Acoustic radiation from the head of echolocating harbor porpoises

An experiment was conducted to investigate the sound pressure patterns on the melon of odontocetes by using four broadband hydrophones embedded in suction cups to measure echolocation signals on the surface of the forehead of two harbor porpoises (Phocoena phocoena). It has long been hypothesized that the special lipids found in the melon of odontocetes, and not in any other mammals, focus sounds produced in the nasal region that then propagate through the melon, producing a beam that is directional in both the horizontal and vertical planes. The results of our measurements supported the melon-focusing hypothesis, with the maximum click amplitude, representing the axis of the echolocation beam, located approximately 5.6–6.1 cm from the edge of the animal’s upper lip along the midline of the melon. The focusing is not sharp but is sufficient to produce a transmission beam of about 16°. Click amplitude dropped off rapidly at locations away from the location of site of maximum amplitude. Based on comparisons of forehead anatomy from similar sized porpoises, the beam axis coincided with a pathway extending from the phonic lips through the axis of the low-density/low sound velocity lipid core of the melon. The significant interaction between click number and hydrophone position suggests that the echolocation signals can take slightly different pathways through the melon, probably as a result of how the signals are launched by the production mechanism and the position of the acoustically reflective air sacs.Keywords: melon, contact hydrophone, sound pressure level, echolocation signals, harbor porpoise, Phocoena phocoena, lipi

A New Acoustic Portal into the Odontocete Ear and Vibrational Analysis of the Tympanoperiotic Complex

Global concern over the possible deleterious effects of noise on marine organisms was catalyzed when toothed whales stranded and died in the presence of high intensity sound. The lack of knowledge about mechanisms of hearing in toothed whales prompted our group to study the anatomy and build a finite element model to simulate sound reception in odontocetes. The primary auditory pathway in toothed whales is an evolutionary novelty, compensating for the impedance mismatch experienced by whale ancestors as they moved from hearing in air to hearing in water. The mechanism by which high-frequency vibrations pass from the low density fats of the lower jaw into the dense bones of the auditory apparatus is a key to understanding odontocete hearing. Here we identify a new acoustic portal into the ear complex, the tympanoperiotic complex (TPC) and a plausible mechanism by which sound is transduced into the bony components. We reveal the intact anatomic geometry using CT scanning, and test functional preconceptions using finite element modeling and vibrational analysis. We show that the mandibular fat bodies bifurcate posteriorly, attaching to the TPC in two distinct locations. The smaller branch is an inconspicuous, previously undescribed channel, a cone-shaped fat body that fits into a thin-walled bony funnel just anterior to the sigmoid process of the TPC. The TPC also contains regions of thin translucent bone that define zones of differential flexibility, enabling the TPC to bend in response to sound pressure, thus providing a mechanism for vibrations to pass through the ossicular chain. The techniques used to discover the new acoustic portal in toothed whales, provide a means to decipher auditory filtering, beam formation, impedance matching, and transduction. These tools can also be used to address concerns about the potential deleterious effects of high-intensity sound in a broad spectrum of marine organisms, from whales to fish

Project report of virtual experiments in Marine bioacoustics: model validation

A series of finite element model simulations are compared against results from various real world marine bioacoustics experiments with the bottlenose dolphin. Three significant results are revealed. 1) Changes in relative position of fat bodies can adjust echolocation beam direction. This is the first evidence of this. 2) Beam direction is consistent despite several elements being present within the sound transmission system within the dolphin's forehead. This suggests that the skull is the primary structural element in the formation of the sound transmission beam, with other elements playing a major role in concentrating or focusing the outgoing beam. 3) There is evidence for focusing of the beam in stages. The model simulations illustrate the narrowing of the sound transmission beam with various level of refinement in structural complexity. It appears as if structures like the melon and air space individually affect the narrowing of the beam, with their combined contributions being significant. All of these results are aligned with, or similar to, results obtained from live animals performing in psychoacoustic experiments over the past fifty years.N00244-09-1-0072

Fin whale sound reception mechanisms: skull vibration enables low-frequency hearing.

Hearing mechanisms in baleen whales (Mysticeti) are essentially unknown but their vocalization frequencies overlap with anthropogenic sound sources. Synthetic audiograms were generated for a fin whale by applying finite element modeling tools to X-ray computed tomography (CT) scans. We CT scanned the head of a small fin whale (Balaenoptera physalus) in a scanner designed for solid-fuel rocket motors. Our computer (finite element) modeling toolkit allowed us to visualize what occurs when sounds interact with the anatomic geometry of the whale's head. Simulations reveal two mechanisms that excite both bony ear complexes, (1) the skull-vibration enabled bone conduction mechanism and (2) a pressure mechanism transmitted through soft tissues. Bone conduction is the predominant mechanism. The mass density of the bony ear complexes and their firmly embedded attachments to the skull are universal across the Mysticeti, suggesting that sound reception mechanisms are similar in all baleen whales. Interactions between incident sound waves and the skull cause deformations that induce motion in each bony ear complex, resulting in best hearing sensitivity for low-frequency sounds. This predominant low-frequency sensitivity has significant implications for assessing mysticete exposure levels to anthropogenic sounds. The din of man-made ocean noise has increased steadily over the past half century. Our results provide valuable data for U.S. regulatory agencies and concerned large-scale industrial users of the ocean environment. This study transforms our understanding of baleen whale hearing and provides a means to predict auditory sensitivity across a broad spectrum of sound frequencies

Left lateral view of the skull bones in a fin whale (<i>Balaenoptera physalus</i>).

<p>Some display transparency has been applied to the squamosal bone so that the tympanic bulla and the dense bony ossifications or “anchors” become visible. Bony skull components that are visible in this orientation are the: occipital (yellow), parietal (white), frontal (red), maxillary (green), squamosal (magenta), tympanic bullae (green), and the “anchors” (white). The dense bony anchors fan out dorsolaterally within the squamosal bones of the skull (see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0116222#pone.0116222.g002" target="_blank">Fig. 2</a>). In this lateral view, the adjacent periotic bones are not visible because they are obscured by the anchors and tympanic bulla.</p

Posterior view of both tympanoperiotic complexes in a fin whale (<i>Balaenoptera physalus</i>), with some display transparency applied.

<p>The periotic bones (yellow) and tympanic bullae (green) are components of each TPC. The dense bony fan-like projections (cyan) are contained within the bones of the skull (salmon). Specifically, the anchors fan out as dense ossifications within the squamosal bone, from a locus at the junction with the juxtaposed periotic bones, and may function to stiffen the connection between the periotic and the skull. The mandibles (pink) are shown for context.</p

Transverse section through the otic region in the head of a fin whale calf.

<p>The bony projections that “anchor” the tympanoperiotic complexes to the skull are cyan. The brain is blue, the skull is salmon colored, and the mandible is pink. The periotic portions of the TPC are yellow, and each tympanic bulla is green. Note that thin bony pedicles form a fulcrum for differential vibration between the periotic bones and the large hypermineralized masses of the tympanic bulla at the distal end of each involucrum.</p

Predicted audiograms for the fin whale calf.

<p>The solid blue line represents the audiogram for the <b><i>pressure mechanism</i></b>. The red dashed line represents the audiogram for the <b><i>bone conduction mechanism</i></b>. The solid black line shows the combined audiograms for the <b><i>pressure</i></b> and <b><i>bone conduction</i></b> mechanisms.</p

Angular oscillation of solid scatterers in response to progressive planar acoustic waves: do fish otoliths rock?

Fish can sense a wide variety of sounds by means of the otolith organs of the inner ear. Among the incompletely understood components of this process are the patterns of movement of the otoliths vis-à-vis fish head or whole-body movement. How complex are the motions? How does the otolith organ respond to sounds from different directions and frequencies? In the present work we examine the responses of a dense rigid scatterer (representing the otolith) suspended in an acoustic fluid to low-frequency planar progressive acoustic waves. A simple mechanical model, which predicts both translational and angular oscillation, is formulated. The responses of simple shapes (sphere and hemisphere) are analyzed with an acoustic finite element model. The hemispherical scatterer is found to oscillate both in the direction of the propagation of the progressive waves and also in the plane of the wavefront as a result of angular motion. The models predict that this characteristic will be shared by other irregularly-shaped scatterers, including fish otoliths, which could provide the fish hearing mechanisms with an additional component of oscillation and therefore one more source of acoustical cues