Abstract: A gas blow-off noise attenuator comprising two or more perforate diffusion members through which gas is arranged to pass in succession wherein the total free area of perforation in each member increases in successive stages to ensure that sonic fluid velocity occurs through one of the stages and at any mass flow within a predetermined range from a low value up to the maximum mass flow through the unit, and wherein the pressure ratio across the final diffusion member is near but above the minimum necessary to cause sonic fluid velocity through its perforations.