Question about NRC ratings - 705 and RockBoard

[RawDepth]

When I look at specs for products like OC705 and Roxul RockBoard, I wonder why the NRC test results only extend from 125 Hz to 4000 Hz. Don't they care about any other frequencies?

And why do the NRC ratings for 4" thickness not equal twice the rating of the 2" thickness? Who is doing the math for these guys, anyway?

 

[recordman]

Well measuring response below 125hz is very difficult and likely to be inaccurate unless done in a very large anechoic chamber which isn't very common. In fact on 99% of speaker freq charts you see the response below 120hz might as well have been a squiggly line drawn by a child. It's also suggesting that response below 125hz in most rooms is going to be a crap shoot. Above 4000hz they are suggesting that it is a given that the product will absorb those frequencies very efficiently. It's not hard to kill such a low energy wave (above 4000hz). So it's fair to say the range they give is very adequate.

4" thickness isn't twice as effective as 2" essentially due to the law of diminishing returns:

"n.
The tendency for a continuing application of effort or skill toward a particular project or goal to decline in effectiveness after a certain level of result has been achieved."

In my experience, the 2" stuff is just as effective in the midbass region when spaced off the wall 2-3" as the 4" stuff placed directly on the wall.

 

[Jeff_D]

When I look at specs for products like OC705 and Roxul RockBoard, I wonder why the NRC test results only extend from 125 Hz to 4000 Hz. Don't they care about any other frequencies?

And why do the NRC ratings for 4" thickness not equal twice the rating of the 2" thickness? Who is doing the math for these guys, anyway?

I suspect that the frequency range is tied to that of human speaking since the majority of this stuff is used to stop sound between apartments and such. Ahh, here ya go...

frequencies of 125, 250 Hz, 500 Hz, 1000 Hz, 2000, and 4000 Hz. These frequencies encompass the fundamental frequencies and first few overtones of typical human speech, and, therefore, the NRC provides a decent and simple quantification of how well the particular surface will absorb the human voice.

-from here... http://en.wikipedia.org/wiki/Noise_Reduction_Coefficient

As far as the the ratings for 2" vs. 4", I don't think its a matter of doing the math correctly. I think its more a matter of that just being the measured result.