[dguarnaccia]

Hi all,

I'm a noobie here at the circle, but was recommended over here after asking a about this over at AV123.  I have a room that is 18' wide by 23' deep and 9' tall. Here is what the room looks like:



Edit: The stuff in the diagram in red is what I was thinking I should do next but I've been told diffusion isn't going to help with the null

I've spent a bunch of time with Room EQ Wizard moving around speakers my sub to find the optimal placement. After dozens and dozens of sweeps, I've got the fronts and sub in the best location I could find. I have the sub and mains Crossed over at 60 hz and I've played with the phase adjustments on the sub and the distance settings of my receiver to minimize the room null I have, but there seems to be a room null as well.

Here is a sweep graph of the Sub only at the optimal position:



and here is a sweep with left, right and sub together:



As you can see, there is a huge null between 70-120.  I'm pretty baffled as to what to try next. I've got my tube traps in the 3 main tricorners, and tubes traps at the first reflection point, but what can I do to try to reduce the room nulls?  Should I just start adding bass traps across the back wall where I have the red areas highlighted?  Does it matter where I place them?

I've looked at the Gik traps as well as the ATS traps before, and I'd likely go that way because I don't think you can build em any cheaper than those guy make em. I'd guess I need 4 inch traps to reach low enough to have an effect at 70 hz but I'd love any advice.

Thanks for reading and helping out a noobie

Best,

Darren

[MaxCast]

Hi Darren, welcome to AC.  First of all, our two acoustic gurus are at RMAF so they may take a bit to offer their suggestions.
What are your 12" bass traps made of?  Have you tried placing all bass traps in the corners?  The best places for bass trapping are in the tri-corners and the ceiling/wall or floor/wall corners.  Usually 2" compressed glass is placed at the first reflection points, walls and ceiling.

Have you tried a different xo point? 

[dguarnaccia]

Hi Max. The Bass traps are a 16 inch and 12 inch tubes of stiff fiberglass, shaped in tubes. Similar of oc703. I followed the instructions found here: http://www.teresaudio.com/haven/traps/traps.html

The 16 inchers are in the front 2 corners, and the 12 incher is in the rear corner. The 4th corner is an open space with a stairwell and I couldn't put em there.  The traps are 6 foot tall each.   I also have 2 traps at the first reflection point as well.

I tried moving the crossover point up to 120, and while it smooths out the bass slightly, I loose a ton of punch from the powered woofers in the mains, as well as the fast bass from the aluminum woofers on my fronts.   I was hoping to get a good response at 60 hz XO if possible.

Couple of questions I have is, would more bass traps solve my null issue?  Do I need my corner traps to go all the way to the ceiling (instead of just 6 foot).  Are the tube traps enough, or do I really need to build/buy superchunk wedges?   If I need more traps, where should I place them?  Along the back wall? Does it matter?  It feels like I've exhasted all my placement options. I've tried the sub in just about every convievable location in that room, and that's the best location I was able to find based on the REQ sweeps.  I've tried moving my fronts out more from the wall, and that didn't seem to have any effect. I've tried moving them inwards and outwards, and that location seems to be the optimal location. So if location changes are out, what are my other options?

I've read everything I could get my hands on, and I'm just plain stumped at this point...

Best,

Darren

[stevenkelby]

Is the null at a room mode(s)? Maybe you need absorption on the walls and ceiling, in the middle of the surfaces, to stop the waves canceling each other out? Or something No idea really!

[ctviggen]

I don't think bass traps at the reflection points do anything.  The reason you put absorption at first reflection points is to prevent (reduce the size of, actually) a second version of the sound from hitting your ears at some time later than the direct sound hits your ears.  If the magnitude of the second version is too high, then it has the possibility of muddying the sound.

Bass traps should be placed either in the corners (where all modes exist) or perhaps at a location where a mode has a high level.  Putting them at the first reflection points is useless, unless that point happens to be where a mode has a high level. 

It's easier to bring down the highs than it is to raise the nulls.  One thing to do is to move your sitting position, as then you may move out of the location of the nulls.  If you can't move the sitting position, move the speakers.  It appears as if you've done this already.  Another thing is to move the traps at the first reflection points to the corners or a high location of a mode.

By the way, tube traps aren't diffusers.  Diffusing 70Hz is a monumental task, since the size of a diffuser is inversely related to the frequency (higher frequency = smaller diffuser).

What I'd do is first take a reading with only the speakers playing and not the sub.  How low do the speakers play?  I think what might be happening here is that you have a null created because the speakers aren't playing in that area.  If you look at your curves, the main speakers are about 10dB higher than the level of the sub, but they don't appear to be producing anything below around 150Hz.  Is that due to a room null or a speaker null?  Are you sure the phase is set correctly? 

[bpape]

Agreed.  Bass absorbers at the reflection point are likely a waste.  They'd likely do better centered on the rear wall where you were going to pu tthe diffuser.

It appears to me that assuming an 80Hz xover, you may have a phase issue in the sub setting.  There is a peak prior to 70Hz that makes the null look worse than what it is in terms of the 80-85Hz down.

What is the slope of your xover and where do you have it set on both sides - the processor and the sub? 

It'll be a bit spotty from me the next few days since I'm out in Denver but I'll check in when I can.

Bryan

[Ethan Winer]

The Bass traps are a 16 inch and 12 inch tubes of stiff fiberglass, shaped in tubes.

Those are simply too small to be effective at the low frequencies you need. I also agree with the others that most are not in good places. Before you buy or build more, I'd try taking the traps you have that are not now in corners, and lay them sideways along as many wall-floor corners as you can manage. You could also try stacking three of the 12-inchers adjacent in some wall-wall corners, with one tight in the corner and the other two on either side. But ultimately you'll need larger traps, and lots of them. I have 48 panels in my 25 by 16 living room. Some are first reflection absorbers and some are diffusors, but most are bass traps. You don't need that many! But you need more than what you have now.

--Ethan

[dguarnaccia]

Wow, great feedback!  I'll start experimenting with the traps at the first reflection points and try them at the back wall, as well as stacked adjactent to the corners.   The XO is being set at the AVR.

I have tried phase adjustment between the sub and mains. This is actually as good as I manage. I tried adjusting the phase on the sub, as well as incrementing the distance setting in the AVR for the sub by 1 ft increments. 

How big would my traps need to be in the corners to be effective?  I thought I read that 16 inch tubes would trap down to 55hz, but is that correct? do I need to stuff them with mineral wool or batting to make them more effective?

Thanks again, and enjoy RMAF! Wish I could be there

Edit: Also, the sub is set to the same level as the mains, it I believe that the differential is due to room nodes.

[dguarnaccia]

ok update...

took some more readings, and moved the first reflection point bass traps around, largely to no effect. There just isn't enough of them is my guess, and as Ethan pointed out, the aren't having any effect at those low frequencies.

So I did some more work on positioning. I pulled my fronts and sub 4 feet off the front wall just to see it's effect. That actually had some impact, though it does look a bit odd (we'll see if the wife will notice )

Here's the new sweep with fronts and sub



The new location introduces a null down at 40ish hz, but it's fairly small...

Here is a sweep of the sub and one of the fronts independently as someone asked...

Here is the sub



and here is the right front by itself




Is there anything I can do to make the tube traps more effective for lower frequencies? How big of a trap would I need to deal with nulls in the 70-100 hz region?

[Ethan Winer]

How big of a trap would I need to deal with nulls in the 70-100 hz region?

It's not only how big they are but how many of them you have. For those low frequencies you need at least two feet wide by four inches thick, and for a 23 by 18 foot room you need a lot of them. I don't know how your traps are made, but the goal is rigid fiberglass or packed fluff fiberglass, as big and thick as possible.

--Ethan

[dguarnaccia]

ah, so it's really the depth that matters then as to what kind of frequencies are absorbed?  Assuming I put 2 right next to each other, or just do pannels in certain areas, it's just a question of volume. The tubes are simply rigid fiberglass in a tube shape with mdf on top and bottom like in the picture    

I'm thinking now that I should open the existing ones back up and put in some mineral wool for additional absorbtion.  That, and I'll need some more panels in the back.

The wife busted me on pulling the speakers out already, not sure if they'll stay pulled out from the wall that far. Might have to move them back a bit...but we'll see once I start putting more traps back up

[jimdgoulding]

Sounds like yer gonna get there alright.  My question concerns the placement of your main speakers.  It would appear that they are very close to the wall behind.  It occurred to me that you might want them on the same plane as the TV.  This is not good for the expansion of a virtual stage, however, particularly in your field of depth, and to the rendering of instruments, too, actually.  If it is as it appears in the drawing, might I suggest that you pull them out more into the room for music.

[dguarnaccia]

Thanks Jim. I tried pulling them out from the wall to 4 feet off the wall. That's where they are currently, and the latest graphs reflect that position. Not sure if I'll be able to keep em there though (WAF). The bass response improved a little, but not as much as I would have expected.

[MaxCast]

Your last graph with the mains and sub doesn't look that bad.  Turning down your sub will get rid of the bottom end rise.  But, you may like it that way, especially for movies.

Have you tried a sweep with the main set of speakers and listening position shifted to the left/right a foot or two?  Or with the sub on a different wall?

Packing your tubes tight will help.  Taking them to the ceiling will help.  Placing a 2'x8'x6" in front of the tube in a corner will help most.

placing 2'x4'x2" on the first reflection points (left,right,ceiling) will help with your imaging and covering the TV with a removeable 2-4" piece will help depth.

It all depends on what you can get away with (waf).  Tell your wife the three front speakers should be in an arc anyway so you need to pull the mains out a bit 

[Ethan Winer]

ah, so it's really the depth that matters then as to what kind of frequencies are absorbed?

It's simply the total mass of absorbing material that you have in corners and other places bass accumulates.

The tubes are simply rigid fiberglass in a tube shape with mdf on top and bottom

The MDF does nothing useful. It doesn't hurt on the bottom, and if the traps went all the way up to the ceiling it won't hurt there either. Otherwise, MDF actually reduces absorption because it blocks sound from getting into the tube through the top.

I'm thinking now that I should open the existing ones back up and put in some mineral wool for additional absorbtion.

Yes, it can only help.

The wife busted me on pulling the speakers out already, not sure if they'll stay pulled out from the wall that far.

The typical audiophile approach of pulling speakers far out from the walls is not necessarily the best method. The only way to know for sure where your speakers should be for the best bass response is to measure using REW while you experiment with placement.

--Ethan

[satfrat]

The wife busted me on pulling the speakers out already, not sure if they'll stay pulled out from the wall that far.

The typical audiophile approach of pulling speakers far out from the walls is not necessarily the best method. The only way to know for sure where your speakers should be for the best bass response is to measure using REW while you experiment with placement.

--Ethan

You could always use your ears as a last resort.

Cheers,
Robin

[jimdgoulding]

I can confirm that putting a 2'x4' panel in front of your screen will take it more out of the picture (no pun intended).  Don't think it is material to your in room bass response, however.  I'm out of my depth here but I think a lot of guys use two subs at different places in the room for the purpose of averaging out null points.  Maybe some guys can speak to that.

Oh, I looked back and see that your screen is a large projection screen.  So, nevermind bout the 2'x4' panel thing.

[dguarnaccia]

Man, I have some work to do, but this is great...I was just stumped as what to do next...

Have you tried a sweep with the main set of speakers and listening position shifted to the left/right a foot or two?  Or with the sub on a different wall?

Haven't tried shifting left or right, I'll try moving the mic over a bit to see if it results in any change.  I have tried the sub in just about every location in that room though, and the front seems to be the best location so far...

So based on all the recommendations, here is my plan:

Stuff all current tubes with insulation/rockwool, and remove the MDF from most of the tubes.
Bring all tube traps all the way to the ceiling
Add Panel bass traps of 4" thickness to the back wall whereever I can fit em
Add more tubes towards the back of the room (I'd do panels, but I need freestanding traps due to the room features)

Did I miss anything else?  I can also keep playing with the speaker placement a bit as well but I think I'm as close to as good as I can get at this point...

[jimdgoulding]

As regards the GIK's, they come in 3.5" and 5" thickness, I think that's right.  You want the 5" ones, I believe.  They may have something other than that they would recommend.  It's been awhile since I've visited their site.

[satfrat]

As regards the GIK's, they come in 3.5" and 5" thickness, I think that's right.  You want the 5" ones, I believe.  They may have something other than that they would recommend.  It's been awhile since I've visited their site.

GIK also makes the nicest little Table Traps. I already have 1 that I use as a plant stand and I have another one on order that I'll use as my recliner stand. AFAIC in regards to room treatment (I look at tweaks the same way), it all adds up, at least to my ears as I have no instruments to look at.


Cheers,
Robin