Spectral analysis of sperm whale vocalizations

Sammanfattning

Sperm whales (Physeter macrocephalus) communicate with one another using an intricate language made up of sequences of clicks. Recent studies have begun to dig deeper into the structure and content of these vocalisations in an effort to detect and extract significant patterns and features. Whether or not the results of such research will eventually allow us to translate sperm whale speech into something akin to a human language is a subject of debate. Regardless of the ultimate outcome, the insight is of interest to many scientific fields, including conservation biology and bio-inspired engineering. For this project, over 1400 audio clips recorded between 1952 and 1995 were processed in order to isolate clicks from background noise. Frequency analysis on over 5000 detected clicks yielded dominant frequencies in the range of 2 kHz to 10 kHz with varying frequency distributions. It was possible to group the clicks into distinct categories based on their frequency contents, dominant frequencies, and number of strongly represented frequencies. This paper illustrates the performance of methods for automatically detecting click onsets in noisy data, as well as an effective approach for fitting an estimated signal envelope to clicks based on a Gumbel probability density function. Several patterns in the spectral features of sperm whale clicks were identified. The recurring patterns in the value and number of dominant frequencies in each click suggest the existence of multiple distinct click types, and several examples of potential click categories are described in this paper. There also appears to be a noteworthy relationship between the geographical location, dominant frequency, and spectral envelope parameters of clicks. Further studies will be necessary to validate and refine the results described here, which suggest that sperm whale vocalizations consist of a more rich and complex combinatorial structure than has previously been described and that this structure varies systematically between geographical locations.