On Some Properties of Low Frequency Sonar Signals of the Dolphin Phocoena phocoena

The study of low frequency sonar signals emitted by Phocoena phocoena on approaching a target, or in avoiding obstacles, shows two types of signals:

– an impulsive shaped one, usually called “clicks”,

– a short one containing some cycles of a frequency-modulated wave.

The spectral characteristics of these two types of signals, show two different properties. The click signal contains a large spectral band, the modulated one, a relatively concentrated spectral density.

This last point suggests that this type of signal is advantageous in long distance propagation. The ambiguity function of these two types of low frequency signals, reveals that both have a strong tolerance to the Doppler effect.

Such signals may be compared with the signal emitted by animals possessing a sonar system and placed in an unknown environment.

For this purpose we have studied the evolution of the sonar emission of a naive Ph. ph. placed in a pool, unknown to him.

During this experiment, some of the observed signals are closely related to a category of signals which provide optimal transmission in a noisy and reverberant medium.