Automatic measurement of sperm whales size: cepstrum analysis through the clicks acoustic energy.

The Sperm whale produces continuously short sound signals, called clicks, to recognize the environment and for the intraspecies communication. Each click has a multipulse structure. The Inter Pulse Intervals (IPIs) represent the travel time of the sound between the air sacs present inside the sperm whale’s head. This characteristic offers the possibility to estimate acoustically the sperm whale size, by measuring the stable delay between the pulses (Nominal IPI). We started from the idea to estimate dimensions through the average of cepstrum analysis for a large number of clicks and we introduced a new method, considering the acoustic energy of the clicks. The script was developed in MATLAB platform. The user must set the analysis parameters, select the numbers of sperm whales recorded, the audio channel, the type of frequency filter and the threshold level to find clicks. The script allows to identify clicks by their acoustic energy, to extract the corresponding time intervals and to carry out a cepstrum analysis, both in waveform and in energy. Finally, it elaborates the average of cepstrum results for all clicks extracted and selects the more representative delays. The goal is the choice of the Nominal IPI values comparing waveform and energy results, with an automatic evaluation of reliability. First tests of this software were realized with the NEMO OvDE station dataset, which is an INFN-LNS project developed in the Mediterranean Ionian Sea. We have chosen the period with more sperm whales recordings and we compared the results with the manual cepstrum analysis, using SeaPro software developed by CIBRA. For the estimation of the animals’ sizes we used different formulas in bibliography. This work has allowed us to analyse the sperm whales population structure in a strategic area. During the months of October 2005 and 2006 we found only adult animals.