Estimating bearings to low-frequency baleen whale calls with closely spaced hydrophones

When processing data from acoustic tags attached to whales, an important question is whether a detected vocalization comes from the tagged whale or a nearby conspecific. For higher frequency species, such as clicking odontocetes, bearings to detected sounds, extracted from the time difference of arrival of the signals on stereo hydrophones, has been used to select clicks on a consistent bearing and assign them to the focal animal. We investigated whether acoustic bearing estimates using tags could be obtained for the lower frequency vocalizations of singing humpback whales and southern right whales. For singing humpback whales, where the signal-to-noise ratio was high (often > 30 or 40 dB) and there was significant frequency content above 1 or 2 kHz, bearings could be accurately measured, with typical TDOA errors being as low as 1 ms, giving bearing errors <2 deg. For right whales, however, there was less high-frequency content in the calls, and SNR was lower (<20 dB), we were unable to extract useful bearing information.