Support for the beam focusing hypothesis in the false killer whale

Abstract

The odontocete sound production systemiscomplex and composed of tissues, air sacs and a fattymelon. 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 distanceand calculated the beamwidth, intensity, center frequencyand 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 narrowerand 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 killerwhale is 'focusing' its sonar beam, producing a narrower and more intense signal than that predicted by spherical spreading.