5.1 About silencers

5.2 Method of measurement

5.3 Attenuation material and cleaning of the silencers

5.4 Ventilation does not have to be noisy

5.4.1 Simple sound analogy

5.4.2 Sound can be absorbed

5.4.3 Where to place the absorption material in the duct

5.4.4 Damping varies with a few simple rules

5.4.5 Noise frequency influences the choice of silencer

5.4.6 The concept of decibel

5.4.7 Humans perception of sound

5.5 Choosing silencers

5.1 About silencers

The Lindab silencers are of the absorption silencer type. The damping ability of absorption silencers is affected by the geometric design of the silencer and the type of damping material chosen. Silencer comprises several such variants, with different properties. The graph below summarises the attenuation of some types of silencer.

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5.2 Method of measurement

The silencers are tested in accordance with ISO 7235 ”Acoustics - Measurement procedures for ducted silencers - Insertion loss, flow noise and total pressure loss”.

ventilation_general_theory_sound_method_of_measurement.svg

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5.3 Attenuation material and cleaning of the silencers

Lindab uses different attenuation materials depending on the needed property of attenuation and cleanability.

Mineral wools: stone or glass wool are always covered by a surface textile to prevent the fibres from coming out of the acoustic media. All the silencers can be cleaned with a rotating nylon brush or a vacuum cleaner.

The silencers made from Lindtec wool or Acutec® polyester attenuation material can also be cleaned with a damp cloth. In the case that mechanical cleaning is needed it is also possible to cover the attenuation material with perforated plate.

Most of the silencers with a round connection are made with perforated plate or stretch metal mesh to protect the attenuation material.

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5.4 Ventilation does not have to be noisy

If you use your common sense, and construct your air treatment system with consideration and good components, you can often avoid problems and complaints.

Fans make noise, this is something you can not do a lot about. But you can prevent the noise from getting into the areas connected to the fan system - you can absorb and damp the noise on the way.

This description does not claim to teach you how to calculate and attenuate noise in a ventilation system - there are books available on this.

This description only aims at providing information about a few simple rules and hints, which together with common sence can be enough for simple installations.

You must have some basic knowledge about how and where noise is generated, transmitted and attenuated in the system, to be table to choose the correct principle and correct components. To take a simple analogy: noise transmission consists of waves in a medium, i.e. air, which we can not see. This is very similar to the way waves spread on water.

Let us examine the analogy, to make the comparison clearer:

5.4.1 Simple sound analogy