In order to understand the design and concept of The Mill, a brief outline of the recording industry in the 1970s will help put it into context.
The global fashion in studio and control room acoustics from the mid 70's to the early 80's was to have an Eastlake studio. Designed by Californian Tom Hidley, his vision was to produce studios and control rooms with a uniform sonic performance in order that albums could be recorded & mixed with confidence in any of his studios around the world without having to take risks on differing sound quality. This philosophy worked to a usable extent, he was tremendously successful, and albums were often recorded in London, overdubbed in New York & mixed in Los Angeles as part of the boom years of the recording industry when time and expense were in good supply.
The key to a Hidley/Eastlake (Westlake in USA) room was the treatment of the reverb time. Resonant bass trapping was employed to make the reverb time essentially flat throughout the audio spectrum, which meant that a speaker with a flat response curve would theoretically show a flat response in the room on a third octave spectrum analyser when pink noise was played through it. Just to get it really flat, third octave equalisers were used on each power amplifier.
The bass absorbers needed to be half a wavelength long and have a large surface area to be effective, which meant that a great deal of space was taken up with the acoustic treatment.
The one main criticism of the Eastlake philosophy of "flat reverb time" was that it just sounded unnatural, as most everyday environments show not a flat but a gently tilting reverberation curve with more absorption of the high frequencies and less in the low frequencies. As a result our ears expect to hear this acoustic environment, and can feel at odds with anything different.
The MIll control room however was quite the opposite of the Eastlake concept. The ceiling was between 4-5ft thick concrete, there was no bass trapping and there was very little fabric/rockwool on the walls to absorb mid/high frequencies. There was a brickwork faced wall around the speakers. And then there was the fireplace, situated under the left hand monitor speaker with a chimney resonating at 75Hz (I measured it once as the boom could be heard in the workshop above).
So you can see there's precious little absorption in the room, but plenty of diffusion. The reverb time resembled something from the winter olympics and no attempt to make the room response flat was made.
I watched Gus and Eddie Veale align the monitors one day. The critical part was done by ear with Gus listening carefully to music and calling for small changes here and there, with Eddie altering the crossover pots and then checking the balance with pink noise and a microphone/spectrum analyser. This process was repeated several times until Gus was happy with the result. The spectrum analyser showed a horrendous curve (after I had been used to setting up Eastlake rooms in a previous company) but closely resembled the reverb time graph.
The monitors at the Mill sounded wonderful, and many engineers recorded there with no subsequent alteration of tonal balance needed. Gus's mixes were often cut flat or with half a dB here and there by Ian Cooper at The Townhouse. So the tapes mixed there appeared to show an accurate acoustic balance.
The explanation for the fact that the opposing Eastlake and The Mill acoustic concepts both appear to have worked lies, I think, in the ability for the human ear/brain to notice room reverberation and take it into account. The fact that the response curve in The Mill control room follows the reverb time graph suggests that the loudspeakers were in actual fact producing a flat response, and the analyser was showing a build up of frequencies that had a longer reverb time. Somehow human hearing seems to adjust for this. Certainly the bass in The Mill could hit you in the chest and flap trouser turnups, especially at the levels Gus played back albums at, yet tapes could be cut flat from this setup.
It was a common occurrence that halfway through side two of an album playback, the Crowns would go in to thermal shutdown as they were being driven so hard. It was very loud and there were no fans near the amplifiers.
The large speakers were designed by Stephen Court and were customised JBL 4130 units, built into the brick faced wall and covered with a brown reticulated foam grille. They were fed from Crown DC300 amplifiers, and contained passive crossovers. The drive units were special JBL 15" LE15B units (two per cabinet) chosen for their extended linear excursion properties, giving low distortion at higher levels. The rest of the drivers were an 8" midrange unit coupled with the popular 2105 slot tweeter.