Need a solution for treating a unusual back wall..

[Chris Shaw]

After reading all of the great info in this forum I think I have a pretty good idea on how to treat my small room.

Dimensions:

12' 4" deep (not including closet which is an additional 27" deep and 12' 6" wide)
10' 10" wide
8' 8" high

(the front and right walls are exterior. the right wall is next to the master bedroom. the rear wall is utility room and hallway.)

Here's very rough layout:



My only question is how to treat the back wall. The 2 doors on the closet and the entry door are very tall with only 6" from the tops to the ceiling. Further complicating things is that the entry door is almost flush to the right wall leaving only 3" of clearance.



I plan to put traps on the door in the primary reflection spots (the red areas on the door). There's just enough room on the left side to put a corner trap but what should I do about that corner/entry on the right side? (Treat the door? The wall? Both? I can't put 4" of 703 on both. One maybe.)

Is there any trap solution for the area above the closet doors? (A small superchunk perhaps? If I overlap the molding I can get 8".)

One last question. Would there be any benefit to hanging a second set of traps on the inside of the closet door as well as on front? (it's full of gear and manuals not clothes.)

Thanks in advance for any advice. I'd be more than happy to give more info if necessary.

Chris Shaw

[Glenn Kuras]

There's just enough room on the left side to put a corner trap but what should I do about that corner/entry on the right side?

You could put a panel on a stand or put a panel on the door and the wall.

Is there any trap solution for the area above the closet doors?

You could straddle a panel up there I think. Kind of like this.

[Ethan Winer]

Originally Posted by Chris Shaw

I plan to put traps on the door in the primary reflection spots (the red areas on the door).

What's in the closet? For bass frequencies the best thing to do is leave the closet doors open. This gives the room two different depths which reduces bass peaks and nulls a bit. If the closet is stuffed with clothes, that should absorb mids and highs pretty well. Or leave the doors open and put proper panels on stands, as Glenn suggested.

--Ethan

[Chris Shaw]

Thanks for the input guys.

Ethan, The closet's full of gear, manuals, tools etc. But the idea of leaving the doors open and putting two traps in front sounds intriguing.

I realized that a stand by the door is probably my best option but I'm also toying with the idea of mounting a corner trap on a hinge on the right wall that I can swing out of the way when the door is open. Any thoughts?

Glenn, I was under the impression that corner traps had to be at a 45 degree angle to be effective. Would a narrow angle as you suggested work as well?

On a different note, would a 414B ULS set in omni mode work as a measuring mic?



I'm off to J&S Supplies here in Brooklyn to look at panel materials. They've got a good deal on 703 2" ($48 a box ), but I'm going to check out some rock wool as well. I'm on a bit of a tight budget.

Once I get going I'll post some pictures.

Thanks again for the advice! There's so much good info on this forum. Spent hours going through it and digesting....

You guys rock. stike

[Glenn Kuras]

Glenn, I was under the impression that corner traps had to be at a 45 degree angle to be effective. Would a narrow angle as you suggested work as well?

It is not that it would not work but just might not work as well. Worth doing though.

[Chris Shaw]

I had a talk today with somebody who has built quite a number of broadband panels using different methods (sealed/open/mass vinyl etc) and he came up with the idea of replacing the doors with 4" panels on hinges. I think that might be a best of both worlds solution. (Will have to find a place to store the doors in my building.)

[jwl]

Just to add to the 45 degree angle discussion, the important part is not that the trap is at a 45 degree angle. The key is to get the trap fully into the corner, and give it adequate airspace behind the panel.

So yes, a 45 degree angle in most instances is best, but as long as you have the trap in the corner with as much airspace behind it as you can give, you'll be fine.

[Glenn Kuras]

Just to clarify, the reason for the 45 degree angle is because it has the larger gap behind the trap. The less the angle the less the gap.

[SaSi_SiDi]

I would be tempted to utilize the closet and doors as a tuned helmholtz absorber. The airspace in the closet (subtracting an estimate of space occupied by gear) would give you one parameter for calculation. You could study the gap between the two doors and use that as the slit.

If that is out of the question, then one option could be to mount floor to ceiling traps on these doors leaving some airspace behind them.

You could have done the same on the entry door, however it opens towards the wall so anything mounted on it's face would obstruct the opening. You could however mount a hinged panel on the pillar next to the door leaving a 4-5" airspace between the panel and the door. You then need to pull the trap before you can open the door to enter or exit the room.

[Chris Shaw]

Started on my traps last week. Read all I could find on this forum (thanks to Ethan and Glenn for their advice). The goal is 16 - 20 traps. Keeping it inexpensive was also a priority.

Here's what I did.

Went with the simple wood frame with 2 sheets of 2" insulation wrapped in muslin.

I spec'ed out 703 but it was a bit too pricey for my limited budget. Went with this insulation made by Thermafiber called Safing. 4lb density. Word to the wise - this stuff is nasty. Really dusty, but once covered in a frame and cloth is nice and clean. Easy to cut as well. I wasn't originally planning on covering the back of the panels but after handling the insulation realized that it would shed so I had to buy more cloth. The muslin I got was 62" wide/ yard which is perfect for these traps.

Went with 1x2x6 lumber from Home Depot. Two pieces make one frame. Measure 48" for the sides and 21" for the top/bottom. This leaves you with two small 3.5" pieces that you can use for stand-offs. This places the insulation slabs exactly 4" off the wall. I stapled the frame together first then reinforced it with 2" screws after drilling pilot holes (to prevent the wood from splitting).

When stapling the cloth I found that stretching it too tight makes the front edges rounded. Pull just enough to keep the cloth slightly taught. When the panel is mounted, you can use Downy wrinkle releaser to get rid of any wrinkles. Just spray on, smooth over, and let dry.

I mounted the corner traps using simple L-brackets. I bent them up slightly so that when the traps were placed on them the brackets would hold them in place against the ceiling. The bottom corner panels were mounted the same way, pressing against the panel above it.

Side panels and clouds were mounted using wall anchors and picture hanging wire. I stacked my speaker stands on top of each other to hold up the panels as I hung them.

I wanted to be able to shift the side panels from side to side if needed. Using a few washers as spacers I mounted a 12" piece of 1x2 on the wall. L-brackets were attached to the wood stand-offs on the panel with leaving a 1/4" lip to hook behind the 1x2 on the wall. Now I can slide the panel side to side to adjust the first reflection point. The stand-offs were mounted close to center of the panels so that panels appear to be floated off the wall.

The front panels, and clouds are full bandwidth absorbers all other panels that aren't placed in first reflection points have a sheet of 2 mil plastic between the insulation and front covering to keep the room from sounding too dead.

The difference was evident as soon as I mounted two of the corner traps. The room had a 20 db dip at around 300hz that pretty much disappeared. As more panels went up the bottom end got much flatter. However a dip at 120 Hz became evident as more traps went up. My desk at the moment is a solid core door on top of two metal filing cabinets. I placed a panel in front of it (where your legs would go under the desk) and the dip was reduced. I'm sure the file cabinets are resonating too. Otherwise everything seems to be relatively flat +/- 6db.


The pictures are pretty self explanatory.

All the ego disks on the side walls have come down...


Materials


Cloth backing


Insulation on Cloth Front


Ready to be stapled


Corner Mount


Frame with stand-offs


L-Bracket on stand-off


Wall Mount


Right Side


Left Side


Back Wall / Closet


Front of room


Here's some measurements made with Room EQ Wizard and a Radio Shack dig meter.

Before....



After...



14 panels up another 6 to go. (Corner next to door, two rear side panels, and 4 ceiling corner traps).


Just gotta work out that dip at 120......

[Glenn Kuras]

Just gotta work out that dip at 120......

Yea but it still looks pretty good over all.
thumbsup

[Chris Shaw]

OK. I've got 22 - count 'em 22- traps installed now but I still have that dip that starts at 90 goes down 15 db at 111 and goes back up at 150. (it seems to be 5db worse now).
This frequency range is a bit critical for me.


There's a dip around 800/1k as well but I think this is either comb filtering from the big desk between the speakers and ref mic or the crossover point on my Genelecs (which makes sense).

-There's a bit of a floor/ceiling standing wave about 2 feet behind the mix position that I could put another trap in. (but would this affect that lo end dip?)

The front and rear traps go from ceiling to about 7" off the floor. Would filling these gaps have any impact?

As always any advice would be appreciated.

[Glenn Kuras]

-There's a bit of a floor/ceiling standing wave about 2 feet behind the mix position that I could put another trap in. (but would this affect that lo end dip?)

It won't hurt but I don't think it will solve it. That is a boundary reflection coming from somewhere. Do you have a reading before you placed the panels? It is kind of hard to explain but you could have had a null and peak canceling each other but when you put a panel up the peak was fixed, but now the null is now there. Over all it does not look like a that big of a null so maybe you can leave it.

[Sha-Zam]

Wow, how would someone deal with a null like that? Is it time to insert a eq in the monitor chain and bump the 115hz up? Would that create some other problems? In that size room, with 22 panels, I would have thought all the problems to be fixed! I'm curious to see what happens as I am getting ready to do some measuring with room eq wiz myself and start the panel journey too.

[macc]

Originally Posted by Glenn Kuras

It won't hurt but I don't think it will solve it. That is a boundary reflection coming from somewhere. Do you have a reading before you placed the panels? It is kind of hard to explain but you could have had a null and peak canceling each other but when you put a panel up the peak was fixed, but now the null is now there.

I'd love an explanation of this... You mean a room mode peak cancelling a boundary-related null, then trapping kills the former leaving the latter?

And if this is the case, is it better to leave the traps out, put them elsewhere, or something?

[Glenn Kuras]

And if this is the case, is it better to leave the traps out, put them elsewhere, or something?

You would want to put them elsewhere. Note this does not happen all the time, but it could.

[Nordenstam]

Did the measurement mic move between the before/after? It seems like a plausible explanation to the worsening result after treatment. It also seems to be a problem that can be fixed by moving the sweetspot and/or loudspeaker position.

[Sha-Zam]

On the site where the REW info and forums are, there is a section about the Behringer feedback destroyer as corrective eq for a subwoofer. You can connect a FBD to the software via midi and it will tell the feedback destroyer what notches and q to use for correction. Having worked with a Behringer feedback destroyer for about 3 years at a church I used to run sound for, I was suprised how well it did. In fact, that particular unit was the only behringer gear I ever used that didn't suck or break (or both). That said, there is still no way I would use it in my monitor chain, but on a sub... might not be a bad idea.

Don't know if it would help your 115hz dip, but it might be worth looking into.

[Victor_Stoian]

using EQ for a very low frequency is ok, but only to reduce a peak by a couple of dB. I would never use EQ trying to raise a dip that is caused by a null.

[Sha-Zam]

Originally Posted by Victor_Stoian

using EQ for a very low frequency is ok, but only to reduce a peak by a couple of dB. I would never use EQ trying to raise a dip ...

OK, thanks. Why is it not a good idea to raise frequencies to counteract a dip?

[RScott]

Originally Posted by Sha-Zam

OK, thanks. Why is it not a good idea to raise frequencies to counteract a dip?

From what I've heard here, if you're getting a null, it's from that frequency hitting your ears from two or more different paths. The paths may be at different phases and angles, and the sum ends up being a low amount due to a near or total cancellation. Say, +1 from the direct path and -1 from bouncing off the back wall.

If you boost a null at that freq, you would be boosting the +1 higher and the -1 lower. All while lowering your headroom and adding an extra process to your monitor signal path.

Supposedly you can use EQ to take out a peak, because if you have +1 and +1, then you can still reduce that frequency and both +1's react in the same manner, effectively lowering your peak.

[Victor_Stoian]

Originally Posted by RScott

If you boost a null at that freq, you would be boosting the +1 higher and the -1 lower. All while lowering your headroom and adding an extra process to your monitor signal path.

Supposedly you can use EQ to take out a peak, because if you have +1 and +1, then you can still reduce that frequency and both +1's react in the same manner, effectively lowering your peak.

, nothing to add to that.

[Ethan Winer]

Originally Posted by RScott

if you're getting a null, it's from that frequency hitting your ears from two or more different paths. The paths may be at different phases and angles, and the sum ends up being a low amount due to a near or total cancellation.

Exactly. And those "collision points" are highly localized. So besides reducing headroom and increasing loudspeaker distortion, EQ boost will also make the response worse elsewhere in the room. Even four inches a way a null can become a peak. Look at the response curves below at 71 Hz (between the 56 Hz and 92 Hz markers) in the graph below. This was measured in a bedroom size space.

--Ethan

[Sha-Zam]

Thank you very much! I can see that this is a very important concept that I have clearly missed thus far. I appreciate you guys taking the time to document this for me and whoever else will be reading. Can I make the assumption based on the above that if a null is pressent, since it is localized it can be countered by re-positioning the monitors and/listening position? I suppose re-positioning the absorption panels could also make a difference. The dip in the the freq. response chart above (post #12) is about 115hz which should still be influenced by the panels.

Also, in that graph (post #12), there is a really big dip that centers just above 1khz. The OP seems to think it is his crossover point, but I am curious if someone with a trained eye would agree? To me, that seems too high for a crossover from a separate woofer to a full range, and too low for a crossover from a full range to a tweeter. If it IS a built-in passive crossover between a full range driver and tweeter, that doesn't seem right.

[AnAverageJoe]

Originally Posted by Ethan Winer

What's in the closet? For bass frequencies the best thing to do is leave the closet doors open. This gives the room two different depths which reduces bass peaks and nulls a bit. If the closet is stuffed with clothes, that should absorb mids and highs pretty well. Or leave the doors open and put proper panels on stands, as Glenn suggested.

--Ethan

Hi Ethan,

Could you give me some more insight on the tip you gave the OP about leaving the doors open to help the bass frequencies in the closet?

Thanks,

Joe

[Chris Shaw]

Wow. I go away for a bit and everyone jumps in.

The dip got a bit more exaggerated as I added panels. I think Glenn is right about the peak/null thing. However I'd like to try removing/changing the desk before I start taking down and moving panels (they were a major pain to put up). I'm thinking there's something going on with my file cabinets and desktop.

There's a bass build up at the back of the room and at the listening position it feels like a null.

I'm starting to wonder if the problem is the lack of traps in the floor/wall corners or the lack of a trap straddling the back corner next to the door or a combo of both.

It'll be few days before I get to tackle this as I'm busy with some mixes.

PS - Ethan, It was great meeting you and Jules at AES....

[Chris Shaw]

Originally Posted by Lupo

Did the measurement mic move between the before/after? It seems like a plausible explanation to the worsening result after treatment. It also seems to be a problem that can be fixed by moving the sweetspot and/or loudspeaker position.

NO , moving the mic around the sweet spot didn't help that much at all. I moved the speakers closer to the wall hoping to boost the low end a bit but there wasn't too much of a change either...

[Chris Shaw]

Now for another question...

All the nasty stuff going on in my measurement from 4k on up.. Is that from the reflections off my desk, hence the comb filtering?

[Glenn Kuras]

Originally Posted by Chris Shaw

Now for another question...

All the nasty stuff going on in my measurement from 4k on up.. Is that from the reflections off my desk, hence the comb filtering?

It very well could be. Try putting a THICK blanket over the desk and see if the problem goes away. If it does then you know it is the desk. If not then.....................

[Ethan Winer]

Originally Posted by AnAverageJoe

Could you give me some more insight on the tip you gave the OP about leaving the doors open to help the bass frequencies in the closet?

I explained it in my All About Diffusion video. The drawing below is from that video, and shows how the different depths each reflect with different time delays. So in this case you have three length modes, which is good, and the strength of each mode is also cut to 1/3 which is even better. The same idea applies to two depths.

--Ethan