Harton Primary School © Kristen McCluskie

By Andrew Parkin (First posted on Adjacent Digital Politics)

I wrote in 2013 about the impending changes to BB93, the document that is the means of compliance with Part E4 of Building Regulations – Acoustic design of schools 1.

After a long protracted ministerial process, the document was finally issued for consultation in February 2014. This document was essentially the Priority Schools Building Programme (PSBP) output specification for acoustics, v1.7 dated June 2013, itself an update of the September 2012 (v1.1) document. By the time of consultation therefore, content had been used to design PF2 schools around the UK and was familiar to school design practitioners.

Not surprisingly, the consultation yielded a large number of responses, 61 in total (compared with 48 for the original BB93 in 2002).

With all the consultation responses back, the process of assessing these is well underway at the time of writing (and should be complete by the time of going live). There were very few new issues raised that had not already been discussed during the past five years. However, there were a number of common issues that have caused the committee to re-visit some decisions that had previously been made.

The two most hotly contested issues were sound insulation and ventilation; both are discussed briefly below. BB93 brought a new descriptor for sound insulation in DnT(Tmf,max),w. Whilst this may sound complicated (and indeed it is difficult to say), it is actually quite simple. DnT,w has been used in Approved Document E for many years and the BB93 version is simply a variant of this. DnT,w is defined as the ‘weighted standardised level difference’ and is a method to describe the sound insulation across an element (e.g. wall or floor) in frequency bands as a single number value. Part of the calculation process involves correcting sound insulation for the amount of reverberation in the receiving room and normalising to a standardised value of 0.5 seconds. What this means in practice is that a test could be carried out in an unfurnished, echoey room but still be compared against the equivalent sound insulation in a furnished space. The BB93 variant simply takes the reference reverberation time and replaces it with the design value for the particular room in a school (e.g. 0.6 and 0.8 seconds for primary and secondary classrooms respectively). The issues with the BB93 version came when actual reverberation times in-situ were less than the design maximum (which is a good thing), but the corresponding sound insulation value was unfairly penalised, sometimes resulting in marginal failures even though the sound insulation was actually fine. So, the committee agreed (by consensus) to use a normalised value to 0.5 s, for simplicity but also to reduce inaccuracies in site measurement. There was significant reaction to this in the consultation which has reopened the old can of worms and discussions relating to the use of Dw (a variant of DnT,w but without any correction for reverberation time) is now being discussed for site measurements.

BB93 was published in 2003, two years before BB101 (indoor air quality, including ventilation). Due to this disconnect there was not a single place that contained both acoustic and ventilation guidance. The plan was to rectify this in the BB93 replacement and therefore, significant detail regarding the ventilation conditions to be assumed in noise break-in calculations was provided. In the first (v1.1) PSBP document this included a ‘deemed to satisfy’ method for determining whether opening windows could be used for ventilation, based on the difference between external and internal noise levels required (with separate values for single – and double-sided ventilation); this was found to be overly restrictive in practice and a ‘mid-season’ condition was assumed for v1.7. The problem is that there is very little evidence to show what attenuation a window that is open for ‘mid-season conditions’ gives; the matter is not helped by the fact that the type, size, number and hinge position of the windows all have a bearing on the calculation. A significant number of consultation responses therefore called for a simplified method, which has prompted the committee to discuss various alternative options. The problem is that there are so many variables, and expressing both acoustic and ventilation requirements in a simple elegant method is in itself a very complex process.

So, thanks to everyone that responded to the consultation. There will never be unilateral agreement on all the issues and it is left to the committee to find a consensus view, using their expertise and experience, assisted by consultation comments.

DfE are keen to publish the completed document in the summer so the pressure is on to reach agreement on these, as well as many other points.

1 http://www.adjacentgovernment.co.uk/pbc-edition-001/soundingout-good-school-design/

Photography © Kristen McCluskie

Find out more about Cundall – http://www.cundall.com/

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Acoustic Engineering, Andrew Parkin

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