Numerical predictions for the reflection of plane sound waves from a pressure‐release sinusoidal surface of large slope are compared with experimental data. The approach introduced recently by Uretsky [J. Acoust. Soc. Am.35, 1293–1294 (1963)] for the calculation of reflection coefficients was adopted with some modification owing to the geometrical parameters of the surface and the frequency of the incident sound. The geometry of the surface and the experiment were such that theory could be tested with three orders of the grating spectra.