Source levels of humpback whales decrease with frequency suggesting an air-filled resonator is used in sound production

Abstract

Source level and frequency are important in determining how far an acoustic signal can travel. However, in some species these sound characteristics have been found to be biomechanically linked, and therefore cannot be modified independently to achieve optimal transmission. This study investigates the variability in source levels and their relationship with frequency in the songs of humpback whales (Megaptera novaeangliae). Songs were recorded off eastern Australia using a fixed hydrophone array deployed on the whales' migratory corridor. Singing whales were acoustically tracked. An empirical, frequency-dependent model was used to estimate transmission loss. Source levels and frequency were measured for 2408 song units from 19 singers. Source levels varied from 138 to 187 dB re 1 μPa at 1 m (root mean squared), while peak frequency ranged between 52 and 3877 Hz. Much of the variability in source levels was accounted for by differences between the unit types, with mean source levels for each unit type varying by up to 17 dB. Source levels were negatively correlated with peak frequency and decreased by 2.3 dB per octave. The negative correlation between source levels and frequency is consistent with the presence of an air-filled resonator in the whales' sound production system.

The effects of noise on the vocal behaviour of singing humpback whales (Megaptera novaeangliae)

“…In this case, source level and frequency cannot be modified independently. In this chapter, published by Girola et al (2019), the source levels and the spectral content of humpback whale songs were measured to investigate how these acoustic characteristics vary for different unit types and to assess whether they are linked or not. A detailed description of the methodology used for the analysis is also provided, which is critical for evaluating the accuracy of the estimated parameters.…”

“…Girola, E., Noad, M. J., Dunlop, R. A. and Cato, D. H. (2019). Source levels of humpback whales decrease with frequency suggesting an air-filled resonator is used in sound production.…”

“…and Girola et al (2019). In particular, recordings were selected if no anthropogenic noise could be detected.…”

“…The analyses were carried out following the procedures described in Chapter 2 and by Girola et al (2019), and summarised here. For each song unit, the location of the singing whale was estimated from the difference in the time-of-arrival of the unit at the different buoys using Ishmael (Mellinger, 2002).…”

“…The fundamental frequencies are between 30 Hz and 5 kHz, and harmonics have been described up to at least 24 kHz (Hafner et al, 1979;Mednis, 1991;Au et al, 2006). Mean-squared source levels vary between 138 and 187 dB re 1 µPa at 1 m (Cato et al, 2001;Au et al, 2006;Smith, 2009), and are statistically different between unit types (Chapter 2 and Girola et al, 2019).…”