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Camberwell Library21 March 20138/ Internal Acoustics8. Internal Acoustics8.1 Overview8.1.1 The reverberation time (T) of a space is a measure of the rate at which the sound decays. Thereverberation time is proportional to the volume of the room and inversely proportional to thequantity of acoustic absorption present. An excessively long reverberation time in a teachingspace leads to a degradation of speech intelligibility. It is defined as the time taken for thereverberant sound energy to decay to one millionth of its original intensity (corresponding to a60 dB reduction in the sound level).8.2 Library and Offices8.2.1 BB93 specifies the reverberation times required for typical teaching spaces which are finishedbut unoccupied and unfurnished. BB93 uses mid-frequency reverberation time which is theaverage value of the three octave bands, 500Hz, 1 kHz and 2 KHz, denoted T mf . Table 1.5 ofBB93 provides suitable upper limit for the reverberation time for each room type.8.2.2 Based on standard assumptions of the proposed finishes (including carpet floor finish), in orderto achieve the recommended reverberation time criteria within the library, calculations indicatethat a total area of 350m 2 of Class A acoustic absorptive material will be required.8.2.3 It should further be stated that at this stage, calculations have not included the intrinsicabsorption that would be provided by books. This is because details of shelving etc are notavailable. A more detailed model can therefore be assessed at a later stage.8.2.4 With the offices, typically, 13m 2 of Class A absorption will be required, assuming typical hardfinishes and carpet floor finish.8.2.5 This can be achieved with a combination of wall and ceiling treatments.8.2.6 Detailed assessment of the proposals for acoustic absorption will be undertaken when moredetails of the proposed finished and general design preferences are clear.10