Support for the beam focusing hypothesis in the false killer whale

Abstract

The odontocete sound production system is complex and composed of tissues, air sacs and a fatty melon. Previous studies suggested that the emitted sonar beam might be actively focused, narrowing depending on target distance. In this study, we further tested this beam focusing hypothesis in a false killer whale. Using three linear arrays of hydrophones, we recorded the same emitted click at 2, 4 and 7 m distance and calculated the beamwidth, intensity, center frequency and bandwidth as recorded on each array at every distance. If the whale did not focus her beam, acoustics predicts the intensity would decay with range as a function of spherical spreading and the angular beamwidth would remain constant. On the contrary, our results show that as the distance from the whale to the array increases, the beamwidth is narrower and the received click intensity is higher than that predicted by a spherical spreading function. Each of these measurements is consistent with the animal focusing her beam on a target at a given range. These results support the hypothesis that the false killer whale is 'focusing' its sonar beam, producing a narrower and more intense signal than that predicted by spherical spreading.

Biosonar signal propagation in the harbor porpoise's (Phocoena phocoena) head: The role of various structures in the formation of the vertical beam

“…Our results do not support the conclusions from Kloepper et al (2012) and Kloepper et al (2015), which suggested adaptive focusing of the echolocation beam and hypothesized that the deformation of the melon causes the biosonar beam to focus. They depicted the melon as an acoustic lens analogous to an optical lens.…”

“…Thus, the popular melon focusing notion is not accurate. Second, with respect to the hypothesis mentioned by Kloepper et al (2012) and Kloepper et al (2015), although the recent data reported the biosonar beam property can be changed by the deformation of animal's forehead (Wisniewska et al, 2015), there could be many factors involved in this complex process (not only by the deformation of melon). The deformation is caused by the action of fibers and tendons of the muscles associated with the melon and it would also alter the shape of other soft tissues in animal's forehead such as the connective tissue.…”

“…There has been new evidences to prove that the animals' biosonar beams are actually not focused in a way suggested by Kloepper et al (2012) and Kloepper et al (2015). The recent work by Finneran et al (2016) demonstrated that focusing was not occurring through high-resolution measurements of the dolphin sonar beam and explained that apparent contradictions between the conclusion from Finneran et al (2016) and the conclusions from Kloepper et al (2012) and Kloepper et al (2015) were likely the result of a misinterpretation of data rather than a difference in biosonar emissions across species. The term "focus" means there is a sign of converging behavior during sound propagation, the odontocetes biosonar beams are directional but not actually focused (converging wave fronts), the term focus is sometimes erroneously used when collimation is meant.…”

“…Furthermore, the shape of the different air sacs can be manipulated by different degrees of inflation to highly affect the shape of the beam. These factors were not considered by Kloepper et al (2012) and Kloepper et al (2015), thus the ideas in the studies are not tenable. It also should be noted that the results in this study only represent the vertical biosonar beam, the roles of structures could be a little different in the horizontal plane during the emission.…”

“…Kloepper et al (2012) and Kloepper et al (2015) suggested that changes in the beam were caused by changes in the shape of the melon affecting its focusing property. Thus, the melon focusing notion has been quite popular although it has never been shown that the gradient in impedance is sufficiently strong to cause sufficient ray bending to have the required focusing property.…”