Mathematics of pulsed vocalizations with application to killer whale biphonation.

Formulas for the spectra of pulsed vocalizations for both the continuous and discrete cases are rigorously derived from basic formulas for Fourier analysis, a topic discussed qualitatively in Watkins' classic paper on "the harmonic interval" ["The harmonic interval: Fact or artifact in spectral analysis of pulse trains," in Marine Bioacoustics 2, edited by W. N. Tavogla (Pergamon, New York, 1967), pp. 15-43]. These formulas are summarized in a table for easy reference, along with most of the corresponding graphs. The case of a "pulse tone" is shown to involve multiplication of two temporal wave forms, corresponding to convolution in the frequency domain. This operation is discussed in detail and shown to be equivalent to a simpler approach using a trigonometric formula giving sum and difference frequencies. The presence of a dc component in the temporal wave form, which implies physically that there is a net positive pressure at the source, is discussed, and examples of the corresponding spectra are calculated and shown graphically. These have application to biphonation (two source signals) observed for some killer whale calls and implications for a source mechanism. A MATLAB program for synthesis of a similar signal is discussed and made available online.