds by rapidly closing one of their frontal chela (claws), snapping the ends together to generate a loud click. The acoustics of the species Synalpheus paraneomeris was studied by measuring the sound produced by individual shrimp housed in a small cage located 1 m from an H-52 broadband hydrophone. Ten clicks from 40 specimens were digitized at a 1-MHz sample rate and the data stored on computer disk. A low-frequency precursor signature was observed; this previously unreported signature may be associated with a “plunger” structure which directs a jet of water forward of the claw during a snap. The peak-to-peak sound pressure level and energy flux density at 1 m (source level and source energy flux density) varied linearly with claw size and body length. Peak-to-peak source levels varied from 183 to 189 dB re: 1 μPa. The acoustic power produced by a typical snap was calculated to be about 3 W. A typical spectrum of a click had a low-frequency peak between 2 and 5 kHz and energy extending out to 200 kHz. The spectrum of a click is very broad with only a 20-dB difference between the peak and minimum amplitudes across 200 kHz. A physical model of the snapping mechanism is used to estimate the velocity, acceleration, and force produced by a shrimp closing its claws.

Audiogram of a harbor porpoise (Phocoena phocoena) measured with narrow-band frequency-modulated signals

The underwater hearing sensitivity of a two-year-old harbor porpoise was measured in a pool using standard psycho-acoustic techniques. The go/no-go response paradigm and up-down staircase psychometric method were used. Auditory sensitivity was measured by using narrow-band frequency-modulated signals having center frequencies between 250 Hz and 180 kHz. The resulting audiogram was U-shaped with the range of best hearing (defined as 10 dB within maximum sensitivity) from 16 to 140 kHz, with a reduced sensitivity around 64 kHz. Maximum sensitivity (about 33 dB re 1 microPa) occurred between 100 and 140 kHz. This maximum sensitivity range corresponds with the peak frequency of echolocation pulses produced by harbor porpoises (120-130 kHz). Sensitivity falls about 10 dB per octave below 16 kHz and falls off sharply above 140 kHz (260 dB per octave). Compared to a previous audiogram of this species (Andersen, 1970), the present audiogram shows less sensitive hearing between 2 and 8 kHz and more sensitive hearing between 16 and 180 kHz. This harbor porpoise has the highest upper-frequency limit of all odontocetes investigated. The time it took for the porpoise to move its head 22 cm after the signal onset (movement time) was also measured. It increased from about 1 s at 10 dB above threshold, to about 1.5 s at threshold.

Transmission beam pattern and echolocation signals of a harbor porpoise (Phocoena phocoena).

The transmission beam pattern of an echolocating harbor porpoise (Phocoena phocoena) was measured in both the vertical and horizontal planes. An array of seven Bruel and Kjaer 8103 hydrophones connected to an amplifier-line driver module was used to measure the beam patterns. The porpoise was trained to station in a hoop and echolocate a cylindrical target located at a range between 7 and 9 m while the array was located 2 m in front of the hoop. The 3-dB beamwidth in both the vertical and horizontal planes was the same at approximately 16 degrees and the beam was pointed toward the forward direction. The individual hydrophones in both the vertical and horizontal arrays measured signal waveforms that were similar throughout the 40-degree span of the array. The porpoise emitted signals with intervals that were 20 to 35 ms longer than the round trip travel time between the animal and the target. The average source level, peak frequency, and bandwidth were 157 dB, 128 kHz, and 16 kHz, respectively.

Demonstration of adaptation in beluga whale echolocation signals

The echolocation signals of the same beluga whale (Delphinapterus leucas) were measured first in San Diego Bay, and later in Kaneohe Bay, Oahu, Hawaii. The ambient noise level in Kaneohe Bay is typically 12-17 dB greater than in San Diego Bay. The whale demonstrated the adaptiveness of its biosonar by shifting to higher frequencies and intensities after it was moved to Kaneohe. In San Diego, the animal emitted echolocation signals with peak frequencies between 40 and 60 kHz, and bandwidths between 15 and 25 kHz. In Kaneohe, the whale shifted its signals approximately an octave higher in frequencies with peak frequencies between 100 and 120 kHz, and bandwidths between 20 and 40 kHz. Signal intensities measured in Kaneohe were up to 18 dB higher than in San Diego. The data collected represent the first quantitative evidence of the adaptive capability of a cetacean biosonar system.

An ecological acoustic recorder (EAR) for long-term monitoring of biological and anthropogenic sounds on coral reefs and other marine habitats

Keeping track of long-term biological trends in many marine habitats is a challenging task that is exacerbated when the habitats in question are in remote locations. Monitoring the ambient sound field may be a useful way of assessing biological activity because many behavioral processes are accompanied by sound production. This article reports the preliminary results of an effort to develop and use an Ecological Acoustic Recorder (EAR) to monitor biological activity on coral reefs and in surrounding waters for periods of 1 year or longer. The EAR is a microprocessor-based autonomous recorder that periodically samples the ambient sound field and also automatically detects sounds that meet specific criteria. The system was used to record the sound field of coral reefs and other marine habitats on Oahu, HI. Snapping shrimp produced the dominant acoustic energy on the reefs examined and exhibited clear diel acoustic trends. Other biological sounds recorded included those produced by fish and cetaceans, which also exhibited distinct temporal variability. Motor vessel activity could also be monitored effectively with the EAR. The results indicate that acoustic monitoring may be an effective means of tracking biological and anthropogenic activity at locations where continuous monitoring by traditional survey methods is impractical.

Echolocation signals and transmission beam pattern of a false killer whale (Pseudorca crassidens)

The echolocation transmission beam pattern of a false killer whale (Pseudorca crassidens) was measured in the vertical and horizontal planes. A vertical array of seven broadband miniature hydrophones was used to measure the beam pattern in the vertical plane and a horizontal array of the same hydrophones was used in the horizontal plane. The measurements were performed in the open waters of Kaneohe Bay, Oahu, Hawaii, while the whale performed a target discrimination task. Four types of signals, characterized by their frequency spectra, were measured. Type-1 signals had a single low-frequency peak at 40 +/- 9 kHz and a low-amplitude shoulder at high frequencies. Type-2 signals had a bimodal frequency characteristic with a primary peak at 46 +/- 7 kHz and a secondary peak at 88 +/- 13 kHz. Type-3 signals were also bimodal but with a primary peak at 100 +/- 7 kHz and a secondary peak at 49 +/- 9 kHz. Type-4 signals had a single high-frequency peak at 104 +/- 7 kHz. The center frequency of the signals were found to be linearly correlated to the peak-to-peak source level, increasing with increasing source level. The major axis of the vertical beam was directed slightly downward between 0 and -5 degrees, in contrast to the +5 to 10 degrees for Tursiops and Delphinapterus. The beam in the horizontal plane was directed forward between 0 degrees and -5 degrees.(ABSTRACT TRUNCATED AT 250 WORDS)

The broadband social acoustic signaling behavior of spinner and spotted dolphins

Efforts to study the social acoustic signaling behavior of delphinids have traditionally been restricted to audio-range (<20 kHz) analyses. To explore the occurrence of communication signals at ultrasonic frequencies, broadband recordings of whistles and burst pulses were obtained from two commonly studied species of delphinids, the Hawaiian spinner dolphin (Stenella longirostris) and the Atlantic spotted dolphin (Stenella frontalis). Signals were quantitatively analyzed to establish their full bandwidth, to identify distinguishing characteristics between each species, and to determine how often they occur beyond the range of human hearing. Fundamental whistle contours were found to extend beyond 20 kHz only rarely among spotted dolphins, but with some regularity in spinner dolphins. Harmonics were present in the majority of whistles and varied considerably in their number, occurrence, and amplitude. Many whistles had harmonics that extended past 50 kHz and some reached as high as 100 kHz. The relative amplitude of harmonics and the high hearing sensitivity of dolphins to equivalent frequencies suggest that harmonics are biologically relevant spectral features. The burst pulses of both species were found to be predominantly ultrasonic, often with little or no energy below 20 kHz. The findings presented reveal that the social signals produced by spinner and spotted dolphins span the full range of their hearing sensitivity, are spectrally quite varied, and in the case of burst pulses are probably produced more frequently than reported by audio-range analyses.

Echolocation transmitting beam of the Atlantic bottlenose dolphin

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 animals 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 animals 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.