Monday, March 26, 2007

Spermaceti Sound

In smaller odontocetes, the tissues involved in sound production are situated above the bony nares and below the blow hole in front of the brain (Mohl 2001). In Sperm Whales, however, the sound-generating structure, the spermaceti sac, has migrated forward and above the melon structure (junk sac) (Mohl 2001). The spermaceti organ, as a whole, acts as a reverberation and sound-focusing chamber; produces pulsed long range echolocation signals (Schenkkan and Purves 1973, Norris and Harvey 1972).

These acoustic pulses are projected, from sperm whales, into the environment through a series of lenses found in the anterior junk (Cranford 1999). Though these structures have different physiological locations within the head, they are anatomically equivalent to the processes and pathways in delphnoids (Cranford 1999). In the porpoise (Tursiops truncatus) glycerides in external tissue act as an acoustic window; in the Delphid (Stenella), sound velocity measure in melon with heterogeneous lipid concentrations; in the Pilot whale, similar heterogeneous lipid composition (Morris 1975).

The sound produced by a sperm whale is composed of different parts. There is a primary event leads to a train of reflected pulses with long enough intervals for the event to travel back and forth between air sacs (reflectors) (Mohl 2001). Then there is the high pressure air from right post-nasal cavity is drawn into the larynx to produce sonar clicks; the sound is then trasmitted to the bones of the rostrum (Schenkkan and Purves 1973). This pattern is a result of a single acoustic pulse reflected multiple times between two acoustic mirrors in the nasal structure (Norris and Harvey 1997).

Observations were made by Mohl (2001) on a pod of 13 beached whales in Denmark as to their sound generating abilities. During these observations he found that a single excitation at the distal sac produced a multi-pulse pattern similar to ones reported from live sperm whales (Mohl 2001). This same pattern was produced by injecting pulses into the junk (Mohl 2001). This suggests that junk and spermaceti sac form an acoustic continuum (Mohl 2001). Pulses injected dorsally (3m behind tip of snout) did not generate this pattern (Mohl 2001). This would then suggest that the purpose of the junk is to transmit the sound and is in acoustic contact with the spermaceti sac which receives it (Mohl 2001).