[gkinberg]

I am considering building some acoustical absorbing panels with variable density. One inch of ~3 pcf (pounds per cubic foot) mineral wool on the exterior facing the room followed by one inch 6 pcf  followed by one inch of 8 pcf material closest to the wall. My goal is to have a broad spectrum absorber, using low density on the surface for good high frequency absorption and high density in the rear for good low frequency absorption. I of course think this is genius but I am a total novice. What is the opinion of someone who actually knows something about room acoustics?

Thanks, Garth

[Rob Babcock]

The conventional wisdom is that variable density isn't worth the trouble and won't really perform any better.  It does make calculations of the effects much less accurate and requires more trial and error.

[Glenn Kuras]

Agree with Rob.  The only way to really know if it works any better is to build them and test it. I think you are going to find that it really is not going to make all that much of a difference.

[MarvinTheMartian]

Garth:
Been there thought that ... I am a "outside the box" WOW,
BUT, once you start to think like sound "logarithmically"
1"+1"+1" will not remotely cut it.

Sound is log Powers of 10 = 1/10th"+1"+10" = 11"+ a smidgin
Reverse the practical WAF limits and you end up with
4"+ 1/4" + 1/40" and you are almost at the thickness of fabric.

Stick with one sheet of 4" oc703 and get 99% of the effect.

Shawn

 

[Big Red Machine]

Gk, A+ for effort.  I like your thinking.  However, it takes very little to tame the nasties and while it would be a fun construction project, in the end will probably not pay off as you suspect.

I have predominantly diffusion in my room and I can achieve this:

[gkinberg]

You guys are a buzz kill! J K, thanks to all for your input. My thought process was the following. I hear some harshness in the vocal range of the spectrum and I believe it to be due to reflections coming from the large bare wall directly behind my head. There are not many places amenable to treatment in my room so I wanted to get the biggest  bang for my buck in the area behind my listening position, hence the desire to have a broad range absorber. I have read that higher density treatments can reflect off axis high frequency, so low density in front and high density in the rear. However, due to the unanimous feeling that it is not worth it, then I can save myself the trouble. Is 703 the general consensus as the best material, because I might lean toward 705. In addition, 4 inches is pushing the limits, I am leaning toward 3 inches. Big difference? In intend to absorb first and then move toward diffusing  second =)

Thanks, Garth

[Russell Dawkins]

Do you have a mattress that you can experimentally stand up against the rear wall to gt an idea of the effect of absorption back there?

To absorb bass more effectively, I think the order of layers you are contemplating should be 1" 3 pcf, 1" 8 pcf, and 1" pcf against the wall. This will allow the  8 pcf panel to vibrate, absorbing bass energy. The main function of the 1" layer of 3 pcf against the wall is to add mass. Even better would be to use your sequence (3,6,8 pcf), but stand the panel off the wall by an inch, so the entire sandwich is free to vibrate, absorbing bass energy even more effectively.

I have done the rough equivalent to this in my last two mastering rooms to good effect, although in the first instance I used 8" of low density fiberglass and, in the second, three layers of surplus wall-to-wall carpet hung from a 2X6 frame and prettied up with a nice decorative rug facing the room. In both cases, though, the absorptive panels were behind the speakers; the rear half of the rooms were/are more diffusive, with much less absorption.
One approach to absorbing bass is to have a massive, lossy diaphragm free to vibrate. Surplus industrial conveyor belt used to be used for this, for example, but my approach is cheaper and easier.

[gkinberg]

Reviving this thread. I have been dragging my feet on this project partly because I’m not much of a DIY’er and folks didn’t think there would be much benefit. I’ve decided to go the more straight forward route and purchase commercially available acoustic panels. I plan to buy ~three 2’x4’x4” panels to start. At least one behind my seating position, maybe one on the ceiling and maybe a half size behind each speaker. I’m asking for input on good quality, high value panels. I’m most familiar with GIK, RealTraps and MSR. Is one company superior or do most just use 703 corning? My main goal is to tame some midrange harshness that I feel is reflection related distortion. As Russell suggested, I did place some fiberglass panels I borrowed from work behind my seating position and it did help some. I’m hoping high quality panels placed in this location and a couple of others will make a large improvement.

Thanks for any help, Garth

[jhrlrd]

Try a couple square meters of 3" Sonex on that wall behind your head. It diffuses and absorbs in a fairly wide frequency range. You can hang it half an inch out from the wall to get a little more bass absorption. I believe its most effective in the midrange.

[gkinberg]

I'm not familar with Sonex. Is this something that I can purchase finished as I don't seem motivated enough to DIY. Also, I'm probably most interested in absorbing for now and looking into diffusion later.

[Glenn Kuras]

We do not use OC but Knauf, which falls under our "Greensafe".  You can read about it here.
http://www.gikacoustics.com/greensafe/

Glenn,
GIK Acoustics

[Jambalaya]

I'm new to this forum but active on others.  Lots of great information, practical applications AND great gear. 
I have a couple of 2" GIK panels (no affiliation), the floor standers GOBO and they are wonderful for moving around easily.  I'm keenly interested in experimenting with thicker panels (I was warned by GIK that I might benefit from the 4" panels). 
Thinking of building a couple of 4" to try and tame a 50Hz room modal hump.  Doesn't seem to matter where I move my subs. 

Great forum IMO.  Look forward to participating more.