Thermodynamic Behaviour of Acoustic Modes
* Presenting author
Abstract:
Thermodynamics provides a physical description of systems with a relatively large number of hidden degrees of freedom. It deals with approximate holistic (macroscopic) representations of true indeterministic and interacting microscopic phenomena, taking into account the ignorance of their observer. In this broad sense the theory is applicable to purely acoustic modes even in a homentropic fluid, when they cannot (or need not) be determined exactly. For example, it has already been noted in literature that the flow of sound energy in the basic statistical energy analysis (SEA) is equivalent to the flow of heat in the state of canonical thermodynamic equilibrium with maximal entropy. In this work we use a fundamental result of quantum thermodynamics to derive the standard equation of state for an ideal gas by considering only acoustic modes in it, without reference to an external thermodynamic theory of the fluid. We also discuss a possibility for extending the applicability of SEA to lower entropy states, that is, non-diffuse sound fields. The entropy difference between such states and the canonical equilibrium introduces a simple correction of the sound level estimated with the usual diffuse-field assumption.