Tissue physical property of the harbor porpoise Phocoena phocoena for investigation of the sound emission process

Abstract

15The process by which sound is propagated in the head of a toothed whale is

Click emission in Dall’s porpoise Phocoenoides dalli, focusing on physical properties of tissues

“…At the ES frontal tip ( x = 31.79mm), the melon acoustic impedance was 1.454×10 6 Pa s/m ( Fig 3 ), closely matching that of seawater, 1.54×10 6 Pa s/m [ 19 ]). This result supports the theory that the melon functions to match the acoustic impedance of seawater for the effective emission of clicks [ 13 – 14 , 20 ]. The distribution of acoustic impedance was more similar to Young’s modulus than it was to density.…”

“…It should be caused by the extremely large difference between the density and Young’s modulus (for example, in DCT, the rate of change of Young’s modulus fluctuated from 400–8000N/m 2 , whereas that of density ranged 0.8–1.2 g/cm 3 ). Conventional study [ 13 ] suggested that focusing distribution of Young's modulus is more important than that of density because when the acoustic impedance changes, the change rate of Young’s modulus would occupy a larger percentage than that of the density. In this study, we supported this idea.…”

“…Young’s modulus, also called as longitudinal elastic modulus is a measure of linear resistance to linear compression and defined as the slope of the linear region of the stress-strain diagram. measurement was performed following our previous study [ 13 ]. Young’s modulus and density were measured from 15 head slices of average thickness 10.97 mm (±0.86 mm SD).…”

“…Phonic lips and dorsal bursae are located on the rostral and caudal walls of the nasal passage; vibration of the dorsal bursae propagates to the melon, a fatty tissue containing connective tissue, which focuses the “sound beam” with the assistance of air sacs, facial muscles, bone and dense connective tissue (DCT) [ 9 , 11 – 12 ]. At the melon’s frontal face the click beam is emitted from a circular aperture, an emitting surface (ES herein, [ 13 ]), comprised of DCT [ 14 ].…”

“…In addition, more acoustic experiments and anatomical studies have been conducted on harbor porpoise [ 9 , 11 , 16 ] than Dall’s porpoises. Previously we measured acoustic impedance in harbor porpoise head tissues, and compared this to seawater in a study that focused on one characteristic of sound propagation: that when acoustic impedance matched between two objects, sound propagated efficiently [ 13 ]. Here, in a similar study, we report the distribution of the acoustic impedance and anatomical structure in the head of Dall’s porpoise.…”