  Acoustic waves in fluids undergoing the transition from sub- to supersonic
flow satisfy governing equations similar to those for light waves in the
immediate vicinity of a black hole event horizon. This acoustic analogy has
been used by Unruh and others as a conceptual model for `Hawking radiation.'
Here we use variational methods, originally introduced by Brizard for the study
of linearized MHD, and ray phase space methods, to analyze linearized acoustics
in the presence of background flows. The variational formulation endows the
evolution equations with natural Hermitian and symplectic structures that prove
useful for later analysis. We derive a $2\times 2$ normal form governing the
wave evolution in the vicinity of the `event horizon.' This shows that the
acoustic model can be reduced locally (in ray phase space) to a standard
(scalar) tunneling process weakly coupled to a unidirectional non-dispersive
wave (the `incoming wave'). Given the normal form, the Hawking `thermal
spectrum' can be derived by invoking standard tunneling theory, but only by
ignoring the coupling to the incoming wave. Deriving the normal form requires a
novel extension of the modular ray-based theory used previously to study
tunneling and mode conversion in plasmas. We also discuss how ray phase space
methods can be used to change representation, which brings the problem into a
form where the wave functions are less singular than in the usual formulation,
a fact that might prove useful in numerical studies.
