The Sonar Signal Transmission System

Abstract

We will now begin to examine another major subsystem of the dolphin sonar, the signal transmission system. In this chapter we will first discuss the general characteristics of sonar signals used by odontocetes, both in open waters and in enclosed tanks, in order to gain an appreciation of the wide variety of signals that are used. The topic of biosonar (biological sonar) signals will be briefly covered in this chapter, mainly as a prelude to the subject of sound production mechanisms. Characteristics of dolphin sonar signals will be more thoroughly discussed in Chapter 7. Although we are generally concerned with sonar signals used by dolphins, we need to pay some attention to other types of acoustic signals they produce. Bottlenose and other dolphins emit a wide variety of sounds not used for active sonar searches. Sound emissions can be classified into two broad categories of narrow-band frequency-modulated (FM) continuous tonal sounds referred to as whistles and broadband sonar clicks (Evans 1967). Whistles appear to be used for intraspecific communications (Herman and Tavolga 1980). These sounds are generally low frequency emissions between 5 and 30 kHz and may last for several seconds. They are often referred to as squeaks, squawks, and squeals, as well as whistles.