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effect of the extra absorber layer

post #1 of 8
Thread Starter 
What kind of sound attenuation improvement at low/mid/high frequency ranges can be expected when adding a 2"-3" thick absorber material such as owens 703 or roxul safe 'n' sound in between two 4" thick brick walls separated by a 6" air gap? (so the final air gap reduces to 4"-3" after adding the absorber layer).
post #2 of 8
OC 703 would be a waste of money in that application. Use that on the inside of the room for acoustic panels. Cheap fluffy insulation is generally recommended. The safe and sound is not that expensive but more than fluffy fiberglass and you would not notice any difference. Here is a chart showing the STC of various materials. Double brick is not shown so I'm not sure how well the entire assembly would do.

http://www.sae.edu/reference_materia...TC%20Chart.htm
post #3 of 8
Exactly. Just throw some R13 in there. The insulation will not make a big deal in any event.
post #4 of 8
Thread Starter 
Is it possible to get similar insulation if i reduce the air gap to 2" and fill it with 2" 703?
post #5 of 8
No. The air volume is a factor in the calculation of the resonance point of the partition. Reducing the air volume will raise the resonance frequency of the wall, and in any event 703 is too dense.

We want the resonance point as low as possible in the walls and ceiling. The lower it is, the lower the frequencies we're isolating. That's an admittedly broad statement, but a good rule of thumb.

Assuming the system is decoupled, the two factors that define the resonance point is the mas and the air cavity volume. Mass is a much bigger driver, so a smaller air cavity is more than offset by an increase in the mass.

The foundation wall, due to the mass of the foundation, will tend to define a pretty low resonance point, so if you were needing closer clearance on the foundation walls, I'd consider using clips and channels and attach to wood furring secured to the foundation.
post #6 of 8
Thread Starter 
Little bit of clarification..

This is what i understood from your post.

The contribution from mass is a much bigger driver for resonance frequency. In that case, if i reduce the air gap to 2" and do not fill anything inside, the resonance frequency (and hence the sound insulation) will not change much. Any small change can be offset by using clips and channels attached to wood furring rather than filling 703.

Is that what your are suggesting?

Thanks
post #7 of 8
A reduction or increase in cavity volume will reduce or increase the resonance frequency of the wall. Big drum vs. little drum sort of thing.

A reduction or increase in mass on the decoupled framing will reduce or increase the resonance frequency of the wall also.

Of the two, the mass will have a bigger effect on the ultimate frequency.

Any hollow cavity will require insulation to deal with cavity resonance.

703 is too dense to use in walls / ceiling.
post #8 of 8
Thread Starter 
Later realized that having a 6" space would not be practical as it will reduce the room space quite a lot. However i still need a pretty good isolation. My plan is to reduce the fap to 3" and put Roxul's Safe'n'Sound 2" between the walls which will result in a 1" air gap. Appreciate if some one can help me in finding sound attenuation at low frequencies for such a setup.
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