While it sounds like a plausible option, I wouldn't give isolating my turntable into a separate room any consideration. First off I don't like the idea of missing the first few notes, although I may not be positioned yet, and second....I didn't pay what I did for my turntable to hide it. If that means giving up quality (which I'm not sold on) so be it.
Several years ago I was concerned with airborne vibrations and decided to try and isolate the turntable. It was already in a 2'x2'x4' cabinet with a 4" wall at the back, 1" plywood shelves at tip and bottom and 1/4" plywood at left and right sides. The front is open.
After considerable research on sound proofing material I specialized 1" think very thick foam and proceeded to attach it to the five sides. I then cut a 1" plywood door, attached the foam, and temporarily attached it to the cabinet to completely seal the turntable. The big problem with attenuating noise are the low frequencies, say under 300Hz. Certainly 1" foam is not going to help much.
I played the first record and there was quite a dramatic change in sound. I was excited an started making plans for a more permanent isolation chamber, however, after a few more days of listening I realized the sound was unbalanced. What could have caused this.
I got out my mic and REW and did some sweeps with the mic resting on the turntable with the temporary door open. I then did the same sweep with the door closed. There was a considerable change between the two sweeps. Door closed measurement was far lower at over 1000kHz, almost fully attenuated. However, a severe resonance had been created below 100Hz (don't remember the exact frequency) undoubtedly caused by creating a sealed box. This resonance must have fed into the turntable and changed the frequency spectrum.
I think it is very difficult to build a sound proof box, unless you are Herzan. I would prefer Bruce's solution or a sound proof area built into the listening room. Today's gear is so incredibly sensitive that I think it is underestimated how airborne noise will affect playback.
My table sits on a Black Diamond Source shelf which sits on 2'x'2'x4' 2" thick maple shelf which is levitated by six Newport airpods. I use a considerable amount of specialized plastic materials as well as Isodamp for additional vibration control, so I am reasonably happy with the control of floor borne vibration.