The nave is 42 m long, 17 m wide and 14 m to the vault: 9,996 m³ for 780 places, or 12.8 m³ a person, which is the ordinary density of a large parish church and about a third more room per head than a concert hall gives. Before the work it measured 4.2 s at mid frequencies with the pews in. That is a magnificent number for plainsong and an impossible one for a sermon, and the parish had been living with the compromise for a century by asking the priest to speak slowly.
The commission was to reach 2.5 s without altering a single visible surface, which in a Grade II* building is not a preference but a condition. What made it possible was the seating. Nine hundred metres of Victorian pine pews were removed under a faculty and replaced with 780 upholstered chairs specified for an absorption coefficient close to the occupied figure — 480 m² of seating block, and the largest single term in the schedule at 384 sabins. The chairs are the acoustic treatment. Nobody looking at the nave can see any.
The remainder is 190 m² of 50 mm mineral wool laid in the void above the aisle ceilings, where it is reached by sound through the existing ventilation grilles and is invisible from the floor, 50 m² of carpet in the chancel, and the untouched plaster, stone and leaded glass. Computed at 500 Hz the schedule gives 2.48 s. Measured with the chairs in and the church empty it came in at 2.44 s. It is not often that a prediction on a historic building lands within two per cent, and we would caution anyone against expecting it: the coefficient for old lime plaster on rubble masonry is a range, not a number, and we were fortunate rather than clever.
We did not chase the bass and would not have been allowed to. The room measures 4.05 s at 125 Hz against 2.44 s at mid frequencies, and a stone church always will — mass reflects, the leaded windows are the only compliant surface in the building, and there is nowhere to put a low-frequency absorber that a conservation officer would accept. This is the right answer as well as the only available one. The long bass is what the organ is voiced into and what the building sounds like; take it out and you have a well-behaved room that nobody recognises. Speech is handled instead by a line array of eight small columns down the nave, delayed to the pulpit, throwing at the congregation and not at the vault. The room is left to do what it has always done, and the loudspeakers do the part it was never able to.