Noise levels Air Conditioning

All air systems have a noise level made up of the following:

  1.  Noise of central station machinery transmitted by air, building conduction, and duct-borne
  2.  Noise from air flow within ducts
  3. Grille outlet noise The first of these can be reduced by suitable siting of the plantroom, anti-vibration mounting and possible enclosure of the machinery.


Air flow noise is a function of velocity and smooth flow. High-velocity ducts usually need some acoustic treatment. Grille noise will only be serious if long throws are used, or if poor duct design requires severe throttling on outlet dampers.

Apart from machinery noise, these noises are mostly ‘white’, i.e. with no discrete frequencies, and they are comparatively easy to attenuate. Where machinery of any type is mounted within or close to the conditioned area, discrete frequencies will be set up and some knowledge of their pattern will be required before acoustic treatment can be specified.

Manufacturers are now well aware of problems to the user and should be able to supply this basic data and offer technical assistance towards a solution. Where several units of the same type are mounted within a space, discrete frequencies will be amplified and ‘beat’ notes will be apparent. Special treatment is usually called for, in the way of indirect air paths and mass-loaded panels. Useful practical guidance can be gained by visiting existing installations before taking major decisions on new plant.