understanding wall/ceiling construction effects

[Jabbaz]

In a typical wall construction you have two panels of sheetrock separated by an air gap. Each panel considered on its own will reflect high frequencies and pass mid's and hi's. How is it that when you layer two of these with a gap between, that mids and lows are also reflected? Can you simplify this system to one panel that has the average density of the system evenly distributed? (i.e one panel that is more dense than air but less dense than sheetrock and of the original thickness)

My ultimate goal is to determine whether there is any practical value to filling walls + ceiling with rigid fibreglass. Ethan's article recommends filling unfinished basement ceilings with fibreglass and covering with fabric. I want to know if there's any point in doing this but covering with sheetrock. Will the sheetrock pass lows and mids that can be absorbed by the fibreglass behind?

Thanks,

Dave

[giles117]

When you fill the walls with rockwool or other insulation, it adds to the dampening effect (with regards to the absorption of the mids and lows inside the wall) to reduce the transmission of sound through the other side.(Increase transmission loss)

So in a nut shell to your last Q

Will the sheetrock pass lows and mids that can be absorbed by the fibreglass behind?

Yes. But for a different Purpose.

Now that is not the reason for Ethans recomendation. After sheetrocking your basement ceiling, you should also cover it with a layer of rigid fibreglass or semi-rigid rockwool to absorb sound. Which is the intent of him having you fill the cavity and cover it with cloth. In a Low ceiling situation it is best to have the ceiling non reflective as it is very close to your listening position.

[barefoot]

Dave,

Here is the ubiquitous transmission loss diagram for various wall arrangements.



You can see that the inner layers of sheetrock actually have a detrimental effect on transmission loss. The way to interpret all this is that the mass of the two outer layers, and the air gap between them, are most important factors for good isolation. Putting insulation between the walls does aid somewhat in direct absorption, but its most important effect has to do with increasing the effective distance between the walls. The insulation alters the thermodynamics of the air compression and expansion as sound waves pass through it. It actually reduces the speed of sound. And the lower sound speed translates to an effectively wider gap between the walls. So basically, the insulation helps to decuple the walls even further.

Thomas

[Jabbaz]

Yes. But for a different Purpose.

Now that is not the reason for Ethans recomendation. After sheetrocking your basement ceiling, you should also cover it with a layer of rigid fibreglass or semi-rigid rockwool to absorb sound. Which is the intent of him having you fill the cavity and cover it with cloth. In a Low ceiling situation it is best to have the ceiling non reflective as it is very close to your listening position.

I already have a single 2'x4' panel above the listening position to absorb mids and hi's - to complete the RFZ. I was hoping I might acheive some additional lower freq absorption by filling the walls/ceiling with rigid fibreglass. I've already put about all the broadband bass traps I can fit in the room. I don't know how much space I have to fill in the walls/ceiling, but it would be helpful to know what frequencies I'd be able to absorb for a givin thickness.

[giles117]

Hmm thought we covered that Question.

Again, the filling the walls with insulation will not help trap bass IN YOUR ROOM. You need to have bass traps mounted built or designed into the acoustical treatment of your room.

As barefoot so beautifully illustrated the infilling of your walls with insulation is to improve transmission loss through the wall, not improve your listening/mix position.

So If you have enuff bass traps you should have no bass control issues INSIDE your control room unless the bass traps you have installed are grossly inadequete for the job. Like the Auralex Lenrd, biggest Lie there is.

Can you diagram your room showing what you have put where or provide us with a pic of the various walls and treatment placements.