Off-axis effects on the multi-pulse structure of sperm whale coda clicks

Sperm whale (Physeter macrocephalus) clicks have a multi-pulse structure, a result of the reflection of sound energy between air sacs in the spermaceti organ. Although previous research revealed that usual clicks (used for echolocation) recorded away from a vocalizing whale's longitudinal axis have waveforms with poorly defined pulse structures, it has been unknown whether sperm whale coda clicks (used for communication) show similar off-axis effects. To address this knowledge gap, a hydrophone array was used to localize vocalizing sperm whales, and the waveforms of coda clicks recorded from different aspects were examined. Coda clicks recorded close to the whale's acoustic axis showed well-defined multi-pulsed waveforms, while those recorded off-axis did not. As for usual clicks, this suggests that sound energy radiates directly into the water upon reflection off the frontal sac.