A commercial electrostatic speaker is now available that provides an excellent and cheap line source of high-frequency sound in the 1000 cps to 20 000 cps range. It can also be operated in reverse to act as a quite satisfactory detector at these frequencies. By replacing the Galton whistle with one of these speakers, and using a second speaker as a detector, the Schilling apparatus can be used for quantitative undergraduate experiments on interference and diffraction, yielding results to better than 2% accuracy.