In some studies of the reflection of sound from submerged models, the sound intensity has been displayed graphically as a function of both frequency and angle. With the frequency swept repeatedly over a chosen range while the angle is swept only once through its range, the intensity is mapped on anXYplotter as a raster of lines varying in width, much as a picture is produced on a TV screen. Many of the patterns obtained in this way exhibit ripples that bear no obvious relation to the expected patterns. The particularly striking ripples observed for reflection from a plate with a corrugated surface led to a study of the reflection from arrays of parallel rods. The ripples are shown to be related to the moiré patterns familiar in optics but have some features that are usually absent from optical demonstrations. Like their optical counterparts, the acoustic moiré patterns may be useful in measuring strains or certain types of displacements.