The application of LEED criteria to UK office fit outs
Acoustic criteria for the design of office fit outs in the UK typically reference documents such as the British Council for Offices Guide to specification, British Standards 8233 and 6472-1 and CIBSE guides A and B4. Over the past decade, it has become more and more common for developments to seek ratings for sustainability schemes such as BREEAM, WELL, LEED and so on.
Where the scheme acoustic criteria are rooted in standard UK guidance, designing for the relevant credits or points is relatively straightforward. Where schemes are based on North American derived criteria, such as LEED, it can be a little more complicated as criteria parameters are sometimes not directly comparable. This article discusses the main parameters used in LEED criteria, and considerations for their application to office fit out projects in the UK.
Under LEED, the sound insulation between neighbouring spaces is given in terms of the composite sound transmission class (STCc) ratings. This is an ASTM Standard metric commonly used in North America; in the UK we normally use the ISO Standard Rw, the weighted sound reduction level.
In this case, the two are more often directly comparable, normally only differing by ± 1 dB (although in certain conditions it can be more complicated). In addition, the performance standards required under LEED and UK standards are also broadly comparable.
Reverberation time criteria for LEED are typically presented in terms of the mid-frequency reverberation time averaged over the at 500 Hz, 1 kHz and 2 kHz octave bands. This parameter is the same as that normally used in the UK, though the numerical performance requirements in LEED for spaces, such as cellular offices, can be slightly more onerous than those typically adopted for office fit outs in the UK.
Building services noise
The building services noise criterion is the most complicated parameter as there are differences in the specified parameters and levels.
To comply with LEED, noise from building services should not exceed maximum background noise levels given in the ASHRAE Handbook: HVAC Applications. These are presented in terms of NC (noise criterion) curves. The comparable parameter in the UK is the NR (noise rating) curve. The NC and NR curves have broadly equivalent A-weighted sound pressure levels, however the values of the curves across the frequency range vary more significantly. A comparison of NC and NR curves is provided in the following figure for NR/NC30 to NR/NC40. To comply with a given curve the measured or predicted sound levels must be below the criterion curves in all octave bands.
A simplified approach often adopted is that the NC curve is equivalent to the NR curve. This is because the A-weighted sound pressure levels of NC and NR curves are similar, and the curves are very close to one another at the 125-250 Hz octave bands, which is typically where fan noise is concentrated. However, from the figure it can be observed that the NC curve is lower than the NR curve at low frequencies, and the reverse is true at high frequencies. Consideration is therefore needed of the type of building services systems proposed, and the consequent character of the sound generated, to determine which end of the frequency range is of concern: NC imposes more onerous limits on low frequency rumble while NR is more restrictive of high frequency hiss.
For spaces such as offices, acoustic conditions for speech would normally take precedence, and the focus would be on the mid to high frequency end of the curves where the NR curve is more onerous. This is a beneficial consideration when designing to LEED, which calls for NC30 in meeting rooms and cellular offices versus the typical UK criterion for these spaces of NR35. If we took a like-for-like approach here (eg, NR30 directly substituted for NC30), we would potentially over-specify the design within the frequency range most relevant to speech.
Certainly, if one lays the NC30 curve onto the NR curves, the corresponding NR value is actually NR34. This indicates that the conversion between the LEED NC criteria and standard UK criteria is around + 5dB, which permits a relatively seamless interchange between the two sets of criteria without significant change needed to the normal design approach and reflecting a logical consistency in the design standards sought to provide good acoustic condition in offices on both sides of the Atlantic.