[jpv]

I have been in the process of adding 2 x 4 panels of 4" thick 6 pound rockwool to my room.
I am adding them to the ceiling as I said in my previous post on the vaulted ceiling.
I have noticed some changes to my responce I don't understand. I am using REW to measure the changes after each addition and I see a evening out of certain freg. as expected. What I don't understand is why when I add absorbtion it created large dips. I remove the panel and the problem is gone. What causes this?

[youngho]

Where are the dips? What are their magnitudes? Can you post some of the graphs? How did you mount the rockwool (4" thick panels should have good absorption between 125 Hz and 4 kHz or so, with more absorption of low frequencies with an airspace behind them)? Some details would probably make it easier for the gurus to answer your question, which seems rather vague. One remote possibility could be that some of the room modes were treated but that the remaining ones are becoming more evident. For example, if you were sitting in the node for the odd-order width modes (sitting equidistant between the right and left wall could put you in the region where certain frequencies cancel out, namely the odd-number multiples of 565 divided by the width of the room), the relative null that a listener would experience could be balanced out if the length or height modes compensated. If the length or height modes but not the width ones were treated, though, then the null would become more apparent. Hope that makes sense, even if it doesn't answer your question.

[jpv]

OK a little more info about the room. the room is 16.8 ft long 13.5 ft wide and the ceiling is 10 high at the peak the side wall is 5 ft.  I sit 5' 8" from the back.  Speakers are proac tabeletts and 2 M&K subs.
I have in the room 4 bass panel traps, 2 corner bass traps (4" rock wool with space behind)
For the ceiling I put up 3 - 2 x 4 4" thick traps there is about 14" of space the the peak For first reflections I have 4  2 x 4 traps (2 on each side.
 
Here is my graph taken using REW and a Radio shack meter
The Green line has only the 4 bass panel traps up. The blue has the corner traps and 2 ceiling traps. the two ceiling traps are locate in front and the center of the room. The purple line is what I got when i put up the last ceiling trap.
As you can see the 2 fregs of 181 and 244  got worse with the addition of the last trap.

[bpape]

Try moving your seat forward 1.5' and putting 2 of the 4" panels on the rear wall directly behind your head.

Bryan

[jpv]

I finally got around to doing what you suggested. I moved by seating position up 1.5 ft and place 2 4" traps behind me. I placed them 3 inches away from the wall. The response did change for the better but got worst at other freq. so I want back to where I was.
I played the 184 Hz freq. using REW and notice that in the front of the room is here the peaks are. They are to the sides by the wall in-line with the speakers and between the speakers. I also noticed if I stand between the speakers the dull improves by 10 db or so.
 Would placing treatment there help? Is the last trap that created the problem not in a good place and should it come down?

[youngho]

Hi JPV, sorry, I'm not one of the experts here, but I'd like to offer a few thoughts and ask a few more questions:

Because you heard a peak in line with the speakers at 184 Hz, it sounds to me like the speakers are sitting in or near an antinode (an area of the room where they would excite a mode or room resonance) for one of the higher-order modes where the listening position is in or near a node (an area of the room where a cancellation occurs) for that particular mode. If that's the case, then moving the speakers forward probably would be the easiest and most helpful thing. As I mentioned before, it could be possible that the treatments you've put have started to address some of the room modes (axial) but left the remaining ones (diagonal or tangential) that much more evident, since they're no longer compensated. The vaulted ceiling makes these a little more complicated than a typical listening box/room. Alternatively, if you can figure out specifically which node is causing the null at 184 Hz at the listening position, you can address it (e.g. diagonal floor-ceiling modes might benefit from a bass trap straddling the middle of the floor-wall edge on the front wall directly in front of or on the rear wall directly behind the listening position). Can you make a diagram of your room floorplan and/or put up some pictures? It would be helpful to know exactly where the speakers (and perhaps the subs) are, as well as the existing room treatments. In the previous thread, you said your room was 18.5' long, but in this one, it's 16.8' long. Because these frequencies are relatively higher, there is also the possibility of comb filtering issues (for example, although it sounds like you put panels at the first reflection points on the side walls, you may not have done so at the angled points on the vaulted ceiling), but this doesn't seem likely to cause the issues you've described.

Young-Ho

[jpv]

Young-Ho,

   Sorry to confuse the situation with 2 diff measurements. The back of my room has small out cove where the door to the room is 18.5 ft. Right behind my listening position the room is shorter. I will take photos and post them tonight. I don't have anything on the first reflection points on the ceiling. I could if you think it would help.
 
John

[youngho]

Sorry, John, I really don't know. The nulls resulting from increased trapping are really weird, but the fact that they get worse with more ceiling trapping leads me to believe that they must represent other modes that are as yet untrapped. However, given what you said about the 184 Hz peak being at the front of the room in line with the speakers, the easiest thing would be to try moving them forward. How far are they from the back and front walls, just out of curiosity?

[jpv]

As promised some photo's of my room. Please excuse the mess, We are working on my daughters room so some of her stuff was move in here while it is being completed. I have three photos . The first one is the front view.

On the left and right corners are two bass traps, you can also see two of the four panel bass traps and a tri corner trap.
My stereo rack is on the left. The speakers are 49.5" out from the front and 48" from the sides. The two subs are 25" from the  front and 55" from the sides. I have two record shelves in the corners that I will remove. I am planning on putting them on the opposite wall in line with the stereo rack.
The two diffusers are for slap echo. The slap echo problem was really bad.

 This is the rear view of my room and the ceiling. There are three traps along the top. The last one is the one that created the dips. They are not completely to against the ceiling yet.  Two diffusers and broad band traps are back there The outcove on the left has a folding door I close while listening

 Here is the rt side of the room with two board band traps. There are 2 on the left also.
Again the side walls are 5 ft and the ceiling peak is 10 ft.
 I hope this gives some more insight into what my room looks like.

[WerTicus]

looking at your sound system and the chart i wouldn't be surprised if that is just where you have a dip in the response.... and that dip was being 'fixed' by sound reflections.  Effectively you need a woofer  

tweeter *check*
mid range *check*
woofer... nope
subwoofer *check*


that would explain the 200hz dip, now that you've fixed the room.

[richidoo]

Could also be speaker placement, pull them out a foot and remeasure. Reflected waves from front wall can cancel direct sound at frquencies that correspond with the distance of speakers to front wall. Usually this 100-200Hz range is affected.

But is this effect constant through the room Bryan? Will moving his chair in a foot as you suggested change the SBIR effect or is it constant anywhere on the centerline?

You could also try your thickest panel absorbers behind the speakers, with a foot of space between them and the front wall. If that doesn't affect the 200Hz dip then it may not be SBIR. Speakers ideally should be as far away as possible, maybe 6 feet if you can manage it.
Rich

[Mariusz]

I have been in the process of adding 2 x 4 panels of 4" thick 6 pound rockwool to my room.
I am adding them to the ceiling as I said in my previous post on the vaulted ceiling.
I have noticed some changes to my responce I don't understand. I am using REW to measure the changes after each addition and I see a evening out of certain freg. as expected. What I don't understand is why when I add absorption it created large dips. I remove the panel and the problem is gone. What causes this?

I have to say, you have one tricky room to work with.
As to your problem. It seems like absorption that was apply to the ceiling affected dips/suckouts even farther........I can not help but wonder if diffusion instead of absorbing had the same effect. Also, as some already said....- placement of speakers and listening postion might help.

[bpape]

Absorbing that peak is the right thing to do.  Generally, when you put up a set of treatments and all of a sudden you get a big null, it's telling you that there was a corresponding peak that was masking this issue.  The null has always been there, it had just been counterbalanced.  If removing just the last panel helps the issue (assuming you can't find out what's causing the null), then that's a small price to pay if you can leave the rest of the peak covered.

Bryan