Here is a summary of Abbey Road Studio 2 from the RTB Book.
Abbey Road Studio 2 is 60'L x 37'W x 28'H reverberation time is listed as 1.2 seconds. I guessing thats without accoustic screens? A dozen bass traps(large wooden boxes) designed to eliminate excess bass rumble on all 4 walls. Accoustic quilts placed throughout walls. 3 Indian rugs placed across the far end of studio to create "dead end", a more accoustically dry space with less echo and reflection. This is end is most often used by The Beatles. Uncarpeted "live end" of room often used for brass & string overdubs. Swing out partitions each 10x20 create temporary walls to box group in. Covered in acoustic tile maked absorption/diffusion effect. This provides more intimate less echoing sound. Thick solid wood floors,non-resonant, but solid surface made sound acoustically warm & pleasant. Painted brick walls had a acoustically reflective sound; however bricks had been deliberately laid irregularly to aid in sound dispersion. What appeared to be a high ceiling was in fact a large structure of decorative rectangle supports with thin panels of cloth strung in the frames. Sound actually reached false dropped ceiling first, travelling through it and into the rafters and ductwork above the studio which served to diffuse and trap it. The room's sound was an interesting blend of of reflection (from the hard surface floor and painted brick walls) and absorption (from the quilt, bass traps, ceiling and rugs). Its sheer size & acoustic properties give it a warm reverberant quality & a short, distinctive "hum" that resonates on any low-mid note produced in the room. While it has neither the bright reflections of a concet hall nor the low rumble of a basketball court, it is far more acoustically neutral. The room adds a dark coloration that emphasises notes & harmonic content.
And about Automatic Double Tracking-
Major summarised account of ADT=
So what was ADT, and how was it acheived? In simple terms,the process involved taking an existing recording, creating a duplicate "image" of it, and then aligning this secondary image either slightly behind or AHEAD of the original signal. Properly done, this created the impression of two seperate signals playing back almost-but not quite- in sync with each other, just like manually double tracking. One of the keys to the process (and the reason it is so hard to accurately recreate today) lies in the unique construction of Abbey Road's Studer J37 four track machines. All multi-track tape machines have 3 tape heads-Erase,Record,Playback. This is fairly self-explantory, but the Record head actually serves 2 functions. When overdubbing onto a track, the Record head also serves to playback the signal from the tracks which have ALREADY been recorded. This allows the musicians to record their new overdub IN SYNC with the existing material. Because of this function, the Record head is also referred to as the "Sync" head. Most modern tape machines have a single output amplifier per track. This amp is auto switched to play back signal from either the Sync head (when recording/overdubbing) or the Playback head (when mixing or playing back the tape). Therefor, the signals of the from the Sync head and Playback head can never be heard simultaneously. By contrast, the Studer J37 used at Abbey Road had TWO seperate amps per track- one dedicated to Sync and Playback. This meant that the signal from BOTH heads COULD be heard simultaneously. As the heads were spaced some distance apart, there was a slight delay between the time a recorded signal arrived at the Sync head and the time it reached the Playback head. This slight time offset and the two SIMULTANEOUS outputs were the vital features that made ADT possible.
Here comes some more summing up=
Take signal from off the record head of the tape and delay it until it almost coincided with the signal from the playback head, you might get two sound images instead of one. Tapping off Sync head signal from the Studer was easy, it came out of the Sync output on the rear of the tape machine and into control room patchbay, where it could be routed anywhere they pleased. What was needed was a way of DELAYING the output of th Sync head so that it was almost perfectly aligned with the signal from the Playback head. The solution was the play and record heads on the BTR2 tape machine were roughly twice as far apart as the ones on the Studer J37. It was worked out that twice one was almost one of the other, if you ran the BTR2 at 30 ips, it was comparable to the J37 running at 15 ips. This stroke of luck provided the perfect means by which to delay the sync output. Taking the Sync signal from the Studer and send it to a BTR2, there we would record and IMMEDIATELY replay it, so that that it was then delayed by the head gap between the Record and Playback head on the BTR2. This delayed signal then emerged from the BTR2 JUST as the original sound was being played back from the Studer.
Because the head spacing on the BTR2 was not EXACTLY the same as the one on the Studer-although it was fairly close- the delayed signal still required some fine tuning. To adjust the delay a varispeed was installed. Using a Levell oscillator and the Vortexion, you subtly altered the speed of the BTR2, effectively changing the delay, so you could then make this signal arrive either earlier or later than the direct signal on the Studer. This ability to have the second image appear BEFORE the main sound is one the key features the makes Abbey Road's ADT unique. Modern delay and chorus systems usually only allow for sounds to be delayed after the original signal, giving only a portion of the effect possible at Abbey Road.
Controlling the speed on the BTR2 gave engineers the power to determine just how close the two signals were to each other. I think the magic figure was 40 milliseconds recalls Gibson, if you got about 40 ms that sounded like ADT. Also contributing to the effect were the very slight motor and mechanical variations which caused the mechanics to drift in subtle ways. This drift was not significant, as these were professional machines, designed to run very solidly. But there WERE slight variations between the two machines that added richness and naturalness to the sound, making the effect difficult to accurately recreate digitally. "It was better because it was so unstable" asserted George Martin. "Someone had to monitor it all the time to make sure the tape speed was correct, which was why it was so good. Now, if you press a button, once it's there, even if its scanned a little bit, it's still alittle bit mechanical."