Convection consists of rising hot and descending cool parcels of gas. In order to assess the effect of such an in homogeneity on acoustic waves a simple model has been proposed by Zhugzhda and Stix (1994). The model consists of a sequence of alternating vertical layers with temperatures T1 and T2 and upward and downward velocities V1 and V2. At the interfaces between the layers the horizontal component of the velocity and the pressure are continuous. This model allows to determine the phase velocity of a vertically propagating acoustic wave. The main result is that, with increasing frequency, this phase velocity approaches the sound speed of the cooler layers. The reason of such a behavior is that the horizontal structure of the wave is oscillatory in the cool layers, but exponential (evanescent) in the hot layers, so that there is a certain amount of wave trapping in the cool layers. An analogous effect of trapping in a horizontal layer has been described by Kahn (1961) for the temperature minimum of the solar atmosphere.