Sheet lead (or some other lead based material) would probably be superb if you could find a type that was still legal - I remember people lining the engine compartments of boats with some kind of lead product back in the days before "lead" became a four letter word.

I've often wondered whether the heavy stuff the dentist drapes you in when you get an x-ray would work.

Change of density/impedance is, as you say, the key.

Edit --- Aha - I found something I had heard of - it's called Mass Loaded Vinyl and is used in place of sheet lead for soundproofing applications. A sandwich of something like this and concrete board or something similar - aybe with an air gap in between might be useful.

I think that's what Sheetblok is. At least that's what it seemed like when I saw a sample of it.

It's placed between studs, preferably offset studs, and then drywall is installed as normal. The Sheetblok must be installed in a way that leaves no gaps for sound to pass through. It reportedly has the sound resistance of five sheets of drywall.

Also, keep in mind that drywall is heavy. For ceiling applications you need to find out if the ceiling joists can support that weight. If you're dealing with a suspended ceiling, then Sheetblok is the way to go.

Consult with the manufacturer to determine how this material should be applied.

Jim's post make the best sense. In practical terms, sound proofing isn't that big a deal - BUT you have choices to make. Density and airspace are required, and both are difficult. Most people, who don't have huge budgets work on the room within a room principal. If the outer skin is lined with a sandwich of plasterboard, insulation board and something like MDF and then the same material is used to inner skin a structure of timber - typically 3x2" it works really well. Audio studio suppliers sell neoprene strip that the timer stands on to help prevent structure borne sound getting through. A second floor, neoprene mounted on the building floor then gets fitted. Double doors that create sound traps work well too. I've built 6 of these for clients now on low budgets, and they work - timber construction is fairly simple, and they are effective enough to prevent an idiot on a drum kit being a nuisance outside the room. Sound treatment on the inside to sort out the standing waves uses commercially available material.

The killer is sealing them. Any tiny airgaps ruin the isolation - so cabling and services need careful management. Worst thing is that sealing the structure also means stopping ventilation. You'll need either aircon (even in england!) or at least some kind of ducting to get foul air out, and fresh in!

Only make any alterations with the co-operation and approval of the owners of the studio. I believe it is being leased.

I'm not suggesting this is the case here, but I know of minor interior studio wall changes being made and the owner was looking for a lease deal breaker because they had a better offer. After 12yrs there, my unsuspecting buddy had no idea this was the case and had to move out. He knew that was going to be so costly he'd have no funds to fight back.
HTH Cheers.