All acoustic samples were taken using a calibrated omni-directional hydrophone Aquarian H2a (sensitivity −191.56 dB re 1 V/Pa; frequency response 25–20,000 Hz) connected to a ZoomH1 Handy recorder (sampling rate 44.1 kHz, 16-bit) operating on batteries. The hydrophone was previously calibrated thanks to a pistonphone
(Brüel & Kjær Type 4228 and Brüel & Kjær 2238 Mediator) using a sound source operating at 250 Hz, results agreed with the manufacturer's declared sensitivity.
Data were further calibrated according to the manufacturer's specifications.