Detailed analysis of two detected overlaying transient components within the echolocation beam of a bottlenose dolphin (Tursiops truncatus).

Josefin Starkhammar,Isabella Reinhold,Maria Sandsten,Dorian S Houser,Patrick W Moore

doi:10.1121/1.5096640

Abstract

Dolphin echolocation clicks measured far off-axis contain two time-separated components. Whether these components overlap and appear as a single signal on axis has received little attention. Here, the scaled reassigned spectrogram analysis was used to examine if bottlenose dolphin (Tursiops truncatus) clicks measured near- or on-axis of the echolocation beam contained overlapping components. Across click trains, the number of overlapping components spatially varied within the echolocation beam. Two overlapping components were found to predominantly occur in the upper portion of the beam, whereas the lower portion of the beam predominantly contained a single component. When components overlapped, the trailing component generally had a higher center frequency and arrived less than 5 μs after the leading component. The spatial relationship of components was consistent with previous findings of two vertically distinct beam lobes with separated frequency content. The two components in the upper portion of the beam possibly result from a single transient click propagating through a geometrically dispersive media; specifically, the slower sound speed of the dolphin melon's core slightly delays the more directional, high frequency energy of the click, whereas the less directional, lower frequency energy propagates through more peripheral but higher sound speed portions of the melon.

Highlights

Measurements of beluga (Delphinapterus leucas) and bottlenose dolphin (Tursiops truncatus) echolocation signals at extreme off-axis angles have been demonstrated to contain two transient components (Au et al, 2012a,b; Finneran et al, 2014; Lammers and Castellote, 2009)
Time differences decreased as the measurement angle decreased toward the main response axis (MRA) of the beam until the measured signal close to the MRA appeared to consist of just one signal
For the case with the bottlenose dolphin, the second of the two signal components measured at extreme off-axis angles had a higher center frequency than the first

Summary

Introduction

Measurements of beluga (Delphinapterus leucas) and bottlenose dolphin (Tursiops truncatus) echolocation signals at extreme off-axis angles have been demonstrated to contain two transient components (Au et al, 2012a,b; Finneran et al, 2014; Lammers and Castellote, 2009). Time differences decreased as the measurement angle decreased toward the main response axis (MRA) of the beam until the measured signal close to the MRA appeared to consist of just one signal. For the case with the bottlenose dolphin, the second of the two signal components measured at extreme off-axis angles had a higher center frequency than the first. Using the timing differences between the off-axis components, various hypotheses have been proposed as to the origin of the odontocete echolocation click. Lammers and Castellote (2009) concluded from their off-axis click measurements that two active sources combine to generate the echolocation click, a proposition that has been supported by Cranford (2011) and Cranford et al

Methods

Results

Conclusion