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Acoustics - Basic Guide to Acoustic Terminology

 

Sound Absorption

Sound in a space (both source sound and reflections of source sound) has its “lifespan” reduced by being absorbed rather than reflected. Both source sound and reflections of the source sound can have their “lifespan” in a space reduced by being absorbed.

Absorption is effected by the sound frequency, the size of the room space and the surface finishes of the room space.

Several terms are used to define performance:

red Absorption Coefficient: This represents the proportion of incident sound energy arriving from all directions that is not reflected back into the room. It ranges between 0 and 1, where 0 is reflective and 1 is totally absorptive.

 

red Class “X” Absorption Class: Classification applied to a material or system defined by EN ISO 11654. Class A is the best classification representing the highest level of absorption between 250 and 4000Hz, and Class E offers the lowest classification.

 

red Sound Absorption - RT Reverberation Time - A measure of the echoic nature of a room. Normally measured in 1/3rd octave or 1/1 octave bands by creating a noise and measuring the time it takes for that noise to decay by 60dB. The longer the reverberation time, the more ‘echoey’ a room sounds.In schools performances recommended by BB93 fall between RT’s of 0.6 and 1.5. The RT performance of a room with given finishes can be calculated using the volume of the room, the areas of each finish and their Absorption Coefficients (a).

A calculation may demonstrate that the roof system alone can achieve the required RT for BB93.This offers savings in the potential provision of other absorptive materials ie acoustic wall or ceiling tiles. Project acousticians may run calculations for each project using the absorption data provided for most Euroclad Systems and for other room finishes.

 

Sound Absorption - RT Calculations


Using data from our extensive test programmes we have run calculations to establish the performances of our tested systems in various common school applications under BB93 RT requirements. These will be published in our forthcoming Acoustic Handbook. The calculations contain limited data on typical wall, floor and glazing elements from Sound Research Laboratories database.

Using the most onerous examples for wall and floor finishes it has been possible to demonstrate the ability of our acoustic roof systems 4.25A2, 4.20A1, 3.25A2 & 2.25A2 to meet many of the requirements of BB93 without the need for further space treatment ie acoustic wall tiles etc. Perforated structural deck based systems eg 5.20A typically perform less well because the area of perforation is more limited and absorption less efficient.

Using a typical sportshall example a deck based system would need some additional absorption provided by other elements ie small areas of carpet, wall treatment with tiles or fibreboard etc. As a rough guide the Absorption Class is a good starting point, with better classification feeding directly into better RT performance. Room size and other finishes also very specifically affect the RT performance and a specialist acoustician should be consulted for each project to seek fully detailed advice. However, the data required by project acousticians is readily available from Euroclad and we are happy to guide and assist. It will not always be possible to meet the RT requirements with the roof alone.

 

Sound Absorption - Some less commonly used terms:


red Practical Absorption Factor: A frequency dependent value of sound absorption coefficient based on measurements in one-third-octave bands in accordance with ISO 354 and which is calculated in octave bands in accordance with EN ISO 11654 : 1997. This helps to even out some of the possible anomalies which can crop up in test data ie it is quite possible to have absorption coefficients greater than 1 from the tests, the ap helps to clarify this.

red NRC Noise Reduction Coefficient: Actually a sound absorption value, the noise reduction coefficient of a material is the average, to the nearest multiple of 0.05, of the absorption coefficients at 250Hz, 500Hz, 1kHz and 2kHz.