A physical demonstration apparatus, inspired by [K. Meykens, et. al., Am. J. Phys., 67(5), 400–406 (1999)] is used to demonstrate evanescent sound waves in a rectangular waveguide. The apparatus consists of a rectangular waveguide, approximately one meter in length, with one optically transparent wall. The waveguide is driven at one end by a pair of loudspeakers whose polarity may be switched. The other end of the waveguide is open. The sound field inside the waveguide may be interrogated using a microphone and an oscilloscope. The apparatus will be used to demonstrate several features of acoustic wave guides, including the propagation of plane waves, the cutoff frequency for non-plane waves, the propagation of non-plane waves above the cutoff frequency, and the exponential decay of evanescent non-plane waves below the cutoff frequency. The educational and teaching applications of this apparatus will be discussed.