Resonant acoustic waves in a stratified atmosphere

The upward propagation of linear acoustic waves in a gravitationally stratified atmosphere is studied. The wave motion is governed by the Klein-Gordon equation which contains a cut-off frequency introduced by stratification. The acoustic cut-off may act as a potential barrier when the temperature decreases with height. It is shown that waves trapped below the barrier could be subject to a resonance which extends into the entire unbounded atmosphere. The parameter space characterizing the resonance is explored.