  We explore an analytic model of the accretion shock in the post bounce phase
of a core-collapse supernova explosion. We find growing oscillations of the
shock in the l=1 and l=2 modes, in agreement with a variety of existing
numerical simulations. For modest values of the ratio of the outer accretion
shock to that of the inner boundary to the shocked flow, the instability
appears to derive from the growth of trapped sound waves, whereas at higher
values, postshock advection clearly plays a role. Thus the model described here
may relate to the different mechanisms of instability recently advocated by
Blondin & Mezzacappa, and by Foglizzo and collaborators.
