In ultrasonic diffraction grating spectroscopy, the grating surface is in contact with the liquid or slurry. The ultrasonic beam, traveling in the solid, strikes the back of the grating and produces a transmitted m = 1 beam in the liquid. The angle of this beam in the liquid changes with frequency and the so‐called critical frequency occurs when the angle is 90°. At this point, the signal of the reflected m = 0 wave—the signal observed in the experiment—increases and this increase is used to characterize the liquid or slurry. © 2003 American Institute of Physics