  We study the acoustic field enhancement through a circular hole in a hard
plate of finite thickness in this work. We derive the boundary-integral
equations corresponding to the acoustic scattering to investigate the
Fabry-Perot type resonant frequencies that induce the field enhancement. The
asymptotic expansions of Fabry-Perot type resonances with respect to the size
of the hole are derived, and the quantitative analysis of the field enhancement
caused by the configuration at the resonant frequencies is presented with both
the enhancement order and the shapes of resonant modes characterized. The
transmission in the nonresonant quasi-static regime is also investigated.
