Acoustic properties of humpback whale songs

A vertical array of five hydrophones was used to measure the acoustic field in the vertical plane of singing humpback whales. Once a singer was located, two swimmers with snorkel gear were deployed to determine the orientation of the whale and position the boat so that the array could be deployed in front of the whale at a minimum standoff distance of at least $10m$⁠. The spacing of the hydrophones was $7m$ with the deepest hydrophone deployed at a depth of $35m$⁠. An eight-channel TASCAM recorder with a bandwidth of $24kHz$ was used to record the hydrophone signals. The location (distance and depth) of the singer was determined by computing the time of arrival differences between the hydrophone signals. The maximum source level varied between individual units in a song, with values between 151 and $173dB$ $re$ $1μPa$⁠. One of the purposes of this study was to estimate potential sound exposure of nearby conspecifics. The acoustic field determined by considering the relative intensity of higher frequency harmonics in the signals indicated that the sounds are projected in the horizontal direction despite the singer being canted head downward anywhere from about 25° to 90°. High-frequency harmonics extended beyond $24kHz$⁠, suggesting that humpback whales may have an upper frequency limit of hearing as high as $24kHz$⁠.