Acoustic Comfort

The science of acoustics, and its application within buildings, can often be complex and confusing for the non-specialist. A seemingly endless array of different criteria and rating methods can add a new complexity to any project.

SAS International is an expert in this field and can support your project, providing guidance and experience to help you specify the most appropriate products for your design that also meet industry and legislative standards.

This information should help explain some of the more relevant acoustic terminologies and technical aspects. In particular, the correct product selection and its application within buildings.


 

Sound Absorption

This is a measure of the ability of a surface to absorb sound, minimising the reflections of sound energy back into a space.

This is important because a predominance of acoustically reflective surfaces in an enclosed space, such as a classroom, can lead to an overly reverberant environment, where 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 listener's ear directly, and can cause the sound to become less clear. The room characteristic that defines this feature is “reverberation time” – technically, this is the length of time (in seconds) that it takes for a sound source to decay by 60 dB.

Different environments have varying demands in this respect, dependent upon the use of the space, and there are a variety of terms used to describe these characteristics.

Two examples are: a radio broadcast studio, where a reverberation time of around 0.2 seconds is required, and the sound can be described as “dry” or “dead”; or, a swimming pool with a reverberation time that could be as long as 3.0 seconds, described as a “bright”, “live” or “reverberant” sound.

Sound absorption is defined as a coefficient, ranging from 0.0 for total reflection to 1.0 for total absorption. The sound absorptive properties of a material are defined in standard BS EN ISO 11654:1997


 

BS EN ISO 11654:1997

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: αw

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, αw, as this allows a 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.

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

Finally, 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, however, being superseded by αw.

sound absorption class



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.