WordNet sense 1 (a barrier that has parallel or crossed bars blocking a passage but admitting air):

grate

WordNet sense 2 (optical device consisting of a surface with many parallel grooves in it; disperses a beam of light (or other electromagnetic radiation) into its wavelengths to produce its spectrum):

diffraction grating

WordNet sense 3 (unpleasantly harsh or grating in sound):

gravel, gravelly, raspy, rough, scratchy, rasping

WordNet sense 4 (having an unpleasant sound):

cacophonous, cacophonic