When modeling an acoustic pressure wave propagating through an interface between two materials, an essential boundary condition is that the materials remain in contact. As a result, acoustic pressure and particle velocity on both sides must be conserved (Equation and 2). Three quantities are considered at each interface: 1) the incident pressure, P incident , which indicates the pressure just as the wave interacts with the interface, 2) the reflected pressure, P reflected , which indicates the pressure of the wave propagating back on the side of the incident pressure, and 3) the transmitted pressure, P transmitted , which indicates the pressure of the wave propagating through the interface. A schematic is shown in Figure . Since each pressure cannot easily be measured at each interface, the reflection coefficient, r, and transmission coefficient, t, are used to describe the reflection and transmission, respectively, of an acoustic wave across an interface. Furthermore, these coefficients are determined by acoustic impedances, Z, of the materials at the interface. This property relates the particle vibrations in a material to the applied pressure and is equivalent to the product of the material's mass density and speed of sound (Equation ). The reflection and transmission coefficients can be easily expressed in terms of impedances by Equation 4 and 5.