5.1 Introduction

Room acoustics/reverberation affects the way a space sounds. A high reverberation time can make a room sound loud and noisy. Speech intelligibility is also a function of reverberation, a high reverberation time causes speech to sound muffled and muddy. Rooms designed for speech therefore have a low reverberation time: ≤1 second.

A high reverberation time can enhance a music hall by adding richness, depth and warmth to music. A higher level of reverberation within a concert hall is therefore critical. Illustration 2 provides indicative reverberation times for a range of building types and room volumes.

The reverberation time of a room is defined as the time it takes for sound to decay by 60 dB after an abrupt termination. The reverberation time of a room is linked to the total quantity of soft treatments and the volume of the room, see below.

Acoustic Properties of Materials 

To control reverberation time, acoustic absorption is used. Absorbent materials conventionally take two forms; fibrous materials or open celled foam, 3. Fibrous materials absorb sound, since sound waves force the fibres to bend and this bending of the fibres generates heat. The conversion of acoustic energy into heat energy results in the sound effectively being absorbed. In the case of open celled foam, the air movement resulting from sound waves pushes the air particles through the small narrow passages which in turn generate a viscous loss along with heat.

Architecturally, fibrous materials and open celled foams are not particularly attractive or robust. It is therefore common to cover these materials with an acoustically transparent finish such as a tissue, cloth, slatted wood, perforated materials; wood, metal, plasterboard and so on.

The thickness of a given material along with properties such as its fibrosity governs the acoustic performance of a product. Finishes within a space are therefore defined in terms of their absorption coefficient. This is a number between 0.0 (purely reflective), for example stone, tiles, concrete and 1.0 (100% absorbent), products with this rating include high performance acoustic ceiling tiles, slabs of mineral wool, etc.

Products such as carpets have an absorption coefficient between 0.1 and 0.3 depending on their thickness. Perforated plasterboard generally provides around 0.6 to 0.7. It is also common to classify absorbent materials in categories, A to E, where A is highly absorbent and E is almost fully reflective.