Noise reduction for air flow devices

A method of reducing noise in a device configured to generate an air flow, wherein the device comprises an air flow duct arranged to convey a flow of air, a gas-filled cavity disposed beside the air flow duct, and a wall separating the air flow duct and the cavity, the wall comprising at least one aperture;

• wherein the method comprises optimising an acoustic property for an acoustic resistive screen that is to be held in tension over the aperture such that the acoustic screen is held taut over the aperture, by:
• determining geometric properties of the cavity, the aperture and the air flow duct;
• determining, based on the geometric properties, a noise reduction for the device for each of a series of values for the acoustic property of the screen;
• and
• comparing the respective noise reductions for the series of values to determine an optimised value of the series.