A small diameter tube with a loudspeaker mounted at one end may be used to measure the normal incidence absorption coefficient, $\alpha_n$, of a sample of material mounted at the other end. The quantity, $\alpha_n$, is defined as the ratio of the energy absorbed by the sample to that incident upon it. The energy reflection coefficient, $\left|R_p\right|^2$, is simply $1-\alpha_n$. For single frequency sound, use the solution to the wave equation to derive an expression for the total sound pressure amplitude as a function of axial location, $x$, in the tube and the pressure amplitude reflection coefficient, $R_p$, of the sample. Let the sample surface be at $x=0$ and the loudspeaker be at $x=L$. Assume a phase shift of $\beta$ between the waves incident and reflected from the sample.