Acoustics

The roof and wall cladding on a building can be used to restrict sound entering (such as aircraft or traffic noise) or leaving (such as factory or manufacturing noise) in order to provide a comfortable acoustic environment for workers or residents. Sound absorption is often important when considering reverberation in spaces such as auditoria, sports halls or swimming pools. Particular levels of acoustic performance may be required by local planning, environmental health of other regulatory departments.

Sound Reduction (SRi R_W dB)

Sounds propagate through the air as a pressure disturbance or wave. The range of sound pressures is very large, approximately in the ratio 1 to 10 million, from the quietest to the loudest sounds, so sound (noise) is measured using a logarithmic scale with units of decibels (dB), to give more manageable values. For example, the sound pressure level in a quiet residential area at night might be 40dB, and in a busy street or workshop it might be 80dB. The minimum threshold of hearing is 0dB and a jet aircraft taking off might be 120dB.

Sound inside a building can escape to the outside (or vice versa) by transmission through the walls, windows, roof etc. The reduction in sound level provided by the building elements is called sound reduction or sound insulation. The performance of a particular construction element such as a roof can be accurately measured in a special laboratory, and the result is referred to as the Sound Reduction Index (SRi). This varies with frequency, but for convenience can be expressed as a single figure, known as the Weighted Sound Reduction Index (expressed as R_w).

These laboratory tests provide the basic acoustic performance data for a construction, and generally indicate the best that can be achieved on a real building. Sound insulation measurements on a completed building may be lower because of flanking (sound by-passing the element), features of the building, and workmanship. In general, the denser the structure; the more sound insulation it provides. In light weight metal clad constructions, the sound insulation is improved by minimising any rigid connection from inside to out, having separate liner and outer sheets, with a soft mineral fibre infill, as in the CABP roof and wall cladding systems, is a distinct advantage. Furthermore, because the systems are site assembled, the individual layers can be tailored in design in order to achieve a particular level of sound reduction.

Various modifications to the basic cladding system are possible to achieve a particular acoustic performance, but this performance cannot be predicted with certainty, and it is essential to carry out laboratory tests to determine the performance accurately.

Sound Absorption (coefficient)

Sound pressure waves can be absorbed by materials such as carpets, soft furnishings and mineral fibre insulation, whereas, hard smooth surfaces will reflect sound rather than absorb it, which will have an adverse effect on the acoustics inside a building because the reflections can increase the internal sound level. Therefore sound absorption is used to reduce noise levels inside a building.

The sound absorption performance of a material can be measured using the same special laboratory procedure as mentioned in Sound Resistance, and the result is expressed as the Sound Absorption Coefficient, across the range of frequencies. This varies from 0 (total reflection) to 1 (total absorption). Standard profiled metal liners do not have a good absorption coefficient, but can be perforated so that the sound can enter the core of the construction and be absorbed by a special layer of mineral fibre insulation. A separate vapour control layer MUST be incorporated into the construction in this case, sandwiched between the acoustic insulation and the thermal insulation layer, however air leakage requirements may dictate that the VCL be located directly over the perforated liner panel, which in turn will reduce the sound absorption performance of the system. Note that perforating the liner will reduce the sound reduction performance of the system, compared to an unperforated liner.

Rain Noise (LIA dB)

A common issue relating to acoustics is the noise created during intense rainfall. Laboratory tests can be conducted in accordance with BS EN ISO 140-18:2006. CA Building Products have undertaken tests on a few constructions to enhance the standard constructions which typically achieve sound intensity levels (L_iA) for insulated panels circa 62dB and generic twin-skin systems circa 50dB.

In terms of rain noise the lower the result the better the performance of the system which is different to sound reduction where the higher results dictate better performance.

Noise Ratings (NR)

Noise ratings or NR values are frequently confused with sound insulation values. There is no relationship between noise ratings (NR) and weighted sound reduction (R_w). Noise ratings were developed in the 1950’s by Kosten and Van Os and are used by building services engineers in the design of air conditioning systems. The calculations are used to assess the acceptability of noise in any area served by the mechanical services systems.

Design to avoid noise flanking

It is important to consider, when including acoustic mitigation within any building design, the risk of noise or sound bypassing any acoustic systems installed and therefore rendering the design without use. In the simplest terms, consider two classrooms with a dividing wall. Including sound reduction within the dividing wall may reduce sound being transferred from one classroom to another however if a suspended ceiling or cavity runs above the wall this will bypass any acoustic mitigation.

Acoustic Design

Acoustics are a complex subject and the building designer should not rely solely on the cladding manufacturer for expert advice on the overall acoustic performance of the building. If acoustics is an important issue, it is essential to have an acoustic consultant on the design team.

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