Bringing clarity to acoustic jargon

The world of acoustics can be baffling to the untrained. How many of us know the difference between attenuation or frequency for example? And what exactly is the sound absorption coefficient? Such is the complexity, acoustics is a subject that has been frequently described as a ‘dark art', particularly when it relates to the application within buildings. In a bid to demystify the definitions and notations for the non-acousticians amongst us, Stuart Colam, Acoustics Advisor of SAS International delves into some of the more basic principles and common acoustic terminologies.

Sound absorption is a measure of how much sound is absorbed by a surface or object. When sound comes into contact with a surface, such as a wall or ceiling that is not particularly sound absorbing, it will be reflected back into the space. This can result in a room becoming noisy or reverberant because the sound is ‘trapped' and continues to ‘bounce around'.  Excessive reverberation results in poor clarity of speech which is problematic in schools and transport hubs, for example. As more sound absorption is introduced into a space, the noise level will reduce and the sound will decay more quickly.

A material's sound absorption properties are described by the sound absorption coefficient (α_s), which is a value between 0 and 1.  A value of 0 means total reflection while 1 means all sound is absorbed by the surface and not returned to the room.  Sound absorption of a surface is not the same for all frequencies of sound. For example, a porous surface like carpet is more efficient at absorbing mid and high pitched sound than low pitched sound.  The sound absorptive properties of a material are defined in standard BS EN ISO 11654:1997.

Sound insulation (sometimes referred to as sound attenuation) describes the extent to which sound is limited when passing through a building element or elements.  The associated term sound reductionis used to define the drop in sound level after passing through an element such as glazing, partitioning or ceiling. This ‘single pass' descriptor is abbreviated as R_wwhere ‘R' refers to reduction and the subscript ‘w' refers to weighted (a type of average). In short, the R_w figure is a simplified indication of the difference in sound level from one side of a building element to the other.

Sound insulation is also quantified in terms of the reduction in level due to a flanking or a double pass route.  The abbreviation D_nfw is used which means a sound level difference via a flanking route that is normalised and weighted. It basically defines how much sound is blocked by passing through the same element twice, such as ceilings, which span more than one room and have a common void.

The fact that acoustic terminology can be confusing to the uninitiated has made it increasingly important for specifiers to ask the right questions to ensure they have been completely understood.  Acoustic comfort in the built environment has become a concern to society and a challenge to designers. The acoustic performance of a space within a building will ultimately have a dramatic effect on the performance of tasks taking place in those spaces.

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