Design Principles

The designer may take the sound reduction indices shown in the table below as a general indication of the levels of sound insulation he should allow for:

Average Sound Reduction (dB)	Approximate Effect

20	Normal speech easily heard

25	Loud speech easily heard

30	Loud speech can generally be overheard

35 (minimum desirable standard for acoustic privacy between rooms)	Loud speech can be heard only faintly and not distinguished

45 and over (minimum standard required for separating walls between dwellings)	Shouting heard only with difficulty

Flanking Transmission

The noise reduction characteristics of a separating structure can be negated by the ability of airborne or structure-borne sound to pass around, over or under the structure at its perimeters.
This type of flanking transmission can be avoided at the design stage or by correct detailing at element junctions, where the use of mineral wool acoustic products can often provide a solution.

Correct Detailing At Penetrations of the Structure

The smallest aperture in a separating wall or floor can negate most of the sound insulating quality of the construction. (An aperture 25mm square would reduce the dB rating of a 45 dB construction to 30 dB).

For this reason, services or pipe-work and duck-work should be adequately sealed where they pass through an element of structure (e.g. between quiet and noisy areas).

In the case of large ducts (e.g. for ventilation), special acoustic treatment may be required. Similarly chasing for seriously impair the sound insulation rating.

The Importance of Good Workmanship

To achieve satisfactory sound reduction, it is imperative that the structure is soundly and carefully constructed with particular attention to the filling of all joints in the construction. Acoustic insulation must be tightly butted to leave no gaps between adjoining sections, slabs, quilts, mats etc. or between the acoustic material and the structure itself.

Internal Partitions and Enclosures