Time and frequency domain characteristics of sperm whale clicks

Abstract

Regular clicks from diving sperm whales, both large bull males and smaller females, were recorded in deep oceanic water off the Azores and subsequently sampled to computer disks for digital analysis. A total of 8540 clicks were marked and analyzed. Simple temporal analysis of the interclick intervals during feeding dives revealed mean click rates for male sperm whales of 1.1713 s-1 and 1.9455 s-1 for females. Fourier analysis showed distinctive peaks in the spectra of bull male sperm whales at 400 Hz and 2 kHz which were stable over extended periods of up to 20 mins. The clicks contained higher frequency components with energy ranging up to at least 12 kHz but not concentrated at any sharply defined frequency. The clicks of smaller female sperm whales showed similar spectral peaks, shifted to 1.2 and 3 kHz, respectively, but these peaks were less pronounced than those in the male click spectra and less stable with time. Higher frequencies were also present up to at least 15 kHz. The previously reported multiple pulse structure of sperm whale clicks is confirmed, but digital filtering reveals this structure to be frequency dependent. Analysis using the short-time Fourier transform confirms the complex time-frequency structure of individual clicks. The frequencies at which the multiples emerge in male and female clicks supports the idea of air cavities in the sperm whale head acting as sound reflectors, although the magnitude of the second pulse at high frequencies suggests some form of off axis distortion. It is also possible that air cavity resonance in the head of the sperm whale may act to reinforce the high-frequency components of the click, and that such components may have superior range and resolution performance in terms of echolocation.

Sperm whale behaviour indicates the use of echolocation click buzzes ‘creaks’ in prey capture

“…Clicks during creaks have an initial inter-click interval of ca. 0.2 s, which decreases to 0.02 s (Goold & Jones 1995). The amplitude of these clicks also appears to decline rapidly, with clicks at the end of the creak more than 20 dB lower in level than the regular clicks (see figure 2; Madsen et al 2002b).…”

“…A clicking whale often pauses for several seconds following a creak before resuming regular clicking. Based upon the acoustic features of these click sequences, creaks appear to be analogous to the terminal buzzes produced by echolocating bats as they close on targets (Goold & Jones 1995;see fig. 8 in Griffin 1958).…”

“…Following the discovery that sperm whales produce intense broadband clicks during dives (Worthington & Schevill 1957), most researchers have argued that sperm whales forage using echolocation (Norris & Harvey 1972;Whitehead & Weilgart 1991;Goold & Jones 1995;Jaquet et al 2001;Whitehead 2003). Sperm whales produce regular clicks (called 'usual' clicks by Whitehead & Weilgart (1990)) at inter-click intervals of 0.5-2.0 s during descent from the surface ( Jaquet et al 2001;Zimmer et al 2003), until the whale begins its ascent to the surface (Madsen et al 2002a).…”

A comparison of model and non-model based time-frequency transforms for sperm whale click classification

“…This was done in order to isolate two dominant frequencies (Goold & Jones, 1995). The separation of the data in a low-and high frequency band followed the same protocol as the filtering in the preprocessing of the data.…”

“…The first approach was motivated by the results of Goold & Jones (1995), who showed that clicks from a single click train can have fairly constant dominant frequencies in at least two different frequency bands. For a male sperm whale, these frequencies can be found around 500 Hz and 2000 Hz.…”