Sound transmission in the porpoise head

Abstract

By using small hydrophones implanted in porpoise tissue and an external sound source, pulsed sound was found to be channeled by the blubber coat and strongly reflected by blubber-muscle interfaces. Implants in the head showed concentration of sound in the proposed sound channel of the throat and jaws, complete acoustic isolation in the peribullary air space, and sound focusing in the melon. Sound-velocity measurements in the melon showed a low-velocity core extending from just below the anterior surface toward the right nasal plug and a graded outer shell of high-velocity tissue. Such structure should beam sound generated or transmitted from the area of the superior narial passages forward and impedance match it closely to the surrounding water.Subject Classification: 80.20, 80.50.Toothed whales (suborder Odontoceti), which include porpoises, dolphins, and a number of moderate to largesized whales, uniformly seek out individual prey items, pursue and catch them, often at night or in the deep sea. Such pursuit and capture is greatly aided by active echolocation, perhaps in all odontoeetes. The requirements of echolocation systems operated by these aquatic animals have resulted in many structural and behavioral modifications.Amongst these are transmission of sound from the forehead and upper jaw, •-3 probably entry of sound into the entire blubber coat and its eanalization into new pathways to the middle ear •'+'ø including the lower jaw and throat region, and the generation of at least some sounds in the soft tissue of the nasal passages superior to the cranium. 7 The lipids of the sound-transmission structures are chemically distinct from nearby lipid deposits. s Thus, both sound-sending and reception systems seem grossly modified from the general mammalian plan. Sounds emitted from the forehead are propagated in highly structured fields with the highest frequencies narrowly beamed directly forward, 3,0, 0 which allows the animal to scan its environment by means of associated head movements. Such scanning movements allow not only selective ensonifieation of the environment but refined reception across echo fields, as the surfaces of the jaws, throat, and sides of the head move and produce frequency-related refractive and refleetire effects. • Internal sound reflection and isolation systems in the form of tissue interfaces and air sacs at the same time promote acoustic isolation between transmission and reception.Seaborne sound hitting a porpoise must first hit the blubber coat except at a few loci where this layer is absent or very thin, such as over flippers, flukes, and the fin. Over the proposed acoustic receptive surfaces of jaws and throat, blubber is well developed. Thus, entering sound first encounters a water-blubber interface.Once inside the animal, it will encounter a complex of underlying tissue layers which we can expect to alter its character. What actually occurs is unknown, though it must be crucial to porpoise hearing. However, sound in the blubber coat appears to become channeled over underlying ...

Topographical variation in lipid content and morphological structure of the blubber in the striped dolphin

“…1). In odontocetes, the melon and the large fat bodies found in and around the mandibular region serve as part of the acoustic pathway and are known as the acoustical window for sound transmission (Norris et al 1961, Norris 1968, Norris and Harvey 1974. The biochemical composition of these "acoustic fats bodies" is different to that of body blubber (Varanasi and Malins 1970, Ackman et al 1971, Litchfield et al 1975, Koopman et al 2006.…”

Support for the beam focusing hypothesis in the false killer whale

“…This implies that the melon serves a crucial functional role for odontocetes. Within the melon, the lipids are distributed in such a manner that there is a variable topography of chemical properties and sound speeds, which together help collimate the sound anteriorly (Litchfield and Greenberg, 1973;Norris and Harvey, 1974). Surrounding the melon is a network of facial muscles (Cranford et al, 1996;Heyning, 1989;Huggenberger et al, 2009;Mead, 1975), which may change the size and/or shape of the melon and associated air sacs (Harper et al, 2008;Huggenberger et al, 2009).…”

“…The melon is ovoid in shape and located slightly right of the midline in the forehead (Heyning, 1989). The melon itself has both an outer core, composed of acoustic fats and connective tissue, and an inner core, composed mainly of acoustic fats (Heyning, 1989;Litchfield and Greenberg, 1973;Mead, 1975;Norris and Harvey, 1974). The acoustic fats are composed primarily of wax esters, which are metabolically toxic to the animal.…”

Active echolocation beam focusing in the false killer whale, Pseudorca crassidens

“…Although the size, shape and lipid composition of the melon differs across genera (Litchfield et al, 1973;Cranford et al, 1996), the presence of acoustic fats in the forehead is common to all odontocetes. The distribution and layering of fats inside the melon act to focus the sound towards the anterior portion of the melon and act as an impedance-matching device for sounds traveling into the surrounding seawater (Litchfield et al, 1973;Norris and Harvey, 1974;Malins and Varanasi, 1975). Additionally, the melon is surrounded and traversed by a complex array of muscles and tendons that may act to change the shape of the melon during echolocation (Harper et al, 2008).…”