Buildings may sometimes have to keep external noise out or keep internal noise in. Three obvious cases are:

• Buildings at or near busy roads and motorways.
• Airports, where the sound of aircraft taking off and landing needs to be reduced or eliminated.
• Where internal process is so noisy that the sound must be contained, for example, engine test houses.

When a sound hits a wall or a partition it goes three ways. Some of the sound will be reflected, some will be absorbed and converted into heat, and some will be transmitted into the room of a building. The fractional reduction of incident sound which is transmitted is defined as the Sound Reduction Index (SRI) or equally known as the transmission loss. This transmission loss for a particular construction is measured in a laboratory based on the methodology provided by BS EN ISO 140-3: 1995.

Sound can also find its way through any weaknesses in the design of a building. These areas of weakness are known as flanking paths and occur as a result of a complex transmission of airborne sound and structural vibration along indirect paths in the building.

In laboratory testing, flanking paths are eliminated and  the direct path is measured. In the real world, the transmission loss values of a structure can be lower due to sound passing through roof voids, small holes and cable glands in the structural system. Doors and windows, for example, are often acoustically weak and can significantly reduce the acoustic insulation of a façade. There is little value, therefore, in constructing a high quality insulating wall or roof without considering the overall acoustic effectiveness.

If there is a need to reduce the build-up of sound and reverberations inside the building, then acoustic absorbing materials should be used. These could be used to cover the interior of a room in a building in the form of a perforated liner sheet. In such situations, typical porous sound-absorbing materials, such as mineral wool or fibre, can be used. Their effectiveness is described by the absorption coefficient, which is a function of frequency, and can be measured based on the methodology provided by BS EN ISO 354: 2003. The absorption in a porous layer generally increases with frequency. High absorption at low frequencies will require that thick layers of absorbent insulation should be used.