Acoustic Comfort

acoustics
standards
field vs lab
practical advice

The science of acoustics, and its application within buildings can often be a confusing experience, with a seemingly endless array of different criteria and rating methods. This section will explain the acoustic issues affecting suspended ceilings. There are two acoustic properties relevant to suspended ceilings; sound absorption and sound attenuation.

Sound Absorption

Is a measure of the ability of a surface to absorb sound, minimising the reflection of sound energy back into a space. This is important as a predominance of acoustically reflective surfaces in enclosed spaces, such as a classroom, can lead to an overly reverberant environment; the sound of a single voice can be less intelligible due to the many reflections of sound from the room surfaces. These reflections occur with a time delay compared to the sound energy that reaches a listeners ear directly and cause the sound to become less clear.

The room characteristic that defi nes this feature is “reverberation time” – the length of time (in seconds) that it takes for a sound source to decay by 60 dB. Different environments have differing demands, depending upon the use of the space, and there are differing subjective terms used to describe the different characteristics. Two extreme examples are; a radio broadcast studio, where a reverberation time of around 0.2 seconds is required, the sound is described as “dry” or “dead”, or, a swimming pool with a reverberation time that could be as long as 3.0 seconds, with a “bright”, “live” or “reverberant” sound.

Sound absorption is defined as a co-efficient, ranging from 0.0 for total reflection, to 1.0 for total absorption. The sound absorptive properties of a material are defined in BS EN ISO 11654:1997, which gives three relevant properties:

• Sound Absorption Coefficient αs

Individual sound absorption figures quoted in third octave frequency bands

• Practical Sound Absorption Coefficient αp

Sound absorption figures quoted in single octave frequency bands

• Sound Absorption Rating αω

A single figure rating based upon the values of αp, compared to a reference weighting curve

Of these values, the most convenient term is the single figure sound absorption rating, αω, as this allows straightforward comparison between two different products. For most environments, specification in terms of the value of αw will be sufficient. The first two parameters are used by acousticians in the detailed modelling of a space to accurately determine its acoustic characteristics.

BS EN ISO 11654:1997 also introduced the concept of Sound Absorption Class, with five categories of sound absorption ranging from Class A to Class E, with Class A offering the higher level of sound absorption. Sound Absorption Class can be roughly equated to the value of αw, however is more properly assessed by plotting the values of αp against a series of reference curves between 250 Hz to 4000 Hz.

An alternative and more traditional method of defining sound absorption is Noise Reduction Coefficient (NRC), which is an arithmetic average of octave band absorption over a limited frequency range. This is no longer the preferred unit of choice being superseded by αω.

Sound Attenuation

Sound attenuation is used to describe the reduction in sound between two spaces separated by a dividing element, with two basic sound transmission paths that will affect the eventual perceived sound level difference. Direct sound transmission is the level of sound passing through the dividing element, and flanking sound transmission is the level of sound passing through surrounding structures. Sound attenuation is measured in accordance with procedures set out in BS EN ISO 140, and defined in BS EN ISO 717. Performance is assessed in terms of third octave band values, with weighted single figure ratings provided to allow ease of comparison. For suspended ceilings, the relevant single figure characteristics are:

Dnfw

Defines the sound insulation value from room to room, where a dividing partition abuts the underside of the ceiling with a plenum (void) above. The laboratory test procedure involves use of a massive partition wall, such that the derived performance is that of the ceiling alone, with no flanking paths.

Rw

This rating defines the level of sound insulation directly through a single layer of material. Whereas Dnfw is a “double-pass” value, Rw can be considered the “single-pass” value, although suspended ceilings are rarely tested to determine this parameter

Sound Absorption

Sound Absorption Class

Sound Attenuation

Relevant standards

There are many different standards that determine acoustic performance in buildings, be they legislative or guidance. The following list describes the most important areas in relation to the acoustic properties of suspended ceiling systems.

Offices

The British Council for Offices provide two relevant documents that include reference to acoustic issues related to suspended ceilings.

The “Office Fit-Out Guide” concentrates on the issue of acoustic privacy between cellular office spaces, with guidance on cell office construction techniques with ceiling sound insulation values of 25dB or 35dB. It also notes that reverberation in offices, particularly large function spaces, can be controlled by use of sound absorbent suspended ceilings, and that the suspended ceiling is the principle means of absorbing sound within an office.

The “Best Practice in the Specification for Offices” provides some objective criteria, by way of reference to the recommendations set out in BS 8233:1999, which state that reverberation times in office areas, or other adjacent circulation spaces, should not exceed 0.4 seconds for fully fitted small rooms of 50m3 or 0.7 seconds for fully fitted rooms of 500m3.

The acoustic characteristics of open plan spaces do not follow normal rules for regular proportioned rooms, and the measurement of reverberation time in an open plan office can be misleading. It is better to follow the rule that optimum acoustic conditions in open plan office spaces are achieved when the suspended ceiling offers as much sound absorption as possible and certainly not less than 0.7αw.

Schools

Building Bulletin 93 (BB93) published in 2003 by the DfES provides a guide for the acoustic design of schools. Its requirements are mandatory, being referenced under regulation E4 of the Building Regulations 2000. Compliance with this regulation must be proved to the Building Control Officer by issue of a comprehensive design report.

BB93 applies to all primary and secondary schools. It does not apply to nurseries (unless part of a school), sixth form colleges (unless established as a school) or higher education facilities. Table 1.5 of Part 1 schedules reverberation time targets that must be achieved in all school spaces, in the most part represented as limiting maximum values (except for corridors and stairwells where appropriate areas of sound absorption are defined by calculation).

Suspended ceilings or acoustic lighting rafts and modules with sound absorbent properties can be used as part of the overall interior design strategy to meet these targets, in combination with other room surface finishes.

Design for good speech intelligibly in classrooms and lecture theatres is recognised as one of the key aspects of BB93, this is important not only for the obvious issue of pupil comprehension, but also to limit teacher fatigue. A mix of sound absorbent and reflective surfaces is recommended in order to promote good speech intelligibility.

Hospitals

Health and Technical Memorandum 2045 (HTM 2045) published by NHS Estates sets out acoustic performance criteria for sound insulation and sound absorption within hospitals, although there is no direct reference within the document to the contribution of suspended acoustic ceilings. Clearly, depending upon the specific design, such ceiling properties are relevant, however this is a complex document and specialist advice should be sought from an acoustic consultant.

Field vs. Laboratory

It is always important to remember that published acoustic performance data is normally derived from laboratory tests, under controlled conditions. When applying this data, there are several factors that need to be remembered, so that expectations of performance are not unrealistic.

Sound Absorption

Unless stated otherwise, sound absorption data will always relate to the basic ceiling system, based on a sample size of around 10 m². This sample will include the relevant amount of ceiling grid as well as the basic ceiling tiles, although it will not include any secondary ceiling features, such as light fittings, air diffusers or loudspeakers. When these are added they will clearly affect the overall level of sound absorption offered by the entire ceiling surface, and any negative effect may need to be taken into account.

For an open plan office ceiling, the effect is probably negligible and can be ignored; however, for ceiling area calculations to BB93 or the Building Regulations, the absorbent surface area is critical and the effect should be accounted for by omitting the relevant surface area of feature from the calculation.

Sound Attenuation

Unless stated otherwise, published sound insulation data is derived from a laboratory test to a defined standard under controlled conditions, and applies only to the element of concern, in this case the suspended ceiling. When this ceiling is installed as part of a system including partitions and raised floors, the composite effect of the elements together needs to be considered. In addition, any ceiling features such as light fittings or air diffusers may contrive to lower performance.

Specific advice on this matter should be sought form an acoustic consultant, however as a rule of thumb, installed ceilings on site offer between 3–5dB less performance than laboratory test figures state. This accounts for sound flanking paths and potential installation tolerances.

Sound Absorption: Practical Advice

Suspended ceilings absorb sound by the passage of sound waves through the perforated face of the ceiling tile, which then interact with the material within the back of the tile. The higher the inherent sound absorption of this material, the better the level of sound absorption will be. Thus an 18mm acoustic pad performs better than an 8mm pad – pad density also has an effect.

The size and pattern of perforations within the tile face also contributes to the overall performance of the tile, with an optimum free area range of between 15% to 22% – beyond this, greater free area does not offer any worthwhile improvement.

Ceiling tile perforation patterns are not important within rooms of normal proportion. Sound absorption data for a particular tile can be taken as published. For open plan office spaces, the normal room acoustic rules do not strictly apply, and the concept of a reverberant field as occurs in normal rooms is not relevant.

It is nevertheless important that the tile should provide optimum sound absorption, with as great a free area as possible. For tiles with a small perforation pattern and/or a low free area, there is a risk of sound over long distances impinging upon the ceiling with “grazing incidence”, such that the sound is not absorbed, and there is less attenuation across the open plan office space that might otherwise have been expected.

Sound Insulation: Practical Advice

Where partitions abut the underside of a suspended ceiling, the quality of acoustic seal at the junction between the two will be critical. Remember that in the laboratory test this weakness is avoided by use of a massive partition and a level of head seal that would be visually unacceptable in an occupied environment.

There may be small sound paths through the ceiling grid to be sealed, as well as shadow gaps against plaster margins. In any event, the partition head must be very well sealed against the ceiling grid (NB: Self adhesive tape may be not be sufficient).

Where partitions are placed “off-grid”, (i.e. the partition junction runs across the centre of a tile as opposed to along a ceiling grid line), the level of sound insulation offered may not match the laboratory test data even after applying the 3–5dB field performance allowance. This is because sound can pass through the body of the ceiling tile via the perforated face, acting as a sound flanking path over the top of the partition head junction.

If such junctions are unavoidable, and sound insulation is important, then you should replace the partition head tiles with non-perforated plain tiles, or replace the tile insulation with a solid sheet material into which the partition head can be positively fixed.

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