Sound absorption in ducts The reduction of airborne noise in a duct is achieved by absorbing acoustic energy or reflecting some of it back towards the noise source. The performance of an attenuator is specified by its ‘insertion loss’ the difference in sound power downstream with and without the attenuator in position. The insertion loss is normally expressed in octave band power levels. The simplest application of sound absorption is the lining of an air duct with sound absorbent material. For a sound to be absorbed, the frequency at which the sound is travelling must be identified so that the correct type and thickness of foam may be established to ensure optimum absorption. Each type of acoustic insulation has its own range of absorption coefficients depending on the density and thickness over the range 25 - 4000 kHz. the absorption coefficient represents the amount of sound absorbed by the material. All sound absorbers work by converting sound into heat. For open cell materials the sound energy is dissipated due to friction between the air particles and the narrow airways.

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