The purpose of this chapter is to outline and illustrate some of the basic conceptual models of sound transmission through a variable sound-speed field, with a particular emphasis on internal-wave-induced fluctuations. The objective is not to focus on rigorous derivations but instead to bring forward physical concepts. This approach involves simplifications and idealizations that will be mentioned but discussed in more detail only in subsequent chapters. Here the first objective is to describe the origin of phase and amplitude fluctuations using the important ray-like process of wave front distortion and the subsequent generation of microrays and interference. From the mode perspective, it is seen how mode coupling generates a form of modal multipathing and interference, the result of which is acoustic phase and amplitude fluctuations. In the limit of strong ray or modal multipathing, the notion of saturation is described where the acoustic field behaves like Gaussian random noise.

These conceptual models bring forward immediately the notion of acoustical resonances, such that the acoustical field is sensitive to select internal waves with specific space and time scales. These resonances will be a function of ray propagation angle and location in the water column, and in the mode formalism they will be a function of acoustic mode number and frequency. From the ray perspective, fully developed ray chaos modifies the simple resonance conditions. With a conceptual understanding of the propagation physics, some of the statistical characteristics of the signals that have been observed in experiments can be described. These signal statistics are described in terms of propagation regimes denoted by the terms unsaturated, partially saturated, and fully saturated.

Origin of Phase and Amplitude Fluctuations

Here building upon a wave front picture of acoustic propagation, the origin of phase and amplitude fluctuations are described. This wave front picture fits naturally into the problems of deep–water, long-range propagation, and high-frequency applications. In one way or another acoustic fluctuations result from interference effects, and in the ray picture one can envision this interference occurring due to extra ray paths that are generated by the sound-speed fluctuations. These rays are termed microrays. In the mode picture, which is strongly relevant to the shallow-water environment, interference occurs from mode coupling that creates new paths for acoustic energy to travel down range in mode space. The simplest case of weak fluctuations is addressed first.