Why do whales communicate at the frequencies they do? Lessons from underwater acoustic signal processing

Abstract

The combination of frequency-dependent attenuation of acoustic energy and the ambient noise field of the deep ocean provides a window of opportunity for acoustic communication ranging from 10 to 1000 Hz. Why do pelagic whale species choose to use center frequencies between 20 and 50 Hz rather than higher frequencies in the window of opportunity? It is not a size limitation, because similar size whales which reside nearby in shallow water have center frequencies of propagation from 400 to 700 Hz. Experience in underwater signal processing has shown that internal wave scattering can be significant for acoustic frequencies above 50 Hz. Here the postulate is presented that deep-water whales (blues and fins) have chosen to send acoustical signals at 20 Hz to minimize internal wave scattering of their acoustic energy. The implications to range of detection will be made from a signal processing point of view and the impact that this would have on marine mammal biology will also be presented. Similar arguments, though based on different physics, will be employed to discuss the shallow-water whales’ (bowhead, humpback, gray) choice of 400 Hz.