The invention provides an acoustic attenuator comprising: a body defining a cavity therein and having at least one open aperture in fluid communication with the cavity; and opposing first and second walls, the second wall being substantially parallel to the first wall, the body comprising at least one of the first and second walls, wherein the aperture and the cavity at least partly define a resonant frequency band across which the body attenuates incident acoustic waves, and wherein the first and second walls are separated by a gap.