NRC? Absorbtion Coefficients?

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Diffusion

Future Astrophysicist
I was just reading a section in a home recording book regarding acoustics and room treatment, and noticed that it seemed like the author was talking about NRC ratings and absorbtion coefficients as two separate systems of measurements, as in, not the same thing. I know that NRC stands for Noise Reduction Coefficient, but doesnt that sound the same as STL (Sound Transmission Loss)? I always thought that NRC had to do with how much a material absorbs, but it sounds as if it means how much a material reduces noise... can someone clarify all this for me? What is the difference between NRC, STL, and absorbtion Coefficients?
Also, if NRC ratings really are how much a given material can absorb sound, with 0 being nothing and 1.0 being complete absorbtion, how can it be possible for a rating to be above 1? I have seen ratings of up to 2.03, but if 1.0 is complete absorbtion, how is it possible to absorb sound that has either already been absorbed, or doesnt exist? Am I making any sense? :confused:
 
Numbers higher than 1 are a function of testing methodology and the formulas used. If I have 8 pcs of material that are each 8 sq ft and I measure the absorbtion and then divide by 64, I get the amount per square foot. HOWEVER, the edges of those panels are also exposed and in some configurations, the backs may also be exposed. BUT I'm still dividing by only 64 even though there could be 80, 90, 100 sq ft exposed and absorbing.
 
so... from what i understand your telling me that while ratings may go above 1.0, anything above 1.0 doesnt really count? or am i misunderstanding you?
 
Diffusion said:
so... from what i understand your telling me that while ratings may go above 1.0, anything above 1.0 doesnt really count? or am i misunderstanding you?
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1 is a theoretical value of what would be absorbed through an open window (presumably all). But because of the edges the panel has more absorbtive area than the supposed window and thus absorbs more than the window.

But you are right, once you are around 1 you are essentially absorbing all and you can't absorb more than all.

The other part of your question is the key that absorbtion and isolation are two entirely different phenomena with two entirely different measures.
 
1 is a theoretical value of what would be absorbed through an open window (presumably all). But because of the edges the panel has more absorbtive area than the supposed window and thus absorbs more than the window.
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I believe diffraction has a bit to do with values over 1 as well.
From what I've been told, due to diffraction at the edges, a corner absorber may also absorb more than the coeffecient rating of the material per square foot, even though the edges are NOT exposed. It is also diffraction that contributes to higher absorption values when panels are spaced in checkerboard configurations. Diffraction is still not well understood, even in labs. And frequency also has a bearing on diffraction. Or so I've been told.
 
i believe it was one of the bass traps at realtraps.com, somewhere around the 500hz area...
 
Can I get a response from Ethan Winer here?
 
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