The fourth floor of Ellerby House is 34 m by 22 m by 3.6 m — 2,693 m³ of open plan for about 170 people — and we held it to the lecture-room band of 0.6 to 0.8 s. That will look like a filing error to anyone who reads the target label, so it is worth explaining: the problem in an open-plan floor is the same problem as in a teaching room. Somebody is speaking, somebody four metres away needs to understand them, and somebody twelve metres away needs not to be able to. Reverberation control is how you make the second of those true, and the band that works for a lecturer works for a desk. The computed figure is 0.73 s.
Getting there in a flat-ceilinged box is mostly a question of not overdoing it. A fully tiled ceiling with carpet throughout computes to 0.50 s here, and a floor that dry is a floor where every conversation carries with unnatural clarity because there is no diffuse field to bury it in. Open-plan offices are made unbearable by too much absorption at least as often as by too little. We tiled 320 m² of the 748 m² ceiling in rafts over the desk clusters, left the rest as exposed plaster on the slab for its thermal mass, carpeted 500 m² and left the circulation spine in hard parquet so that footfall reads as movement rather than as intrusion.
The 220 m² of heavy plate glazing is the one element working against us, at 0.04 across the speech range, and it is why the 125 Hz figure sits at 1.11 s. Nobody notices. Low-frequency reverberation in an office is inaudible against the ventilation, which is the point at which room acoustics stops and noise control begins.
Most of the fee went on the roof. The landlord’s chillers and two AHUs sit on a deck twelve metres from the nearest residential window across the mews, and the planning condition was 10 dB below the measured background of LA90 41 dB at night — which is to say 31 dB at the facade, from plant with an unattenuated sound power that put 50 dB there. We took 19 dB out with a 3.5 m acoustic screen on isolators, attenuators on both AHU discharges, and low-speed fan selections on the chillers that cost the mechanical budget about nine per cent in capital and gave most of it back in running cost. We monitored for a fortnight after commissioning across four positions rather than the one the condition required, because a single position and a single night is a number you can be unlucky with, and one we would rather find ourselves than have found for us.