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#### mbegala

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If two different materials are stacked upon each other, what is the absorption rate of the combined materials as compared to the individual materials? Is it additive?

#### BasementBob

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mbegala:

You'd have to test it to know what the new absorbtion would be.

If material #1 had an NRC of 0.6, and Material #2 had an NRC of 0.8, then putting them on top of each other wouldn't give NRC of 1.4. They can't be added that way. As a limiting factor (not really accurate but as a rule of thumb), think of NRC as a % of energy absorbed -- there's no way to absorb more than 100% of the energy, and absorbing more than 99% of the energy is unlikely.

NRC is a weighted average of absorbtion of sound energy over many frequencies.

The addition of a second porous material (mineral wools) will increase the energy that's absorbed, moreso at the lower frequencies than the higher.

Some guess follow:

a) if it's more of the same material it will behave as the same material twice as thick

b) if it's porous over solid (3/4" plywood or drywall), then the solid can be ignored.

c) if it's solid/membrane over porous, then it's a membrane absorber.

#### mbegala

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BasementBob -

Thanks. I wasn't sure if there was a mathematical equation that applied.

#### Terry Montlick

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Quote:
 Originally Posted by mbegala If two different materials are stacked upon each other, what is the absorption rate of the combined materials as compared to the individual materials? Is it additive?
Unfortunately not. The only case when the individual transmission losses are additive is when the layers are an effectively infinite distance apart (not very practical for most construction ).

Regards,

Terry

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