The interference and diffraction effects of sound waves of frequencies above 4,000 per second are studied quantitatively, using as detectors both sensitive flames and the ear. A telephone energised by oscillatory currents produced by a thermionic tube circuit is used as a source of sound. This source has distinct advantages over other high pitch sources such as the Rayleigh bird call and other forms of whistle. Many of the experiments can be done with the sensitive flame fed by gas at ordinary pressures. Two forms of sensitive flame are employed, one responding to points of maximum air density variation, and the other to maximum motion of the air. The experiments on interference show results analogous to the optical interference experiments of Lloyd, Fresnel and Newton, and very close agreement is obtained between measured and calculated positions of points on the hyperboloid fringe systems. An experiment is described in which results analogous to certain Heaviside layer effects are obtained. Diffraction near various obstacles and a sound zone plate are studied. Some observations are recorded on the fatigue of the ear and binaural audition.