This Demonstration shows how a propagating plane wave interacts with an interface between two acoustic media. The incident wave impinges on the boundary from the top and causes a reflected wave that interferes with the incident wave. The incident wave is transmitted into the bottom layer as a refracted wave with its own propagation velocity. The incident and refracted wave angles

(angle of the wave normal relative to vertical) are related by the well-known equation

,

where

is velocity and the subscripts 1 and 2 denote the top and bottom media. The incident wave is taken to have amplitude 1. If

denotes the reflection coefficient for the scattered wave amplitudes, then the wave motion in the top layer is described by (Towne, 1988)

,

while, with

denoting the transmission coefficient, the motion in the bottom layer is described by

,

where

and

are the horizontal and vertical wavenumbers, with the subscripts 1 and 2 for the top and bottom layers, respectively, and

is the frequency in radians. Here all lengths are in meters.

The coefficients

and

are computed for variable angles of incidence and velocities with

,

,

assuming, with no loss of generality, that the densities of the two media are equal. This definition puts

in the range

, while

has the range

. Because of wave interference, the top media can have amplitudes in the range

. The overall amplitudes are scaled in the plots such that black is -2, white is +2, and medium gray is 0. Note that amplitudes appear to be continuous across the boundary. This physical fact is the basis for the equations for computation of

and

.

D. H. Towne,

*Wave Phenomena*, New York: Dover Publications, 1988.