Detailed Design Considerations

Building Regulations

The 2003 edition of Approved Document E introduced a new requirement for the sound insulation of internal walls within houses, flats and rooms for residential purposes. The requirement is for all internal walls between a bedroom or room containing a WC and another room to have a minimum sound insulation of 40 dB Rw.

Internal partitions do not need to be site tested, laboratory tests are acceptable.

Sound Insulation

Sound insulation, otherwise know as sound reduction, is the prevention of noise being transmitted from example by erecting a partition or wall.

The Sound Reduction Index (SRI) of a building element describes the resistance of that element to air-born sound transmission. This is usually expressed as Rw dB. This is a single figure value based on a weighting system defined in BS EN ISO 717-1: 1997.

When considering sound insulation between rooms is the same dwelling air-born sound is the main concern for designers.

Air-borne Sound

Air-borne sound sources produce noise by vibrating the air immediately around them. Typical sources include the human voice, televisions, musical instruments, sound systems and noisy dogs.

The ability of an element of construction to resist the passage of air-borne sound energy through it is largely determined by four factors:

1. The sound absorbency of any cavities in the construction.

2. The degree of structural isolation between the two outer surfaces.

3. The mass of the construction.

4. The air tightness of the construction.

Increasing the mass of a wall will improve its sound insulation, but the amount of extra weight that can be safely supported by a construction is often limited so other design approaches are usually employed i.e, absorbent materials and improved isolation.

How mineral wool absorbs sound

Sound waves passing through mineral wool are continually being deflected and broken up by the individual fibres. The fibres vibrate with the sound energy, which is converted into heat.

How mineral wool works in a cavity

In an unfilled stud partition, the plasterboard and cavity alone are providing the sound insulation. Depending on the performance, this can require extra layers of plasterboard or wider, specialist studs.

Adding mineral wool improves the sound insulation by distorting and breaking up the sound waves and dampening any deflection of the plasterboard (mass spring effect). Utilising a mineral wool infill can achieve the required performance without the need for extra layers, or high performance plasterboard.