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M-S-M systems. Air springiness question.
Old 14th February 2020
Here for the gear

M-S-M systems. Air springiness question.

I'm in the final stages of constructing the isolation for my studio- this forum has been a tremendous resource.

My outer walls are high density concrete block with an interior air gap and independent stud with two layers of high density 15mm Fermacell gypsum board. All good.

However, one of the concrete walls has a 15cm air vent hole in it. Outside is pretty quiet and not going to intrude noise wise. I have a choice to block or not block this hole- not blocking would allow ventilation to the interior of the M-S-M system, which building control have recommended.

So my question is this: in an M-S-M system, my understanding is that the -S- bit, the air spring, had to be completely trapped in order to create the 'spring'. Is this not true?
I read Stuart's brilliant explanation of the principal of MSM and the fact that the wider the gap, the lower the resonant frequency of the system.
Avoiding triple leaf wall question
But what if the air within that system is/isn't under pressure i.e trapped or not trapped? Surely that affects the f0? Actually the less 'tense' the air, the lower the frequency, so a pressure release is actual a good thing. Or am I thinking this all wrong?
Old 14th February 2020
Here for the gear

Ah just found this post from Sebg:

I did a test many years ago in an Acoustic Laboratory with a double stud wall. tested the TL of the wall. Then cut a hole in one leaf of the wall, to ventilate the air cavity. Attached an acoustic duct to the hole so that TL would not go down due to leakage.

It was a test of how much the stiffness of the air cavity might be reduced since the wall cavity volume was now connected via an air path to the whole room (on one side of the wall).

Perhaps the hole wasn't big enough but there was no effective change to the M-A-M and TL performance.

M-A-M is a low-ish frequency, usually. Large wavelength. Probably fairly stiff locally.

I think this answers my question well: the air in an air gap does not have to be airtight in order to be springy. (but obviously it needs to be sealed to avoid airborne sound intrusion- a different point)
Old 15th February 2020
Lives for gear
Kyle P. Gushue's Avatar
Here Rod discusses an instance where someone wanted to vent a wall to eliminate it from being a leaf. An air spring doesn't just disapear if there's an air leak, the leak like a pressure relief valve. Just like a tire with a slow leak can still be driven around on.

Id still personally prefer to block the whole because i dont like weird stuff in assemblies unless they are required for some good reason.

"Perhaps a little - but it still creates a back pressure that will affect the TL values of the wall.

Think about it - you have sound pressure build up against a wall inside a room with no wall in front of it. You have sound pressure build up in a Helmholtz Trap even if you use 1x4 boards spaced with 4" air spaces between them - what kind of pressure relief from sound pressure do you think your little vents are going to be?"
Old 15th February 2020
Gear Addict

If you know the volume and the size and lengte of the hole you Can calculate the resonance frequency conform a bass reflex loudspeaker system or helmholtz resonator.
Old 15th February 2020
Here for the gear

I definitely won't block it then. I guess this is an unusual scenario- we are reducing flanking noise in the wall, not airborne, so it's not a problem having a smallish vent in the exterior skin.

Thanks all.
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