Simultaneous in-air versus in-water measurements of humpback whale breathing sounds

Abstract

Humpback whales produce an assortment of sounds ranging from moans and songs to surface-breaching, breathing, and foreflipper flaps. During a sea trial of Northeastern University’s coherent hydrophone array at the Great South Channel in US Northeastern coastal waters, the HF subaperture consisting of 32 hydrophone elements at 0.375 m spacings was deployed vertically with half of the hydrophones in air and the other half submerged underwater on September 6th, 2021. Many instances of humpback whale breathing sounds were recorded over several hours of observation. Visual sightings and video recordings were used to coregister sounds recorded on individual hydrophones of the HF subaperture. Here, we examine and compare the simultaneously recorded humpback whale breathing sound spectra, bandwidth, and duration for in-air versus underwater hydrophones. Whale distances to the vertical HF subaperture could be calculated from curved time of arrival differences due to close proximity of the array. These distances are applied to correct the received underwater sound pressure level for transmission loss and estimate underwater recorded whale breathing sound source level. A consistent broadband dip in the measured underwater sound spectra with null centered at 15 kHz is investigated by propagation modelling, considering both modal interference and attenuation from loud bubbles.