[joeq]

F Alton Everest's Master Handbook of Acoustics is a good book

I think you are on the right track putting your hallway up as an insulator between you and the compressors. But low frequency noise is going to travel through the frame of building no matter what your walls do.

Decoupling and mass are your only hopes. Room within a room.

Is the guy an 'asshole' for any other reason than because he requires air compressors to do HIS work? Can he be reasoned with? flies/honey/vinegar etc. etc. Maybe instead of fighting with him you could bribe him.

Or is the vibe already poisoned? How much does the landlord actually give a crap about your problems with this guy's compressors?

Paying to insulate, elevate or even relocate his compressors could be significantly cheaper than trying to build a wall that would block the sound. Even if you have to pay more than just his 'costs'. Buy him a newer, quieter compressor?

You did not specify how far along you are in construction or how locked-in you are to your current situation, but it is my firm belief that excessive local noise is the most intractable problem in any studio build and that finding a different place almost always ends up being the preferable solution.

I hate to be a downer, but I speak from bitter experience and the close observation of the bitter experience of others.

You may recoil in horror at the thought of starting over somewhere else, but if your alternative is 3 or 4 years of misery and unsatisfactory recordings and THEN starting over somewhere else anyway, NOW which do you prefer?