C.1 Introduction

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 Regulation E2 and require less additional absorption than Method A. This Appendix demonstrates by means of a worked example the differences between Method A and B.

C.2 Technical

C.2.1 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.

C.2.2 The total absorption area AT 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.

C.2.3 For n surfaces in a space, the total absorption area AT, can be found using the following equation.

AT = αS1 +αS2 +…….αnSn

C.3 Provision of absorptive material

C.3.1 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.

C.3.2 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.

C.4 Method B calculation

C.4.1 Absorption areas should be calculated for each octave band. The requirement of Regulation E2 will be satisfied when the appropriate amount of absorption area is provided for each octave band between 250 Hz and 4000 Hz inclusively.

C.4.2 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. 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.

C.5 Worked Example

C.5.1 The following section describes the application of Method A and B to an entrance hall of a building (refer to Diagram C.1). Each calculation step is to be rounded to two decimal places.

C.5.2 Application of Method A

In accordance with Method A (see paragraph 5.5.2) 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.

C.5.3 Application of Method B

Provide a minimum of 0.2 m2 absorption area per cubic metre of the volume.

Calculation to Method B is described in steps 1 to 8 of Table C.2. 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, αceiling, 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.

Reverberation control – Method B calculation

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