At the Institute of Acoustics’ annual conference, held in Milton Keynes in 2019, we presented our new method for designing and testing a type of space that has become common in new school buildings: a dining area within an atrium or greater-than-single storey height space. These spaces are often used for many other purposes at different times of the day.
Requests for alternative performance standards (APS) or “derogations” from the reverberation time criterion of BB 93 are common for these spaces. The APSs often cite the 1.0 second requirement for Dining halls as being impractical to achieve given the surface area available, and overall surface area to volume ratio. APSs are not typically based on the acoustic requirements of the users, but rather what the contractor may like to conveniently install
Design Solution: use of DIN 18041:2016
DIN 18041:2016 is a German standard that provides guidance on controlling the acoustic response of a room by suggesting an appropriate reverberation time or quantity of sound absorption relative to room volume. School dining rooms are described within category B5 of DIN 18041, with a suggested absorption per unit volume (A/V) depending on the room height .
BB 93 aims to provide minimum performance standards in line with English Building Regulations; the German standard aims to create conditions ‘suitable’ for the designated use. BB 93 provides a single criterion irrespective of room dimensions, DIN 18041 provides a graduated approach with a reducing absorption per unit volume target as room height increases.
Where the average ceiling height (i.e. volume divided by plan area) exceeds 4.5 m, it can become increasingly onerous to achieve a 1 second reverberation time. A reduced performance, approximately equivalent to DIN 18041 Category B3 may be appropriate to adopt as a minimum performance target. It is suggested on the basis of allowing the notional reverberation time to increase above 1.0 seconds when the average ceiling height exceeds 4.5 m.
Commissioning Solution: use of Strength, G
While reverberation time is the most common parameter used to quantify the acoustic response of a space, issues arise in non-diffuse spaces where reverberation time measurements can be influenced by late reflections and room modes. However, our research has shown that this reverberation “defect” does not affect the build up of sound levels in the same way.
An alternative measurement strategy is to use sound Strength, G. This is a direct measure of the relationship between sound power and sound pressure level in a space, which is the acoustic quality of interest. Although it is defined as the difference between a particular source and receiver location, and the equivalent at 10 m in the free field, a convenient distance may be adopted for commissioning measurements and the appropriate value determined.
Strength can be measured with an omni-directional sound source that has been field calibrated, although laboratory calibration can reduce uncertainty. The equations for calculating Strength as a function of distance between source and microphone, room geometry and mean absorption coefficient, are given in the paper.