The objective of this chapter is twofold: to introduce the sounds of interest, i.e. the sounds made by different cetaceans, and to develop techniques suited to describe the different sound categories. I interlace sound descriptions and processing techniques to allow the reader to experiment with the Matlab scripts, maybe to change some parameters, or to visualize the results differently, etc.

I initially classify the sounds made by cetaceans into two categories, according to their observed functionality. For the purposes of PAM, cetacean sounds are conveniently divided into ‘echolocation clicks’ and ‘communication signals’.

Typical echolocation clicks are made by toothed whales during foraging. They are short pulses of significant intensity, highly directional and used for locating food.

Typical communication signals are whistles or complex call sequences emitted by dolphins and baleen whales. They are relatively tonal or pulsed signals with a varying degree of spectral variability and with potentially low directionality. This differentiation does not suggest that echolocation signals are not useful for communication, but it emphasizes a difference in the primary function of the sound.

Cetacean clicks are next described both in time and in the frequency domain; a time–frequency description is developed for tonal and pulsed cetacean vocalizations. This chapter also discusses the directivity of echolocation signals and the expected source levels of cetacean sounds.