are higher density fiber sheets better at low frequency?

Posted: Fri Jun 20, 2008 1:44 pm

Hi,

I was looking back through my copy of Master Handbook of Acoustics by F. Alton Everest and found a graph showing the effect of density on the absorption properties of fiber sheets.

I was under the impression that higher density fiber sheets were better at low frequency absorption, but that if you go too high then mid and high absorption suffers because it becomes so dense its slightly reflective.

However, looking at the graph from his book it shows the opposite. The lower density fiber is better at low frequencies than the higher density, and the high density is better at high frequencies.

Am I wrong about this, or is the key for the graph back to front?! Or do you think its just inaccurate below about 400Hz?

Appreciate it if you can put my mind to rest on this matter!

Posted: Fri Jun 20, 2008 6:26 pm

I think the point of the graph (per the caption) is that there really isn't much difference between low and high density (at least between 1.6 and 6 lb/cu ft). I don't know if that graph is theoretical or empirical (measured), but I'll go out on a limb and guess that it is empirical - in which case the measurement accuracy may be in play here. The three curves are probably for all practical purposes equivalent.

Note that at 125 Hz, the "middle" density is the least absorbing, so even if the differences are real, they aren't linear.

What else does the book say about the topic?

Posted: Fri Jun 20, 2008 7:48 pm

Hi,

It says relatively little actually. It says that density doesn't make much difference. Then ti says that with very low density absorption suffers because the fibers are so far apart, and that for very high density the interstices are so small that absorption suffers due to surface reflection.

So I suppose as you point out, the graph is not to show the extremes, but just to show the normal range of density where it doesn't really have much effect. I think I just kind of expected the graph to show the situation where there is an effect!

Posted: Sat Jun 21, 2008 9:50 am

Density affects bass absorption, and so does FRK facing. See my Density Report for much more on this.

--Ethan

Posted: Sat Jun 21, 2008 3:43 pm

Thanks Ethan, good stuff.

This topic sponsored by: