Sound transmission in the nose of the sperm whale Physeter catodon . A post mortem study

Abstract

During a sperm whale stranding at Rømø, the Wadden Sea, Denmark, on 4 December 1997, we were notified in time to start acoustic transmission measurements in the spermaceti complex 1 h after the specimen was seen alive. Frequency-modulated sound pulses, sweeping from 30 kHz to 10 kHz in 25 ms, were injected at the frontal surface at two positions: at the distal sac, and at the center of the junk (a compartmentalized structure below the spermaceti organ). A hydrophone next to the projector served as receiver. The analyses of the recordings show a repetitive, decaying reflection pattern at both projection sites, reminiscent of the multi-pulse click peculiar to sperm whales, although with minor differences in the duration of the intra-click intervals. This experimental evidence supports the Norris and Harvey (1972) theory of click generation in the spermaceti organ. Accordingly, the click is composed of a primary event, followed by a train of reflected pulses, spaced by the time required for the event to travel back and forth between air sacs (reflectors) at each end of the organ. The results also show that the junk readily transmits sound and probably is in acoustic contact with the spermaceti organ.