Signal processing techniques for acoustic measurement of sperm whale body lengths.

Waveform cross correlation and cepstrum analysis were used to demonstrate possible techniques to measure pulse intervals within sperm whale sonar clicks. The structure of sperm whale clicks takes the form of a series of decaying broadband pulses separated by a time interval that is a function of sound velocity in spermaceti oil and the length of the spermaceti sac within the whales' head. Click signals were bandpass filtered and waveform cross correlation used on the filtered signals to obtain maxima in the correlation function. Such maxima occur when successive pulses within the filtered click waveforms align after time shifting of the replica waveform by integer multiples of the interpulse interval. As an alternative approach, cepstrum analysis was used on the spectra of individual clicks, which were found to contain ripples with periods corresponding to the reciprocal of the interpulse interval. Variable signal quality lead to the conclusion that neither method was reliable for spot measurements of IPIs from individual clicks. However, calculating IPIs by either method for several hundred clicks in 6-min sequences, and smoothing the results with moving averages, allowed realistic mean values to be obtained and interpulse interval trends to be observed with dive time. Interpulse intervals were generally found to decrease with dive time, in accordance with known sound velocity characteristics of spermaceti oil under increasing pressure. Mean values of interpulse intervals obtained by cepstrum analysis for each click sequence were used to estimate body lengths of the respective animals.