(New) DIY Soundproofing plans

[Wurlitzer]

OK, after my previous postings about ventilation schemes for a double-skinned soundproof building, I sat down with the friend who is helping me build my new room, and tried to get to the root of the problem. Our thinking went something like this:

1. Sound is carried by air. If you let air out of a room for ventilation, sound will/can/might go out with it. This is the essence of it.

2. As sound contacts solid surfaces, some of it is absorbed and turned into heat. In a ventilation system, these surfaces might consist of the air duct itself (which will absorb more sound the greater its sheer length); corners and turns; or semi-porous barriers made of rockwool or whatever, through which the air has to pass.

3. HOWEVER, anything like this that absorbs sound, also impedes the air flow and thus makes it harder for the fan to move the air. Getting a solution that blocks enough sound from leaving the room may end up making the fan unable to push air at all (thus no ventilation), or require a fan so strong and fast that it would be unacceptably noisy.

Have we got the basic physics right so far?

If so, here's our tentative solution:

What if we used all these sound-absorbant factors in designing the air duct - length, corners, insulating barriers - and accepted that to push and pull air past these, we will need several very strong fans, moving very fast and making noise.

We then REPOSITION the fans further down the air duct, to a place well insulated from the inside of the studio, so the noise they make doesn't matter!

In my case, developing the design I posted on the other thread, I'll have two air ducts, one for intake one for exhaust. In the exhaust one, shown below, there will be two mega-super-ultra-industrial uber-fans (shown in red) installed within a little brick box at the edge of the studio wall. These will pull the air down through the air duct, round a couple of corners and out the end of the brick box. There will be rockwool or similar insulation (shown in grey) at the hole in the inner wall, and between the two fans themselves, through which they have to pull the air.

The fact that they will have to work extremely hard to do this won't matter, as they'll be out of earshot of the inner room. They shouldn't even be that loud outside, as they'll be encased in brick. The electrical wiring to power them could run up the air duct itself into the studio through the same hole.

The intake fan will work exactly the same way but in reverse.

Whatcha reckon, on both practicality and soundproofing?

[tmix]

What If you simply make the duct size bigger to allow for less restriction, but still use baffles and turns? As I see it sound is not like dust particles that float on the wind. Simple put enough absorptive obsticles in front of thesound waves and let the air flow around the turns.

[sharward]

You'd need to make sure the fans were accessible for servicing, that's for sure.

You'd also have to be careful not to firmly connect your inner and outer leaves via the duct. Flex duct may have to be used for a portion of your runs.

Have you seen my silencer designs?

--Keith

[Wurlitzer]

You'd need to make sure the fans were accessible for servicing, that's for sure.

Good point. Maybe a hinged top to the brick encasement.

You'd also have to be careful not to firmly connect your inner and outer leaves via the duct. Flex duct may have to be used for a portion of your runs.

Yes, flexible plastic has been sugessted. I presume I would then maybe put some flexible neoprene or something around the duct where it joins the outer wall, to minimise transmission of vibration.

There's no way to absolutely decouple the duct from the outer wall, for the simple reason that it has to pass through it to get air from the outside world! This IS a shame, as with the split foundation I do actually have TOTAL decoupling between the two leaves of wall in every other respect. But I'm thinking something like this will be the best compromise. Unless anyone's got any better suggestions . . .