Time Frequency Analysis of Dolphin Clicks Reveals Fine Temporal Structure
One of the most effective ways for a dolphin to perceive its environment for the purpose of prey detection is to use its active and passive (listening) sonar capabilities. Acoustic energy propagates in water more efficiently than any other form of energy, so that an active sonar capability is ideal for life in an aquatic environment. Dolphin sound emissions can be classified into two broad categories: narrow-band-frequency-modulated continuous tonal sounds, referred to as whistles, and broadband sonar clicks that have durations between 50 and 200μs (Evans 1973). Passing air back and forth through a set of nasal passages, which creates a narrow beam of sound, first produces the click sound. This sound is then passed through a fat-filled cavity in the head called the melon. In the melon, the sound is culminated so that the dolphin can actively direct the clicks without wasting energy due to scattering the sound. When searching for a target, dolphins usually emit bursts of clicks referred to as a click train. The interval between clicks appears to be a function of the distance from the target. Other factors such as the difficulty of detecting a target, the presence or absence of a target of interest, and the animal's expectation of finding a specific target also affect the click train.