Broadband Panels - air gap vs none

The problem with 2" 703 is that it's only really effective down to about 500 Hz. A 2 inch air gap behind it increases it's influence at 500 Hz. 3" 703 will probably go a little lower in freq. but I haven't investigated that.  If you increase the gap too much it starts doing strange things above 500 Hz without much improvement at that point. I'm shooting from memory here so I could be off a bit. It's been a long time since I read the white paper on the absorption and installation properties of 703.  If I remember correctly the air gap behind them shouldn't exceed 1/8 the wavelength of the lowest frequency the absorber is effective at. 

I don't think any thickness of 703 alone is going to address anything at 100 Hz or below unless it is feet thick and it will take a lot of coverage to do it.  If you have a peak around 40 Hz and a dip at 100 Hz it is likely that if you can address the peak, the dip will decrease.  100 Hz is the first harmonic of 50 Hz and that is close enough to 40 Hz to likely have some influence, especially after the wave has been around the room a few times.  Spaces between the panels will have little to no affect at 40 Hz or 100 Hz since that's not being absorbed anyway. That will take some low freq. absorbers like helmholtz or diaphragmatic to get down into the 40 Hz to 50 Hz range and actually do something.  Low freq. absorption in a small room is almost pissing in your boots.  It takes a lot of space to absorb low frequencies and the more space the absorbers take up the worse the problem gets and requires more of it.  IMO it is best to just get familiar with the low frequency inconsistencies in your room and make adjustment decisions based on that knowledge.  Monitor and listening position placement is a big deal.

I have 14, 2' X 4' X 2" panels of 703 in my 14.5' X 13' X 10' room and it made a huge improvement in the flutter echo I had but that's primarily mid and high frequencies. I did some testing but it wasn't very technical. I put up a reference mic in the middle of the room, clapped my hands and recorded the results. Then I added 1 absorber and recorded that, then 2, then 3 etc. all the way until I had 14 of them placed around the room leaning flush against the walls. Then I pulled them out at the bottom and recorded the results again. When listening to playback of those tests in headphones there was a slight improvement with them pulled away from the walls a couple inches. Take into consideration that this is all in a spare bedroom with a bed and some other furnishings. I don't have any low freq. treatment at all. It would take up too much valuable real estate. My stereo image is pretty good and the flutter echo is gone and that's what I was after so I solved the problem I was trying to address.

Adding the absorption definitely helped the sound of the room. It's nowhere near completely dead but the bed in the room probably adds to the absorption somewhat too. To do low freq. absorption effectively takes up a lot of space that I am not willing to give up. All of the corners are occupied so they're unusable.

I have too much stuff all crammed into the space and could use even more if it were available. There's a full sized RT3 Hammond and Leslie, 4 guitar amps and cabinets, my desk, rack, and monitors, a dresser, and another cabinet in the room. I have another Leslie in the living room I would like to move in here but there just isn't room for it. The wife agreed to let me use it as my studio as long as it can still double as a guest room. I'm happy as long as it sounds decent and I have a place to play. It does that.

The photos were before I had my monitors set back up.  I have sold the tape machine since then and the gear setup has changed a bit but it's still very similar.

This is when I was making them.

i.postimg.cc/XXM14b60/Acoustic-03-Frame-Assembly-Jig.jpg

i.postimg.cc/mD56p9dk/Acoustic-04-Some-Frames-Done.jpg

i.postimg.cc/4yH2D3M5/Acoustic-06-703-In-Frame.jpg

i.postimg.cc/QdDsqL9P/Acoustic-10-9-Done.jpg


i.postimg.cc/YCdVLT1Q/Acoustic-19-4-On-Rear-Wall.jpg


i.postimg.cc/NFvPwtcv/Acoustic-22-All-Done.jpg

Finished the building of the new panels. Due to issues with depth, I had to stagger the panels so the monitor speakers could set in a little bit. Each panel has a 2" air gap behind it. Going across from left to right, they are: 12", 6", 12", 6", 12". So the greatest depth is 14". I couldn't spare any more room than that. I've have not had time to do any testing yet but hope to over the next week. Unfortunately, the green panel in the center doesn't look as good as the others. But, it works fine. The picture is from when I started putting stuff back in place, but still not in final placements.

My panels are like the GIK old style panels. I wanted as much exposed surface area so there is no solid frame on the sides. From back to front, this is what I have:
1) frame made of 1x2, covered in muslin, to keep fibers in
2) three 4" pieces of OC703
3) plastic corner bead (perforated) - to keep the edges clean looking
4) Twill material - wrapped around and stapled to the frame

I wish I had read a bit more and tried a combination of softer insulation  (safe & sound) along with the 703. But I was under a time constraint and couldn't be down more than a few days. I will find out if it helped the room. Even a 1-2dB reduction of the peak at 40hz and the dip at 100hz would be a win.

I have read that too but only at certain frequencies and up to a point.  It does increase the absorption amount up to a point but the further away from the wall they are starts introducing strange shifts in the the frequency bands they absorb.  It doesn't/can't make them absorb lower frequencies.  That requires a different medium or design.

Did sweep tests with Sonarworks and also Arc Studio. The new wall completely took out the peak at 40hz. It reduces the null at 100hz as well but not as much. Like most rooms, the major issues have always been under 200hz. It's much improved now but I still need room correction to really hear the low end correctly.