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Sound Absorption and Reverberation

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While contemporary interior design favours the use of hard, reflective surfaces, the consequence is that reverberation times within buildings are on the rise. Ever smaller rooms within buildings compound this, as developers seek to maximise the use of available land space. Where occupants are concerned, reverberation in the very least will be mildly disruptive, and at its most extreme, can cause physical discomfort. This article discusses the drivers behind the rise of the reverberation challenge, and offers some advice on reducing reverberation times in various types of development.

Reverberation, sometimes known in layman’s terms as ‘echo’, is a sound phenomenon created when sound waves are reflected by surfaces within a room, such as floors, walls and ceilings, so that one source of sound seems to become several.

In general, the harder the surface, the stronger the reflection, and the more severe the reverberation. Hard surfaces are prevalent in modern building design because of the absence of soft furnishings and the increased use of tiled and wooden finishes. In buildings such as schools and offices, reverberation can result in disruption, as it inhibits the clear transmission of sound such as speech, which is then heard less clearly by the listener.

The concept of reverberation

Reverberation is measured in units of time, and the reverberation time RT60 is defined as that taken for the sound pressure to fall by 60dB after the source of the sound has ceased abruptly. Reverberation time can be calculated based on the volume of a room using the well-known formula devised by Wallace Clement Sabine. For room volumes less than 1000m3, an approximate reverberation time can be calculated using either of two formulae.

If the average absorption coefficient αm is less than 0.3, Sabine’s formula is used:

T = 0 . 16V/A

where T is the reverberation time in seconds, V is the volume of the room in m3 and A is the total absorption in metric Sabine’s (the total absorption is defined as the absorption coefficient of each surface multiplied by its surface area in m2).

If αm is greater than 0.3, the Sabine-Eyring formula is used:

T = 0 .16V/ {Aair - ST ln (1 - αm)}

where ST is the total surface area in m2 and Aair is the acoustic absorption of the air volume.

Reverberation times are often described in the six octave bands between 125 and 4000 Hz. The longer the reverberation time and the lower the frequency, the more difficult the sound is to control.

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Planning and control

While some level of reverberation is required in a room for sound to be heard effectively, there is a point when it actually becomes disruptive, particularly in educational buildings. To help overcome this, legislative guidance has been introduced in the form of BB93, which sets the standard for reverberation time in newly built schools at 0.8 seconds. However, a good teaching environment will typically have a reverberation time of 0.4 to 0.8 seconds, while a poor reverberation time could be anything between two and 12 seconds.

Where reverberation time is unacceptably long, the usual method of control is to absorb the sound directly. Different types of absorber are available, the selection of which will depend on the level of absorption required.

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Natural sound absorbers

Included in this group are upholstered furniture, curtains, carpets and cushions. Thanks to the open cell structure of soft materials, they naturally absorb sound and prevent some of it from reflecting and travelling onwards. This is why the absence of soft furnishings, in line with minimalist styling, has made reverberation a modern acoustical challenge.

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Porous absorber / anechoic material

These are made from open cell or fibrous materials that dissipate the sound energy within their structure. The sound is deflected inside the material, the friction from which converts it into low-level heat. In terms of performance, the thicker the material, the higher the absorption capacity, because there will be more cells through which the sound energy can deflect. An absorptive material would not be a practical control method at low frequencies, as the wavelengths are relatively long and the absorption coefficients low. It may therefore be necessary to consider some form of acoustical barrier where the problem is predominantly at low frequency.

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Resonant absorber

This dissipates sound energy in a similar way to a porous absorber, in that friction converts the sound energy into low-level heat. However, a resonant absorber does so by allowing vibrations of the air in an enclosed space. The volume of air also helps mute the sound, offering a two-pronged approach to reverberation control. Where very high levels of reverberation are apparent, a porous absorber with a resonant element would be used.

The best practice would be to account for reverberation at the start of a development, by ensuring that the room volume is of a sufficient size - certainly more than 25m3 - and not including too many reflective surfaces. Designing a room with a suspended ceiling will also help as this creates a void that will act as a resonant absorber.

In reality, apart from developments governed by BB93, remedial cases emerge because the acoustics within an environment are not always considered as important as the control of sound between spaces: Part E of the Building Regulations (for residential developments) is one example, although a calculation of the required absorption area in common parts of a residential building is called for. This may change in the future as reverberation in an apartment can raise sound levels to such an extent that it travels between dwellings, a problem not currently dealt with at the site testing stage.

Reverberation amplification results in a higher starting sound level than would be considered normal. For example, a television in a normally furnished apartment emits 70dB into the room. However, the same television in an apartment with no soft furnishings whatsoever could reach as high as 76dB. This increased sound level could result in audible sound in neighbouring dwellings. Problems occur for a number of reasons, including the actual noise levels being higher than anticipated, or the reverberation arising as an issue in non-teaching areas, such as dining rooms.

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Focus on solutions

Depending on the environment, there are a number of different acoustic solutions that can be implemented, incorporating either porous or resonant absorbers. Where remedial work is necessary, the constraints and the practicalities of an environment must be considered along with available budget. A reverberation control method appropriate for a school may well be different from the method used in an art gallery or theatre.

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Measuring performance

The absorption performance of reverberation control measures is classified against the British Standard ISO 11654: 1997. The performance is measured as a noise reduction coefficient (αw) which relates to a percentage reduction in sound: if αw is unity, 100% of the sound energy is absorbed. The sound absorption classes are summarised in the table below. The laboratory testing to determine the values according to ISO 11654 is carried out using the random incidence method to BS EN ISO 354 (2003).

Sound Absorption Class αw
A 0.90; 0.95; 1.00
B 0.80; 0.85
C 0.60; 0.65; 0.70; 0.75
D 0.30; 0.35; 0.40; 0.45; 0.50; 0.55
E 0.25; 0.20; 0.15
Not classified 0.10; 0.05; 0.00
 

Acoustic Wall Coverings

Reverberation is commonly controlled using acoustic wall coverings. These are available in various formats to accommodate flexibility of use and design requirements. For example, tiles or rolls might be used to achieve total wall coverage, and as these are available in a wide range of colours, they can become part of the interior design. Some materials do not fray when cut, which means logos or even a detailed mural can be created on the wall area. If total coverage is not feasible or appropriate, a few tiles can be installed on the wall and used as a pin board.

Acoustic wall coverings are also ideal for areas where there is likely to be impact, such as sports halls and corridors, both being areas where reverberation is a common problem. While acoustic wall coverings are available in a range of absorption classes, they are most frequently used where Class C absorption is required. As they are applied after construction, acoustic wall coverings are suited to both new build and remedial works.

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Wall panels

An alternative to complete or partial wall coverage is to install bespoke acoustic panels. These offer superior absorption to the wall coverings owing to their increased thickness, and are generally used where Class A or B absorption is necessary.

As they are manufactured to order on a bespoke basis, they can be colour-matched and sized to meet any particular requirement on site. This means that effective reverberation control can be achieved without changing the existing interior environment. Acoustic panels can be installed in a variety of different ways, but the two most common methods of installation that offer non-visible fixing systems are fixing clips and fabric-covered metal strips. For increased acoustic absorption, the panels can be installed on timber battens to create an air void behind the material. Panels can also be installed as a suspended product, where fixing to the wall is not feasible or will not deliver the required performance. The performance of the acoustic panel is determined by several variables such as the type of backing, depth of air gap, and method of suspension.

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Summary

  • Reverberation is (roughly speaking) another term for echo
  • Hard surfaces generate more reverberation
  • Soft surface absorb sound, reducing reverberation
  • Reverberation time is calculated using Sabine's formula
  • The longer the reverberation time, the more disruptive the effect
  • In schools, a reverberation times must be less the 0.8 of a second
  • Natural sound absorbers include carpets, cushions and other soft furnishings
  • Sound absorbing construction solutions include open-cell fibrous materials and Helmholtz, diaphragmatic and membrane resonant absorbers
  • Absorption performance is measured is classified against BS ISO 11654:1997
  • Sound absorption class is graded A-E

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