The two primary projects are (A) propagation in a periodic waveguide and (B) scattering of sound by sound. Project (A) is a combined theoretical and experimental study of acoustic Bloch waves in a rectangular, air-filled waveguide that is periodically loaded with reactive branch elements. A small- signal study of the system was done the previous year. This year second harmonic distortion was investigated. A quasilinear solution of the generalized Westervelt equation, with certain terms neglected, showed that although the fundamental Bloch wave is progressive, the second harmonic Bloch wave has both forward traveling and backward traveling components. Experimental measurements generally confirmed the theoretical predictions. In project (B) an analytical study was completed of the effect of various source boundary conditions on the scattered field. The project is mainly experimental, however. The tank facility where the experiments on scattering will be carried out, was completed. A series of three different experiments is planned. Besides the progress on Projects (A) and (B), some work was also done on (C) an upgrade of the Nonlinear Acoustics Laboratory and (D) measurements of ellipsoidal focusing of spark-produced N waves.