It’s not only “interesting” it’s basic acoustic physics. If the owner is interested in isolating his rooms he should read Home Studios Build It Like The Pros by Rod Gervais. Rod was the contractor who build many studios including The Power Station (now Avatar I believe). As well as lots of practical advice including construction drawings there is also a great section at the back of myth busting to save you wasting money on stuff that doesn’t work.
Thank you very much, again.
Sad thing is I don't think he's going to rebuild the four rooms. He is not making a lot of money these days. I'm willing to pay for a nice sounding rehearsal room, like the ones I used to play in when I was in Berlin, but the rest of the band don't care too much about that (and this rooms are the only ones close to the night life area, you know).
Anyway I'll tell him all this valuable information. Maybe he could start, at least, thinking about it.
I'd really like to read that book you talk about. Let's see...