Rockwool mineral wool works in two distinct ways to reduce noise, either by impeding the transmission of sound through an element of the structure or by absorption of sound at the surface.
Transmission loss or noise insulation is the reduction of the amount of sound energy passing through a wall, floor, roof, etc., and is a property of the element as a whole. It is expressed in decibels (dB).
Noise may be due to airborne or impact sound, and both must be taken into account where appropriate. Impact sound is a very important factor, for example in intermediate floors, whilst airborne sound would be the predominant factor in separating walls and partitions.
Particular attention should be paid when designing elements to reduce sound transmission, to the ways in which sound may penetrate at the edges of the element, or through doors, windows etc. This 'flanking sound transmission' can be a problem if it is not taken into account at the design stage.
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Noise absorption is the reduction in sound energy obtained when the sound is reflected from a surface. In general, the harder the surface, the more sound is reflected. Noise absorption is expressed as a factor between 0 and 1.0. If a surface absorbed no sound, its coefficient of absorption would be 0. If 100% of the sound is absorbed, the coefficient is 1.0. 50% absorption, therefore, would be expressed by the sound absorption coefficient of 0.50.
The structure of the fibres in Rockwool makes it an ideal product for use as a sound-absorber, with characteristically high coefficients over a wide frequency range.
The test used is the reverberation room method which gives a good rating assessment. Due to room differences and inherent material variations, measured absorption coefficients show wide variations and it is not practical to place reliance on differences in coefficients of less than 0.15.
When used as a sound absorbent lining within a room, the sound reduction that can be obtained with Rockwool usually ranges between 5 and 10 dB. The actual reduction depends on the room details etc, and an assessment by an acoustic consultant should be obtained. An appreciable airspace incorporated behind the acoustic slabs improves the low frequency sound absorption coefficient.