NRC > 1.0?

[Vagabond]

On several of the fiberglass and mineral wool spec sheets I have seen, NRC values are listed at the standard 125-4k frequencies as being >1.0. I have heard two explanations for this:

1) Values >1.0 are possible in random incidence absorption tests (for a reason I am not aware of). In normal incidence absorption tests, 1.0 is the max and the NRC is the % absorption of the material (whereas this is not the case in random incidence tests).

2) Absorption at the edges of the material, particularly for thick materials, accounts for the >1.0 absorption coefficients.

Which if either is correct?

Also, I am looking at two products. One is very flat NRC ~1.0 from 125 to 4kHz, the other has a big hump in the 250-500 range (NRC~1.3) but stays near ~1 at 125 and above 1000. Which is better? I was thinking flat broadband absorption would be better but the 2nd one is the only one I can get in 16" widths without special ordering.

Thanks!

[knightfly]

That has been more than amply discussed here

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6446

Also, you need to read and follow forum guidelines listed in the very top link in this forum; thanks... Steve

[Vagabond]

Sorry, I have modified my location per guidelines (I am an internet anonymity freak). Also here is some added information for my application:

These will be for broadband absorbers meant to kill first reflections in a music listening environment. I live in an apartment and move every few years, but my current listening room is 12'x17'. I listen on one side of the 17 foot dimension so the listening room/setup is assymetric, and I only really have to kill the reflection off the right 12' wall (no windows). I also might want some absorption perhaps on the back wall for slap echo and perhaps on the ceiling for its first reflection point. I would like 16" wide panels for aesthetic reasons.

Sincere apologies,

Vagabond

[Vagabond]

Thank you! I have read through the linked page and many of the linked discussions contained therein. I didn't know what a hornet's nest I was getting into! I'm still not sure of the answer to my question (which is better) but at least I have a greater understanding of how NRC's can be >1.0. Thanks! And if anyone could help with the other question (which material is better) I would appreciate it.

[knightfly]

Can you provide links or descriptions of the two materials? Kinda hard to evaluate without knowledge... Steve

[Vagabond]

Both are 4" thick mineral wool from 2 local suppliers, the first is 6pcf and I couldn't find density information on the 2nd but I think it is 2.5pcf.

Code: Select all

Freq              125   250   500   1000   2000   4000
Material #1       0.99  1.01  1.10  1.03    1.03   1.05 
Material #2       0.97  1.28  1.25  1.10    1.10   1.09

The increased absorption at 250 and 500 in particular for material #2 is what I was wondering about - is that absorption hump desireable or not?

[knightfly]

That would depend on what you're doing with it; a room needs to be evaluated in its entirety before this can be decided. You're looking for a smooth decay time at all frequencies, and everything in the room (including the walls, floor, ceiling) affects this.

Also, the RT60 time for a room, in order for the room to sound "right", needs to vary depending on room size and the type of music you listen to/record in that room, and whether it's a CR or a recording room.

Bottom line is, the more you tell us about what you're trying to do the closer we can get to a usable answer... Steve

[Vagabond]

Well, one problem is that I am a college student, and move around a lot - so I can't really say what my room is exactly like, or rather what it will be like within a year. Any place I live for the next few years will likely be the same mediocre apartment construction with a living/listening room of similar dimesions (maybe 10'x15'), cut pile carpet on the cheapest pad they can buy, drywall on 2x4 studs on 16" centers assuming they bothered to build it to spec, textured and painted walls, overstuffed fabric sofa that's hard to get through doors. I do always get 1st floor apartments though, so concrete floors are certain. My thinking was that since equal decay time is desireable, to go with the material with flatter NRC. But I also have heard that it is easy to make the highs 'too dead' while not adequately absorbing the mid/lows. So maybe it would be better to go with the material with the hump in the midrange for a typical apartment environment. Again, I have no practical experience with this, and I know that with such vague information it is impossible to give anywhere near a certain answer. If I could just get an "Eh, I think I might go with that one over the other one, maybe, in that type of environment" from someone with more experience than me, I would be happy .

One advantage I have is that my measurement software is extensive, more than 2k invested in an acoustic measurement system to be exact. So once I am in a place, I can take measurements and figure out if I need more or less absorption, problem frequencies, etc. But I would like a general-purpose absorber to use as a starting point.

Oh, and this would primarily be for a wideband absorber to kill out first reflections in a stereo listening room (aka my apartment living room).