Studio Three is a live room of 621 m³ — 12.5 m by 9.2 m by 5.4 m — built inside a 1930s light-industrial shed on a street with a bus route on it. It has no seats and no audience; it holds a string section, a drum kit, or one singer and a piano, and the brief was half a second. It is also the clearest illustration in our portfolio of why we publish two reverberation figures rather than one.
The schedule is 160 m² of 100 mm mineral wool, 90 m² of timber panelling on battens, a timber floor on joists, and the remaining walls in plasterboard on studs. Total surface area is 464 m². The absorption at 500 Hz is 186 sabins, which means the mean absorption coefficient of the room is 0.40 — four times what a plastered hall runs at. Sabine’s equation gives 0.54 s. Eyring’s gives 0.42 s. The measurement, made with the room empty and a swept sine, came in at 0.48 s, between the two and nearer Eyring.
That gap is not an error in either equation; it is the assumption underneath Sabine’s showing at the surface. His derivation treats absorption as continuous, which is true enough while the room returns most of the energy it is given and stops being true once it does not. In a room this absorptive, sound is being taken out at each reflection in visible steps, and a formula that smooths those steps predicts a longer decay than you get. We design live rooms to Eyring and quote both, because the size of the disagreement is itself a reading: if the two figures are within five per cent you have a reverberant room, and if they are twenty per cent apart you have made something quite dead and should check that you meant to.
The 100 mm wool depth was not a comfort choice. A porous absorber only does work where the air is moving, and the air is stationary at the wall and fastest a quarter of a wavelength out from it; at 250 Hz that quarter wavelength is 340 mm, which is why 50 mm of the same material would have given us 0.55 at 250 Hz instead of 0.85 and left the room boxy. Depth is the only route downward, and the client paid for 50 mm of extra depth twice over in the tracking sessions.
The bus route is dealt with by mass and by a break: a 300 mm inner leaf on isolation bearings, structurally independent of the shed, with the services routed through flexible connections and the ventilation running at 1.8 m/s in the terminal ducts. Nothing about that is visible in the reverberation figures, which is worth saying plainly, because it is the confusion this practice corrects most often. Insulating a room from a bus and treating a room for a cello are two different jobs done with two different materials for two different reasons.