The Cheyne Room is a 420-seat recital hall of 3,600 m³ built into the concourse deck of a working interchange, eleven metres above a running tunnel and nine metres from a bus stand. The room acoustics were the straightforward part of the commission and are recorded here because they have to be: 24 m by 15 m by 10 m, 8.6 m³ a seat, 900 m² of timber panelling on battens over a 230 m² seating block, and a computed 1.52 s at 500 Hz against a recital target of 1.4 to 1.6 s. Chamber music, song, and a Steinway D that the room is generous to.
Everything else was vibration. A train passing under a building does not arrive as sound; it arrives as motion in the structure, which the structure then radiates into the room as a low rumble somewhere between 30 and 80 Hz — inaudible on any A-weighted meter and completely audible to an audience during a slow movement. You cannot treat it. There is no absorber for a floor that is itself the loudspeaker. The only remedy is to break the path, and it has to be designed with the structural engineer at concept stage or it cannot be designed at all.
The room sits on 96 steel-spring bearings with a vertical natural frequency of 4.5 Hz, chosen so that the isolation is effective from about 7 Hz upward and the whole box is decoupled from the deck across the entire range the tunnel produces. Every service crossing the break is flexible; every duct has a break; the two stair cores that appear to touch the room do not, and there is a 25 mm gap behind their linings that the fit-out contractor was told about in writing four times. We measured the completed room during a full night of engineering movements and got a maximum of VC-E at the stage edge, with the resulting airborne rumble at NR 18. The design target was NR 20.
The one thing we would do differently is the bus stand. We assessed it early from the operator’s timetable, which showed 40 movements an hour, and specified the facade accordingly. The stand as operated runs closer to 60 in the evening peak — which is a better outcome for the city and a worse one for the room — and the facade is now about 3 dB short of what it should be during a 6 p.m. concert. We were assessing against the information we had, but we had not asked what the stand might grow into, and we should have. It is a question that now appears in our brief template for every transport-adjacent job.