Bertel Møhl

Sperm whale clicks: Directionality and source level revisited

In sperm whales (Physeter catodon L. 1758) the nose is vastly hypertrophied, accounting for about one-third of the length or weight of an adult male. Norris and Harvey [in Animal Orientation and Navigation, NASA SP-262 (1972), pp. 397-417] ascribed a sound-generating function to this organ complex. A sound generator weighing upward of 10 tons and with a cross-section of 1 m is expected to generate high-intensity, directional sounds. This prediction from the Norris and Harvey theory is not supported by published data for sperm whale clicks (source levels of 180 dB re 1 microPa and little, if any, directionality). Either the theory is not borne out or the data is not representative for the capabilities of the sound-generating mechanism. To increase the amount of relevant data, a five-hydrophone array, suspended from three platforms separated by 1 km and linked by radio, was deployed at the slope of the continental shelf off Andenes, Norway, in the summers of 1997 and 1998. With this system, source levels up to 223 dB re 1 microPa peRMS were recorded. Also, source level differences of 35 dB for the same click at different directions were seen, which are interpreted as evidence for high directionality. This implicates sonar as a possible function of the clicks. Thus, previously published properties of sperm whale clicks underestimate the capabilities of the sound generator and therefore cannot falsify the Norris and Harvey theory.

Estimating source position accuracy of a large-aperture hydrophone array for bioacoustics

A linear error propagation analysis was applied to a hydrophone array used to locate sperm whales [see Mohl et al., J. Acoust. Soc. Am. 107, 638–648 (2000)]. The accuracy of two-dimensional (2D) and three-dimensional (3D) array configurations was investigated. The precision in source location was estimated as a function of inaccuracies in measurements of sound velocity, time-of-arrival differences (TOADs), and receiver positions. The magnitude of additional errors caused by geometric simplification was also assessed. The receiver position uncertainty had the largest impact on the precision of source location. A supplementary vertical linear array consisting of three receivers gave information on the vertical bearing and distance to the sound sources. The TOAD data from an additional receiver as well as from surface reflections were used to form an overdetermined location system. This system rendered positions within two standard deviations of the estimated errors from the original 3D array.

Target Detection by Echolocating Bats

Echolocating bats form a highly diversified group. Their different types of sonar signals have been proposed as a base for classification and identification (e.g. Simmons and Stein, 1980, Ahlen, 1981). The “design” of the sonar pulses of a given bat species is believed to reflect adaptations or trade-off’s between various properties such as detection sensitivity, ranging, clutter and noise rejection, and inconspicuousness. Operating at the lowest signal to noise ratio, with parameter estimation being higher order processes (Urick, 1983, Altes, 1984) detection can be argued to be the fundamental process of a sonar. Measures of detection sensitivities — or detection thresholds — therefore are informative characteristics of the sonar of a given bat species.

High Intensity Narwhal Clicks

The hypothesis that some odontocetes use their sonar not only to find prey, but also to debilitate it (Norris and Mohl, 1983) requires that odontocetes produce sound pressures in excess of 230 dB re. 1 μPa (Zaegaesky, 1987; Hubbs and Rechnitzer, 1972). While maximum source levels1 (SL) of clicks recorded from trained Tursiops (Au et al., 1974) and Delphinapterus (Au et al., 1987) are only a few dB short of this value, there is a gap of 50 to 120 dB between the debilitation threshold and the SL’s reported for odontocete clicks in nature (Levenson, 1974; Watkins and Schevill, 1974; Watkins, 1980a, b).

A large-aperture array of nonlinked receivers for acoustic positioning of biological sound sources

A system of independent recording units that can be used to form an arbitrarily large acoustic array is described. Position of units and timing of signals are obtained from Global Positioning System (GPS) with precisions within 2.5 m and 50 microseconds, respectively. An integrated hardware and software solution is presented and results reported from a four-unit feasibility test in shallow water. Sound sources at a distance of 2 km were located within 2 to 138 m of GPS-derived positions.

Sound production in neonate sperm whales (L)

Acoustic data from two sperm whale neonates (Physeter macrocephalus) in rehabilitation are presented and implications for sound production and function are discussed. The clicks of neonate sperm whale are very different from usual clicks of adult specimens in that neonate clicks are of low directionality [SL anomaly (0°–90°) <8 dB], long duration (2–12 ms), and low frequency (centroid frequency between 300 and 1700 Hz) with estimated SLs between 140 and 162 dB//1 μPa (rms). Such neonate clicks are unsuited for biosonar, but can potentially convey homing information between calves and submerged conspecifics in open ocean waters at ranges of some 2 km. Moreover, it is demonstrated that sperm whale clicks are produced at the anterior placed monkey lips, thereby substantiating a key point in the modified Norris and Harvey theory and supporting the unifying theory of sound production in odontocetes.

Sperm whales (Physeter catodon L. 1758) do not react to sounds from detonators

A number of observations show that sperm whales (Physeter catodon L. 1758) react to various man-made pulses with moderate source levels. The behavioral responses are described to vary from silence to fear. Click rates of five submerged male sperm whales were measured during the discharge of eight detonators off Andenes, northern Norway. In addition, the behavioral response of a surfaced specimen was observed. Click rates of the submerged whales and the behavior of the surfaced specimen did not change during the discharges with received sound levels of some 180 dB re 1 microPa peRMS. The apparent lack of response to the discharges could be due to similarity between sperm whale clicks and detonations. Accordingly, it can be speculated that the discharges may have been perceived as isolated clicks from conspecifics.

The detection of phantom targets in noise by serotine bats; negative evidence for the coherent receiver.

SummaryUsing a target simulator three serotine bats,Eptesicus serotinus, were trained to judge whether a phantom target was present or absent. The echolocation sounds emitted by the bats during the detection were intercepted by a microphone, amplified and returned by a loudspeaker as an artificial echo, with a delay of 3.2 ms and a sound level determined by the overall gain and cry amplitude. The cry level of each pulse was measured and the echo level received by the bat was calculated. The target was presented in 50% of the trials and the gain adjusted using conventional up/down procedures. Under these conditions between 40 and 48 dB peSPL were required for 50% detection (Figs. 2, 3).In a subsequent experiment the phantom target was masked with white noise (No) with a spectrum level of −113 dB re. 1 Pa·Hz−1/2. The thresholds were increased by 7–14 dB. Energy density (S) of a single pulse was measured and used to estimate S/No, which ranged from 36–49 dB at threshold. Theoretically the coherent receiver model predicts the ratio between hits and false alarms observed for the bats at a S/No of ca. 1–2 dB. Since the bats require 40–50 dB higher S/No (Fig. 3), this is taken as negative evidence for coherent reception (cross correlation).Furthermore, a strong sensitivity to clutter was found since there seemed to exist a fixed relationship between thresholds and clutter level.

Sound Radiation Patterns in the Frequency Domain of Cries from a Vespertilionid Bat

SummaryUltrasonic cries from an immobilized vespertilionid batMyotis daubentoni, were recorded simultaneously in front of the bat (on-axis), and at various off-axis angles. The differences between the on-axis and off-axis spectra were computed and related to the theory of directional emission from a baffled rigid-piston radiator. This theory implies zero radiation at a specific frequency for a given piston diameter and off-axis angle (Fig. 1). The bats difference spectra showed notches of ca. 25 dB with properties as the zeros of the piston theory (Fig. 7). The structure of off-axis cries was found to be qualitatively predictable from the on-axis cries, using the piston theory (Fig. 4). The difference spectra had a complex, oscillatory fine-structure that could not be accounted for by the basic piston theory.

Characteristics of biosonar signals from the northern bottlenose whale, Hyperoodon ampullatus.

The biosonar pulses from free-ranging northern bottlenose whales (Hyperoodon ampullatus) were recorded with a linear hydrophone array. Signals fulfilling criteria for being recorded close to the acoustic axis of the animal (a total of 10 clicks) had a frequency upsweep from 20 to 55 kHz and durations of 207 to 377 μs (measured as the time interval containing 95% of the signal energy). The source level of these signals, denoted pulses, was 175-202 dB re 1 μPa rms at 1 m. The pulses had a directionality index of at least 18 dB. Interpulse intervals ranged from 73 to 949 ms (N = 856). Signals of higher repetition rates had interclick intervals of 5.8-13.1 ms (two sequences, made up of 59 and 410 clicks, respectively). These signals, denoted clicks, had a shorter duration (43-200 μs) and did not have the frequency upsweep characterizing the pulses of low repetition rates. The data show that the northern bottlenose whale emits signals similar to three other species of beaked whale. These signals are distinct from the three other types of biosonar signals of toothed whales. It remains unclear why the signals show this grouping, and what consequences it has on echolocation performance.