Amplitude gratings diffuse less well than phase gratings but work at lower frequencies

Evidence

Amplitude gratings provide some diffusion, although they cannot be as good at diffusing as phase reflection gratings. But because of their size, they are useful at low frequencies. It also is possible to develop curved diffusion structures, although there are no simple mathematical recipes for them. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2106