http://www.nrcratings.com/
 
Noise Reduction Coefficient (NRC):
The NRC is a single-number index determined in a lab test and used for rating how absorptive a particular material is. This industry standard ranges from zero (perfectly reflective) to 1* (perfectly absorptive). It is simply the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz) rounded to the nearest 5%. 

*(Based on the testing methodology, and depending upon the material's shape or surface area, some products can test at an NRC above 1.) 


While NRC is widely used and accepted, it can also be abused or misunderstood. Be aware of the following items before specifying a particular material based on NRC:

• The NRC rating is an average of how absorptive a material is at four frequencies (250, 500, 1000 and 2000 Hz). This rating is appropriate for assessing how well a material absorbs sound within the speech frequencies, but can be inadequate for sound generated by music, mechanical equipment or other low-frequency sounds.
• Because this rating is an average, two materials with the same rating might not perform the same in identical applications.
• The NRC is based on lab tests. Because the lab is a near perfect environment that is rarely duplicated in everyday applications, some products will not test the same in the field. Certain factors, such as installation variables, are not accounted for in the lab. A product that receives high ratings in the lab may not perform as well in the field.
• Make sure the mounting procedure used in the tests is consistent with your intended installation if you expect the same results.
• NRC does not have anything to do with the material's barrier effect (STC). Click here for our tutorial on the differences between NRC and STC.
• Communication of NRC ratings by manufacturers can be misleading and sometimes deceitful for the following reasons:
  • Some manufacturers will quote absorption at the more-desirable higher frequencies. NRC should be based only on the absorptive characteristics at 250, 500, 1000, and 2000 Hz. Make sure the product data you're reviewing is at these frequencies.
  • A manufacturer of a wall carpet product could provide an NRC rating of .80, which is extremely good. But, if there is fine print be sure to read it, you may see that this rating was achieved while the carpet was installed over fiberglass. In this installation configuration, the fiberglass, not the carpet, acts as the sound absorber. Without the acoustic material behind, the wall carpet will probably only achieve an NRC of .20.

It is advisable to seek an un-biased third-party, such as an acoustical consultant, for NRC confirmation on a particular product and installation. Click here to find an acoustical consultant in your area. 

NRC ratings exist to simplify things for builders, and aren't referenced much by acousticians, who consider them obsolete, inadequate and largely irrelevant for studio applications. 
 
There are two main limitations to NRC values. First, they don't even bother measuring below 250 Hz, where your biggest problems lie. Second, they don't take into account the thickness of materials, only surface area. Fine for picking out ceiling tiles for your office, but not very informative for studio treatments.
 
An NRC rating of 0.65 might sound impressive, suggesting an absorption efficiency of 65% and clearly beating out drapes and carpets. But it's only half as effective as a 4" 703 panel of the same height and width. Yes, proper absorbers will have an NRC rating greater than 100%.
 
 

all i can say is, they work.
 

For comparative purposes here are the OC 703 published values.
I used some typical "street" prices (www.acoustimac.com) to calculate a price per square foot. 
The Audimute Sound Absorption sheets are $1.66/sq.ft.
 
thickness    125hz    250hz    500hz    1000hz    2000hz    4000hz   NRC    $/sq.ft.
1"                   0.11       0.28       0.68         0.9           0.93         0.96          0.7      0.96
2"                   0.17       0.86       1.14         1.07         1.02         0.98          1         1.40
4"                   0.84       1.24       1.24         1.08         1              0.97          1.15    3.25
 
 
 
 

Hi Dave,
 That's an interesting website. I was not aware of Acoustimac. I have been buying rock wool at the cash and carry counter of a large insulation distributor which is located approximately 600 feet from Acoustimac's address for the past 15 years.
 
 :-)

hey mike, I've been looking into Roxul (I ALWAYS forget the name,  saying sheetrock instead) cuz a lot of DIYers seem to jump on it for sound absorption paneling. I am planning to build some permanent wall mounted frames backed with rock wool... does anyone have some tips? A pricing estimate?
Based on my gear choice, the room I'm using at the moment (and will be for a long while) is currently more equipped for recording than monitoring/mixing (I don't own a pair of monitors) but I do want to be prepared for the future. I've read that bass traps need mass (somewhere here I think said it a while ago), so what are good methods for building bass traps and corner panels?

I just have mine sitting in the corners in slip covers that were made out of cheap fabric.
 
Today I was tracking some congas instead of mixing so I picked up 4 pieces and moved them into my recording room and placed them against the walls where the congas were.
 
It seems so much easier than making frames and hangers and stuff like that.

I forgot to mention that I used sound blankets to cover the pieces of the trap kit in the recording room so that those drums' resonance and ringing didn't effect the work we were doing with the congas.
 
The point being that having a variety of materials to work with is practical and useful.