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Figures
- Fig. 1.
The experimental set up consists of a linear array of three hydrophones (A, B, C) suspended between a buoy and a lead weight. The distance between the hydrophones is 4 m, and the first hydrophone is at a depth of 4 m. Theclicking animal is localized from the time- of-arrival differences (t1, t2) of the same signal at the three receivers. The rotationally symmetric position of the sound source S(x,y) is given by the interception of the two hyperboloid surfaces (H1, H2). Analogue signals are amplified and band-pass filtered in the conditioning box before digitisation in the Wavebook 512, writing to the memory of a laptop.
- Fig. 2.
Example of an ensonification event, during which an echolocating Grampus scans each of the three receivers (A,B,C). Note how the ensonification moves from hydrophone C to B to A. Full amplitude of the y-axes corresponds to an apparent source level (ASL) of 222 dB re. 1μ Pa (pp). Local maxima on each of the three channels are likely to represent signals on or close to the acoustic axis of the sound beam. X marks a single click displayed in detail in Fig. 3.
- Fig. 3.
Three versions of the same Grampus click X from Fig. 2 are displayed. (A) The presumed on-axis version of the click with a high apparent source level (ASL), short duration (left) and a smooth, broadband spectrum (right). (B) The same click recorded 5° off the recording aspect of A. The duration is longer, the ASL is lower, and the bandwidth and f0 are reduced compared to A. (C) The same click recorded in an aspect of 10° compared to A. The duration has increased, ASL, f0 and bandwidth have decreased compared to A and B, and a number of notches are seen in the spectrum.
- Table 1.
Source parameters from wild and trained Grampus and Pseudorca with maximum values from trained Tursiops for comparison
Grampus Pseudorca Tursiops Parameters Wild Trained Wild Trained Trained SLpp (dB re. 1 μPa, pp) 220 (202—222) 200 (170—216) 220 (201—225) 220 (155—225) 225 SLrms (dB re. 1 μPa, rms) 207 (190—210) NA 208 (190—215) NA 210 SLE (dB re. 1 μPa2 s) 164 (147—166) NA 163 (145—168) NA 167 τ (μ) 40 (30—75) 50 (40—70) 30 (18—55) 30—50 50 f0 (kHz) 75 (58—91) 57 (53—83) 49 (33—68) NA 100 fp (kHz) 49 (42—110) 48 (27—104) 40 (26—79) 30—125 115 RMS-BW (kHz) 25 (19—31) 25 20 (12—29) NA 25 —3 dB BW (kHz) 27 (15—84) 40 (30—84) 35 (15—76) 5—80 50 —10 dB BW (kHz) 66 (20—124) 100 63 (39—89) NA 60 -
Values give range. NA, not available.
SL, source level; pp, peak to peak; rms, root mean square; E, energy flux density; τ, signal duration; f0, centroid frequency; fp, peak frequency; RMS-BW, rms bandwidth.
Data for the trained Grampus are from Philips et al. (2003).
Data for the trained Pseudorcas are from Thomas et al. (1988a), Thomas and Turl (1990), Brill et al. (1992) and Au et al. (1995).
Data for Tursiops are from Au (1993).
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- Fig. 4.
Dynamics of the sound generator. Three examples (A–C) of on-axis clicks from Pseudorca are displayed (top) along with their spectra (bottom). All three clicks consist of one cycle with a short duration. While the fp values are rather constant, it is seen that the f0 values and the bandwidth are positively correlated with the source level.
- Fig. 5.
Audiograms of Grampus (blue line) and Pseudorca (red line) along with representative spectra of on-axis clicks from each species on a relative dB scale. Note that high ambient noise levels masked the maximum sensitivity of the Grampus audiogram, which explains the large difference in threshold between the two species (Nachtigall et al., 1995). Coloured bars signify frequency range of the centroid frequencies of on-axis clicks from the two species recorded in the wild. The Grampus audiogram is from Nachtigall et al. (1995) and the Pseudorca audiogram from Thomas et al. (1988a).
- Table 2.
Estimated detection ranges of different prey items based on detection capabilities of a captive Pseudorca, and the source parameters derived in the present study
Predator SL (dB re. 1 μPa, pp) f0 (kHz) Noise (dB re. 1 μPa2 Hz-1) Prey TS (dB) Detection range (m) Pseudorca 220 50 35 Tuna (1 m) —301 210 Pseudorca 220 50 35 Dolphin (Tursiops) —202 320 Pseudorca 220 50 35 Small squid (20 cm) —503 80 Grampus 220 75 32 Small squid (20 cm) —503 85 Grampus 220 75 32 Large squid (80 cm) —403 130 -
SL, source level; f0, centroid frequency; TS, target strength.
It is assumed that detection is limited by ambient noise, and that the receiving system of a Grampus performs like that of a Pseudorca.
TS for a Tursiops may not represent TS for smaller delphinids preyed upon by Pseudorca, but it is the only available TS for a dolphin.
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↵1 Bertrand et al. (1999)
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↵2 Au (1996)
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↵3 Medwin and Clay (1998)
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