As detailed in Technical Guidance Document E Method B takes into account the actual absorption power of the surfaces of the enclosure prior to the provision of additional absorbent material. This allows the amount of additional material which is required to be calculated and directed at the sound frequencies at which it is most needed. In some cases, Method B should allow greater flexibility in meeting the requirement of reverberation control and require less additional absorption than Method A.
This Appendix demonstrates by means of a worked example the differences between Method A and B.
For an absorptive material of surface area S in m2, and sound absorption coefficient α, the absorption area A is equal to the product of S and α.
The total absorption area A~T~ in square metres is defined as the hypothetical area of a totally absorbing surface, which if it were the only absorbing element in the space would give the same reverberation time as the space under consideration.
For n surfaces in a space, the total absorption area A~T~, can be found using the following equation.
Provision of absorptive material
The requirements of Technical Guidance Document E require the provision of the following:
For entrance halls, provide a minimum of 0.2 m2 total absorption area per cubic metre of the volume. The additional absorptive material should be distributed over the available surfaces.
For corridors and hallways, provide a minimum of 0.25 m2 total absorption area per cubic metre of the volume. The additional absorptive material should be distributed over one or more of the available surfaces.
Method B calculation
Absorption areas should be calculated for each octave band. The requirement of reverberation control, as detailed in Technical Guidance Document E will be satisfied when the appropriate amount of absorption area is provided for each octave band between 250 Hz and 4000 Hz inclusively.
Absorption coefficient data (to two decimal places) should be taken from the following:
For specific products, use laboratory measurements of the absorption coefficient data using I.S. EN ISO 354 Acoustics - Measurement of sound absorption in a reverberation room. The measured third octave band data should be converted into practical sound absorption coefficient data αp in octave bands, according to I.S. EN ISO 11654 Acoustics -Sound absorbers for use in buildings -Rating of sound absorption;
For generic materials use Table C.1 of Technical Guidance Document E as provided below. This contains typical absorption coefficient data for the common materials used in buildings. This data may be supplemented by published octave band data for other generic materials.
Table HE7 - Absorption coefficient data for common materials in buildings - Extract from TGD E
The application of Method A and B to an entrance hall of a building are detailed below, Diagram C1 of Technical Guidance Document E, provided below provides a visual representation of this. Each calculation step is to be rounded to two decimal places.
Diagram HE46 - Worked example - Extract from TGD E
Application of Method A
In accordance with Method A as detailed in Technical Guidance Document E, for entrance halls, the absorbent material should cover an area equal to or greater than the floor area, with a Class C absorber or better, rated according to I.S. EN ISO 11654.
Therefore, cover at least 20 m2 (i.e. 4.0 m x 5.0 m) with a Class C absorber or better in order to achieve the appropriate level of reverberation control.
Application of Method B
The requirements of Technical Guidance Document E required the provision of a minimum of 0.2 m2 absorption area per cubic metre of the volume.
The calculation of Method B is described in Steps 1 to 8 detailed in Table C2 of Technical Guidance Document E provided below. In this example, the designer considers that covering the entire ceiling is a convenient way to provide absorption. The aim of the calculation is to determine the absorption coefficient, α~ceiling~, needed for the entire ceiling.
In this example, the absorption coefficients from Method B indicate that a Class D absorber could be used to cover the entire ceiling. This can be compared against the slightly higher absorption requirement of Method A, which would have used a Class C absorber or better to cover the ceiling.
Table HE8 - Example calculation using Method B - Extract from TGD E