The narrower the channel of communication between the interior of a vessel and the external medium, the greater does the independence become. Such cavities constitute resonators; in the presence of an external source of sound, the contained air vibrates in unison, and with an amplitude dependent upon the relative magnitudes of the natural and forced periods, rising to great intensity in the case of approximate isochronism. When the original cause of sound ceases, the resonator yields back the vibrations stored up as it were within it, thus becoming itself for a short time a secondary source. The theory of resonators constitutes an important branch of our subject.
—Lord Rayleigh 
Dissipation is ever present in real devices but is not a fundamental phenomenon. It arises because real physical systems are never completely isolated and thus interact with their surroundings. Real physical systems are open. This view of dissipation is reflected in the usual way it is dealt with in quantum mechanics,1 which we described in Chapter 4. Instead of treating the open system directly, we treat the enlarged system composed by it and its environment. This large system is closed, so we can apply quantum mechanics in the usual way. The effect of dissipation then comes to light when we trace over the inaccessible degrees of freedom of the environment.
Cavities are no exception. A real cavity is always dissipative. In this chapter we consider this dissipation ...