Question, Double leaf + a cupboard?

Hi, Firstly thanks to all the contributors this site, such a great source of info and experience and a real help to someone like me starting from scratch. This is a newbie post , my question is kind of specific but below a quick background, if you'd like more info please let me know and I'll provide as much as I can. Background Im leasing two old offices in a warehouse back to back to use mainly as an after hours studio and also for some activity during office hours. Each office is 4.35m (length) X 3.35m (width) X 3.3m (ceiling) and also considering opening up the dividing wall down the track. Meanwhile, I want a degree of sound isolation in one room to protect the corridor and a neighbouring office (enough to bring moderate monitor levels, acoustic guitar strumming/singing down to a level outside the room that blends with competing traffic / building noises around an industrial estate). I'm building isolated walls in front of the existing single leaf old wood partition walls with 110mm gap (including insulation) before the existing wall so the aim is Mass Air Mass in a two leaf system. My question is this, the wall facing the corridor is fitted with a cupboard that lies flat on the office wall and juts out in to the corridor space, its about 500mm deep. With the doors closed I think this then makes a triple leaf system with the existing wall and the new internal wall where the cupboard is. ie. MASS ( new internal isolated wall) -AIR -MASS (existing cupboard doors closed) AIR (cupboard interior) - MASS (back of cupboard) Should I a) seal the doors air tight? or b) actually leave them slightly open so the cupboard air space becomes part of the larger single cavity? Also would stuffing the cupboard with insulation be a good move to help stop it becoming some kind of bass resonator into the corridor? Any advice would be much appreciated! cheers Tim

The cupboard could become extra depth for trapping by removing the doors and shelves and filling with insulation and covering with cloth. Question is, what does the rest of the construction look like in terms of isolation? Ceiling and walls?

Thanks Glenn, I like the idea converting the cupboard to extra trapping, will take that approach , if there is a wall in front of the cupboard does it require cloth covering or is it more to protect us humans from floating fibres? In terms of the rest of the structure I'm bound a bit by rental restrictions and economics for the time being. I've just discovered the wall that I share with my neighbor is actually a thinner 10 - 15mm wood partition with a 25mm gap (no insulation) then a double or triple brick wall. The brick wall also extends up beyond the ceiling (which is plasterboard). I should've picked this up earlier, its a structural wall. This wall is where a second story begins in the rest of the building (2nd story unoccupied) so happily I think there is already some decent blocking for sound flanking over the ceiling and already a fairly solid mass to help my cause. I had noticed that I couldn't hear anything coming from that office unless I walked right past the door on to the corridor. Muggins here already has the track and stud with insulation up but I like the effect that this has and was thinking of installing wood panels and leaving some of it as insulation covered in cloth for some internal sound treatment ( it has definitely helped with the overall sound I just have to take some of the ring out of the steel studs!) The wall opposite backs on to my other room so I am not too concerned about isolating it in that direction through the ceiling or partition wall at this stage though there would be some flanking to the corridor. I think anything loud enough to effect the room two doors down would happen after hours. They sometimes have the radio blaring and I can hear it in the room that shares their wall but not in the room I'm trying to proof at the moment. Luckily the corridor in the other direction also has a door that I can close to help reduce the sound to the room I'm trying to protect. The 4th wall backs on to the street which is again a wood partition wall with a small cavity and single or double brick but also an old leaky window. I think the street noise is enough to help take the focus off any noises escaping that way for the time being but may just add mass to that wall. If I build an internal wall with a cavity there I am definitely looking at a triple leaf set up. I've pent some time installing window plugs etc and the traffic noise is greatly reduced (not perfect but manageable). Thanks again, any further thoughts appreciated. Tim

if the corridor behind the cupboard is ok for letting sound into, then that can help with relieving some of the bass. conversely you could end up getting noise in... the cloth is to keep the fibers in and allow the air flow into it. if necessary some panels in front of any room absorption can help support HF in the room. just ensure the key reflection points are covered by absorption and/or angles. adding some additional layers of drywall may be what you need to increase the isolation but maybe now is the time to run some tests. using some sine sweeps and white noise to assess isolation and some tapping (metal and rubber hammers) to check flanking paths.