DIY construction methods of hang-able acoustic panels (not fixed frames) - Page 5

agreed ^^

mtx,
when you did your last round w/ the cloud - was the cloud positioned directly above the listening position or was it at the ceiling reflection point?

Hi guys,

I've been following a number of the acoustics threads here for a while, though I haven't been very active on the AVS forums. Mike suggested that I share some of my work on room acoustics with everyone here, so here are my corner bass traps:

I followed a similar construction technique that Mike posted for his corner bass traps, but made mine from pink fluffy R-19 instead and wrapped the fabric around the outside of the threaded rod. Here is the photo journey:

My traps are 24x24x34 inch triangles, are about 3.5 feet tall, and probably weigh less than 10lbs each. Since I need access to one of the corners for a doorway, I made them stackable and moveable. I started by cutting triangles, drilling holes for the threaded rod, and inserting T-Nuts for the sides that stack on one another:


Then I cut 23" squares of insulation, cut them diagonally for triangles, and clipped off the corners so they fit snugly between the threaded rods:



A wire mesh made from separated Cat5 wire goes between each layer to keep the insulation from sagging over time. The numbered arrows indicate the direction of winding the wire to support the insulation. A bead of solder keeps the wire from unwinding. Each layer of insulation is about 5" thick.


Here is the first one all stacked up. You can see the supportive wire mesh wrapped around the threaded rod on each side:


And then with the Kraft paper glued to the front with spray adhesive:


Then, turn the trap upside down to affix the cloth wrapping - a two-pack of curtains from the giant W for $15. Each pack is enough for two traps.


The cloth is stapled to the underside of the top plate to prevent sagging over time:


When the fabric is fully secured to the top panel, turn the trap right side up again and pull the fabric around to the back. Trim off the excess and then just pull it tight and use a desk stapler to hold the fabric together in the back:


The staples produce a few ripples in the sides, but you won't see those once you put it in the corner. The front looks nice and clean:


Here is a closer shot of two of them stacked together:


And finally, the entire back half of the theater. With a few bean bag chairs up front, we can comfortably seat 10-12 people. The colors are a little off from a combination of CFL and flash lighting:


Below are the before and after REW plots. The purple trace is the original measurement with no traps and no EQ. The yellow trace is the difference made by the bass traps alone. The traps took 5dB off of the room-induced peak at 45Hz without sacrificing anything else in the audible range:


This is the original waterfall plot made by REW - no traps, no other corrections - just a mess with room modes at 45Hz and 90Hz:


And here is the waterfall plot after ONLY the traps are put in place. I was surprised by how much of a difference the traps made in the decay:


After several days of tweaking the parametric equalizer with the traps in place and a first order high-pass filter in place, here is my "final" room response curve - flat from 7Hz to 100Hz, plus/minus 3dB :'( The peak at 105Hz won't ever really happen because the preamp crosses the LFE channel at 60Hz.


And the "final" waterfall plot - nice and smooth. If I adjust the waterfall graphing limits in REW, the entire response curve is down by 20dB within the first 100-120ms with the exception of a 2-3dB narrow bump at about 23Hz. I am really pleased with this result:


It sounds great! The EQ and high-pass filter reduce (but don't eliminate) the incidence of amp clipping, but still provide enough punch to cause visitors to literally jump up from the couch :P

All of my other projects are on my web page, which I think is linked in my signature.

Eric
Edited by Eric2000 - 6/14/12 at 9:32am

That's good work, Eric. I think that is the first implementations of the pink fluffy bass damper I've seen detailed out (mine uses fluffy but has a foam core so I only have one thickness of fluffy which requires no support--but is probably less optimal). I've seen suggestions to use bird netting to support the layers but the wire thing is easier for this type of construction and definitely inexpensive. I've built some boxes with several horizontal layers of fluffy but they aren't really tall enough to need support for each layer.

Do you have before and/or after sweeps for a variety of listening positions?

And could you post a 0-300Hz waterfall taken with one of the mains and the subs on? I'm interested to see what really happens to that 105Hz thing in real life (among other things).

Just saw your question Mike:
I'd guess the answer is that it depends on the source material. Yes, it's said that we only "hear" in 1/3-octave bands, but by the same token I personally don't think it's too difficult to hear individual bass notes at half-tone intervals. I suspect the 1/3-octave research was done with filtered noise or some methodology that didn't provide all that much possibility for frequency-volume selectivity for the ears. I bet a walking bassline that changed volume a lot would be pretty noticeable. And if you check out the wiki entry for piano key frequencies, they go down to ~27 Hz with pretty small frequency spacing. I do think the specifications for monitoring environments specify 1/3 octave filtering for their judgments -- and I think that is fine for overall tonal balance. But I'd have to go digging around to find enough information to really have a great answer and would be interested to hear if you have gotten any answers from people who are better schooled than I, LOL. Anyway my feeling is that 1/3 makes complete sense above the Schroeder frequency where our ears are good at removing comb filtering effects, but might not be tight enough below it. However since the octaves also get smaller down low... I guess I could be way out in left field, as it were, on that.

I've seen the suggestion for bird netting as well. I have some, but don't have a good idea how I would have used it to support the insulation between each layer and still have it look neat. The wire just seemed easier and more straight forward. I don't have any sweeps for other locations. Most of the time, it's just my wife and I and we both fit in the sweet spot

I also didn't do any measurements with the mains and the sub together. Just after I took all of my measuring equipment down, I thought of both 1) measurements at other seats and 2) integration with mains - go figure. Due to the layout of my equipment (most of it is in the room on the back-side of the screen wall) it is a major project to rig the IB for measurement and adjustment again. I'll eventually get around to it, but it's not likely to be anytime soon.

The sub is an IB that is mounted two feet behind the screen wall and the mains sit about 3-4 feet out from the screen wall, so I suspect that same peak at 105Hz won't happen with the mains. I need to do some crossover tweaks to my center channel at some point, so I do plan to measure the mains before too long.
Edited by Eric2000 - 6/14/12 at 11:19am

Again Eric, nice implementation of the pink fluffy,....nice time domain results

mind the compression of the insulation; low GFR material should be loosely filled. have fun!

whoollllyyy begggeezzuss...

This may be a dumb question but...Is there an reason why hang-able acoustic panels are nearly always 2'x4' other than convenience of the available material sizes? Meaning is the some wave length reason?

I realize the size works nicely to cover a reflection point in relation to seating position and speaker position. But than again so could other sizes.

I have a specific situation that I'd like more of a 2x3 horizontal panel to work in my design.

Nicely done DIY!!

Great Post Guys!

Thanks for the info it will save me alot of trial & error.

Now its time to get building!
Edited by Davide Feltrin - 4/1/13 at 2:28am

Question. I plan to make broadband panels and bass traps. I can get 4 inch mineral wool with density of 8 pcf. I know greater the density better bass absorption, but is it going to hurt my broadband absorption?

Thx for the info. According to that website, 4 inch 8 pcf mineral wool has pretty good broad band absorption.

The values given by manufacturers for materials are random-incidence (diffuse field) coefficients, which are noticeably higher than the absorption of a single-incidence situation. Diffuse-field coefficients are not generally applicable in small-room acoustics. You need single-incidence measurements or estimates from the angles you intend the absorbers to work at, which are not as easy to come by. More info in Toole, p.474 and figure 21.9. Probably the best free way available today to estimate what you might really see for single incidence is to use the Whealy calculator and find the gas flow resistivity for the proposed material. There is a thread on gearslutz attempting to collate all the flow resistivity values available in once place which is probably the easiest way to get an estimate for a proposed material. Of course testing in-situ with the ETC method that mtb described also works.

You could probably consider the random incidence coefficients a hint; some data (even if only obliquely applicable) is better than no data of course.

"I know greater the density better bass absorption, but is it going to hurt my broadband absorption?"

Greater density = "better" bass absorption (assuming better means higher, or moar) is not really the case. I strongly suspect 8 pcf is overly dense, but since I haven't looked into absorption of rockwool that much I can't really say what might be better (if you are sticking to rockwool for whatever reason - without knowing your design limitations there's really no way for someone else to make suggestions).