Inter Pulse lnterval (IPI) measurements on clicks recorded from diving sperm whales have been confirmed to be useful in assessing the whale's body size (Gordon 1991, Goold 1996a, 1996b). Goold (1996a, 1996b) developed a cepstrum-based method to accurately measure the IPIs and to assess the animal size by taking into consideration the acoustic transmission properties if the spermaceti oi1 under different temperature and pressure conditions. To extensively apply IPI measurements, we developed a program, based on our custom real-time Digital Signal Processing Workstation (Pavan et al. 1997), to show in real-time both the spectrogram (spectrum vs time) and the cepstrogram (cepstrum vs time), optimised for this special purpose, of the click sequences received by means of a towed array. The cepstrogram based method resulted very sensitive and capable of displaying low level clicks otherwise difficult to see on the spectrogram and the real-time approach proved very helpful in browsing long recordings as well as in discriminating and counting different whales clicking at the same time. We extended the analysis on sighted and tracked whales to consecutive dives. Our results were consistent throughout whole dives and never showed the scattering of IP1 values previously by other authors, even though we confirmed IPIS slightly varied according to the variation of spermaceti properties with the progression of the dive. Therefore, spot measurements on short sequences of clicks should be considered reliable. Even though the method proved very effective, some problems still remain unresolved, in particular those concerned with the influence of long range sound propagation paths and of the relative position and orientation of the clicking whale in regard to the hydrophone. This research was carried out within a project granted by the Central Inspectorate for Sea Protection of the Italian Ministry of the Environment (1989-1996).
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