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post #1 of 34 Old 02-05-2016, 12:56 PM - Thread Starter
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Bass Traps : How many and where to place them

I'm hoping to find someone that really understands bass traps and how they interact with subwoofers in a room. From my research it seems like based on where your MLP is relative to the walls and subwoofers in the room, bass traps can be strategically used to eliminate nulls. My MLP is about 2' from the back wall, which I believe is part of the problem, but it's a fixed position so I'm looking into options. I've got two big nulls. One at 18hz and one at 125hz. There's plenty of places around the room to toss panels up. Do you just fill the room with panels or is it more complicated than that. It's also a dropped ceiling basement. I've seen the ATS ceiling panels. I couldn't justify filling the ceiling with them, but I could do above the speakers if anyone thinks that would be helpful. Curious if this is a war worth fighting, and if it's one to be fought with a sniper rifle or a hand grenade. Thanks in advance.
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post #2 of 34 Old 02-05-2016, 01:05 PM
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You should ask in the audio theory forum. There is a lot of help in the rew thread
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post #3 of 34 Old 02-05-2016, 01:13 PM
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125hz makes perfect sense with 2ft distance to the back wall. I'd concentrate there. That's a quarter wavelength suck out. You need to absorb. That's the only way. I'd also investigate other reflections that might coincide with 2ft and see if they're ganging up on you.

18hz is going to be very difficult to fight with traps. What is your setup? Can you post the measurements? I wonder if you only have one sub, adding a sub to fill in the 18hz zone might be worthwhile. Then again, maybe you're running a 25hz tune and 18hz isn't realistic. That's a ways down there.
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post #4 of 34 Old 02-05-2016, 01:41 PM - Thread Starter
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I'll check the theory forum. Thanks for the tip. @tuxedocivic , I've got 2 flex 12's with a 17hz tune in the front and 4 vbss 18's in the rear tuned to 15hz behind my couch and between the wall.


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post #5 of 34 Old 02-05-2016, 03:22 PM
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The fact that those subs are tuned pretty much at the null is quite strange. Are you sure they're at those tunings and not fighting each other? Do you have measurements?
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Originally Posted by tuxedocivic View Post
The fact that those subs are tuned pretty much at the null is quite strange. Are you sure they're at those tunings and not fighting each other? Do you have measurements?
Sounds like there is a phase issue around the tuning.

Mike, do you get the null with just one group of subs running?
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post #7 of 34 Old 02-05-2016, 04:14 PM
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Bass traps go in corners. In a rectangular room you have 12 2-wall corners and 8 3-wall corners. The more you can put treatment in the better, but any is better than none. It is possible to overdampen a room, so it's good to throw some diffusion panels in as well. There are several different types, such as superchunk, limp mass, tuned membrane, and helmholtz resonators.

Read: Acoustical Treatments Master Thread
http://www.gikacoustics.com/articles/

Most acoustic treatment companies have educational articles; they're a great place to learn. Bear in mind that they're all trying to sell you something, but the information is good. And then look through what everyone is doing for their theater rooms in the dedicated theater forum. There are some professionals on there offering advice and providing input on people's builds.
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post #8 of 34 Old 02-06-2016, 01:18 AM - Thread Starter
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So here's a graph. Maybe less than scientific, but I figured having the mic close to a speaker would give a fairly clean representation of what it's doing. Then the lower line is what I'm hearing. This is the 2-12's up front without the 18's.

So with the 18's I can get the graph looking tolerable up to a 120hz xover, with the exception of the suck out at the bottom. The 18's by themselves do the same thing.

I started reading through the linked thread and my eyes were completely glazed over by the 5th page. I'm not looking to replace my walls. I guess my question is, do I just start tossing up panels and the like wherever I can fit them? I'll keep reading but I guess I assumed there's be simetjing aimed at the bass heads. Dirac does a decent job with the rest of the speakers. Just looking to clean up the bass.

Thanks


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post #9 of 34 Old 02-06-2016, 03:08 AM
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Quote:
Originally Posted by Mike Schacht View Post
So here's a graph. Maybe less than scientific, but I figured having the mic close to a speaker would give a fairly clean representation of what it's doing. Then the lower line is what I'm hearing. This is the 2-12's up front without the 18's.

So with the 18's I can get the graph looking tolerable up to a 120hz xover, with the exception of the suck out at the bottom. The 18's by themselves do the same thing.

I started reading through the linked thread and my eyes were completely glazed over by the 5th page. I'm not looking to replace my walls. I guess my question is, do I just start tossing up panels and the like wherever I can fit them? I'll keep reading but I guess I assumed there's be simetjing aimed at the bass heads. Dirac does a decent job with the rest of the speakers. Just looking to clean up the bass.

Thanks


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Are you buying pre made bass traps or DIY'ing them yourself? What style and material are you planning to use for your bass traps?
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post #10 of 34 Old 02-06-2016, 07:07 AM
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Mike, you have probably done this, but it's worth asking before you go down the bass trap road. Have you gone through mtg90's time alignment and integration thread (linked in my sig, it's not very long) to try to get your subs all working together before you use Audyssey/Dirac?
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post #11 of 34 Old 02-06-2016, 07:12 AM
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I've been over at GearSlutz forum, studio design. I think you will find a good understanding once you follow a few studio builds.




https://www.gearslutz.com/board/stud...ing-acoustics/ Heres a great studio build from the ground up








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post #12 of 34 Old 02-06-2016, 09:11 AM
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Mike can you post a graph with the flex-12's in one color and the VBSS in a different color let's see how they look together. Just take a sweep of both setups at your mlp like you've been doing. Then maybe another with your lcr and surrounds to.
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post #13 of 34 Old 02-06-2016, 09:21 AM - Thread Starter
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@ Pete, that's what I've been working from. I was not able to get rid of that null by adding subs. Not to say it would be impossible to do with subs, but I'm not interested in adding any more subs at this point. I get that null with just the 12's and just the 18's, so in theory combining them is not going to solve for that problem.
@kgveteran and @chadamir I'm finding a lot of ground up threads. I'm looking at this as a $500 problem to make a first impact and a $1000 problem over 6 months to fix. I'm not interested in refinishing my basement. The graph shown is 2 12's running in phase with one another grouped together under my television directly across from the MLP. I pulled it out and swapped polarity of one of the subs to verify phase was correct. As best I can tell, it's not a phase problem.
@rsvr79 and @Tip24/96 I'm not against DIY if it accomplishes the same thing for less money. It seems like the roxul Rockboard 80 is the best bang for the buck. The Gilford of Maine fr701 seems like the go to in covering. I've read a few DIY tutorials, and it looks within my ability to pull off.
That being said, the GIK stuff seems pretty reputable and depending on how much I need, might not be too expensive. The acoustic room kit number 1 would be in my budget, and then I could DIY some panels down the road if necessary.
@tuxedocivic looks like the tuning might be a little off, but the dip doesn't appear to be a problem with the box or a combination of the two subs fighting, but a problem with the room.

Couple of questions...
I could have a 4" gap between the subs behind the couch and the wall it sits on. Is there any benefit to filling that gap with rockboard? If the subs were pushed up against the wall, would that be better?

Most of the threads I've read have been focused on an overall sound improvement. I'm really happy with how well Dirac has tamed LRC and surrounds. I'm not necessarily looking to fix those things. Is there a way to focus on this gap between 15 and 20? Am I reading the graph wrong? My assumption is that room gain is pushing the graph into the 15,16,17 range and that the null is chopping into that. The 18's get into the 12,13,14 territory, so I have the potential for some great low frequency stuff, if I can fix that notch. When I mix the 18's with the 12's the dip at 90 goes away, and if I cross at 120hz the 125 dip isn't such a big deal. Obviously if all of that gets cleaned up in the process, it's a win, but I really don't want to spend a grand trying to fix the low stuff, and not be able to do it. Any advice here would be greatly appreciated.
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post #14 of 34 Old 02-06-2016, 10:11 AM - Thread Starter
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@eng-399 not even getting into LCR yet. I'll run the other sweeps tonight and post them, but I think that just complicates things and I don't want to get into chasing my tail. This isn't a phase issue at the moment, it's a placement issue at best and a room issue at worst. I can phase/EQ them +-4db down to 18 or so. If this null can't be corrected, then that's as good as it gets, and I can live with that. I'm engulfed by bass at the MLP and I smile ear to ear.

However, if I'm a few hundred bucks in DIY panels away from the 13hz, then I'd be a fool not to do it. Is this a battle worth fighting?
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post #15 of 34 Old 02-06-2016, 10:24 AM
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Bass Traps : How many and where to place them

Mike I have every type of fabric sample for the material for your bass traps or absorbers if your going the diy route. I have gom and dmd and so on do you want to see what these look like? As for your lcr I was wondering how your subs look rolling into your lcr fq and up.
I have a null around the hz area and had Matt myg90 give me a hand and there was nothing I could do to fix it. It was my room that's bad at that fq.
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post #16 of 34 Old 02-06-2016, 10:29 AM - Thread Starter
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Originally Posted by eng-399 View Post
Mike I have every type of fabric sample for the material for your bass traps or absorbers if your going the diy route. I have gom and dmd and so on do you want to see what these look like? As for your lcr I was wondering how your subs look rolling into your lcr fq and up.
I have a null around the hz area and had Matt myg90 give me a hand and there was nothing I could do to fix it. It was my room that's bad at that fq.
I'll definitely take you up on the fabric samples if I go the DIY route. My (R) as in LCR is still being serviced... don't ask. So I'm honestly just not there yet. @eng-399
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post #17 of 34 Old 02-06-2016, 10:30 AM
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Greetings Mike


I had somewhat similar problems with my listening position being up against a wall, and with an overly 'live' room in general (all hard surfaces), so put in some room treatment and was very pleased with the result. You can see more in my build thread, linked in my signature, below.


I spent a bunch of time on the GearSlutz forum. as others have suggested, and found it really informative. The punch-line I came away with is that building a single-frequency tuned trap was possible, but that I would be better served with broad-band absorption. So, I ended up with one big floor-to-ceiling corner-trap that gets down into the 50-80Hz region, and then two 4'X8'X3" panels to help with room reflections.


I would agree with others that are advising you to explore phase issues first, something I would also benefit from doing.


Good luck!


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post #18 of 34 Old 02-06-2016, 10:46 AM
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Quote:
I'm hoping to find someone that really understands bass traps and how they interact with subwoofers in a room. From my research it seems like based on where your MLP is relative to the walls and subwoofers in the room, bass traps can be strategically used to eliminate nulls. My MLP is about 2' from the back wall, which I believe is part of the problem, but it's a fixed position so I'm looking into options. I've got two big nulls. One at 18hz and one at 125hz. There's plenty of places around the room to toss panels up. Do you just fill the room with panels or is it more complicated than that. It's also a dropped ceiling basement. I've seen the ATS ceiling panels. I couldn't justify filling the ceiling with them, but I could do above the speakers if anyone thinks that would be helpful. Curious if this is a war worth fighting, and if it's one to be fought with a sniper rifle or a hand grenade. Thanks in advance.
Here is the bass trap section: https://www.gearslutz.com/board/bass...nels-foam-etc/

Great reading ! Ethan knows his stuff as well as the other members

I'll be doing 24"x24"x90" corner traps, probably three as one corner has a door. Plus more broad band absorbers at the first reflection points, Roxul seems to be the way to go

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post #19 of 34 Old 02-06-2016, 10:57 AM
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I am fighting the same general fight right now: trying to get my room to sound better and kill the nulls. I am far from an expert here (so take my advice with a grain of salt), but I can tell you how to get started. First, take off all eq'ing, disconnect your highs, and start at the very beginning. Run your front subs first with the mic at the MLP and see what the challenges are there. Then do just the back subs and see what those challenges look like. You can solve most of your problems with this data from the room acoustics standpoint. That way you aren't eq'ing everything to compensate or letting a tune from an untreated room affect your measurements. Just adding a few treatments to the big walls will make a difference in the overall curve, and putting them in the right place will make a bigger difference. Once you get it as flat as you can without any eq, just with treatments, then you can start compensating with eq again.

For nulls and peaks at MLP, look at the center of the null or peak and then use that to find quarter and 3/4 wave distances where you could be getting reflections. Then hit that spot with a bass trap and measure again. 2" of oc703 will reduce the reflection by quite a bit, and 4" will do a really good job. 1st reflection is easy, second and third not so much but they have less energy by then so not as big of an effect. Your best bet is to just set up the mic and REW and take your best guess and place a trap there and check. Then move it around. You should see some fairly noticeable results each time you hit a reflection point.

A big 18 hz null screams a phase cancellation because the wave is so long (something like 62 feet long at 68 degrees ambient temp), so first reflection would have to be a long way back. On the other hand, a speaker playing the same frequency at 46.5 feet or 15.5 feet away (or out of phase but 31 feet away) will cause a null. If you run any 1 of your subs and see the null, then it is your MLP in the room, but if you don't have it until you add other subs, it is a phase/delay issue. If it is the room, absorbing 18hz is hard to do, so big thick bass traps might help but it won't completely kill it even if you find the offending reflection point(s). Whatever you do, don't try to compensate with eq'ing, you will only either damage your subs or take all your power away from the rest of the frequencies it is trying to play.

My room is 12'9" wide and 14' long and I am doing a total of 14 bass traps of varying size, 6 2'x3' traps on the big walls, 2 2'x4' traps up to ear level to the left and right of the couch (back wall), 2 3'x4' traps to the left and right of the couch (perpendicular), 2 2'x4' traps on the ceiling at first reflection point, and 2 4'x4' traps to the left and right of the front stage. From there I will determine if I need more (or less). Already the 6 2'x3' traps are up and it makes a huge difference (calmed two peaks by over 10 db each), and those are just general treatments not even placed at exact reflection points. But I have a small room, so my issues are very pronounced and no amount of eq will fix them.

BTW, you will save HUGE money doing this DIY. My 2'x3' movie poster traps cost me around $50 each to make with printed posters on the fabric, fabric wrapped frames, and 2" of OC703. The same thing purchased would be 5-10 times as much. All in, my 14 bass traps will cost me around $700 total and I will have enough to make a couple more small ones by the time I am done. The same would cost you several thousand if purchased outright.
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post #20 of 34 Old 02-06-2016, 11:02 AM
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Originally Posted by Mike Schacht View Post
@ Pete, that's what I've been working from. I was not able to get rid of that null by adding subs. Not to say it would be impossible to do with subs, but I'm not interested in adding any more subs at this point. I get that null with just the 12's and just the 18's, so in theory combining them is not going to solve for that problem.
If you measure elsewhere in the room is 18hz fine? Tried flipping the polarity on all subs? Did the 18hz null exist when you had the ho18's?
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post #21 of 34 Old 02-06-2016, 11:12 AM - Thread Starter
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Hey guys, I realize the traditional reasoning, that this null screams phase, but that graph isn't of multiple speakers. I get the same null when I run the 18's alone as I get with the 12's alone. I tested each speaker individually. It's there. With the 18's it's even worse because there's more sound on the back end. As far as the 2 HO18's I had before. @corradizo They were sealed and they were fizzing out by the time they got there so it wasn't as obvious. And if I'm being super honest, I don't think I ever had the delay right with those. I've learned a lot since then.
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post #22 of 34 Old 02-06-2016, 11:42 AM - Thread Starter
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Thought maybe a lay out of the room might help.

Floor is carpet. Below grade basement. Dropped Ceiling. All walls are drywall except rear wall which is stacked stone.


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@dkersten assuming 20hz was the center of the null, what math is done to determine 1/4 and 3/4 wave distances? I see this type of thing mentioned once and again, and I really don't understand how that works.
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@corradizo just hit the easy button! So I have a first mode at 19hz.

So does this mean I can use traps/panels to fix it? I feel like we're getting somewhere... Maybe?

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post #26 of 34 Old 02-06-2016, 02:10 PM
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@corradizo just hit the easy button! So I have a first mode at 19hz.
The link shows your first order modes at 24Hz, 28Hz, and 71Hz. Where are you seeing 19Hz on the mode calculator?
Quote:
So does this mean I can use traps/panels to fix it? I feel like we're getting somewhere... Maybe?
You're going to try to absorb bass at 19Hz?

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post #27 of 34 Old 02-06-2016, 02:24 PM - Thread Starter
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@sdurani the link is to a program that you can download and enter your room dimensions into. After entering my room dimensions I got 71, 19, and 33. I'm here asking whether this is something to bother fighting. The more I'm reading the less likely it's looking that there will be a fix for this problem.


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post #28 of 34 Old 02-06-2016, 03:01 PM
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@dkersten assuming 20hz was the center of the null, what math is done to determine 1/4 and 3/4 wave distances? I see this type of thing mentioned once and again, and I really don't understand how that works.
The length of a soundwave is calculated by λ = c / f, where c = speed of sound in m/s and f = frequency (λ is in meters). At 20 degrees C speed of sound is 343m/s (68 degrees F). For 18 hz that is about 19 meters (or 62 feet) for a full wave. the quarter waves are the ones 180 degrees out of phase, so at those lengths you will get cancellations from reflections. Using those quarter lengths you can find a flat surface that distance from you and that will likely be the reflection point you need to kill in order to stop the null. If you placed a mirror at that point and moved it up or down until you saw the subwoofer, that is the exact point it is reflecting. That might be behind you, above, below, or to the left or right.
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post #29 of 34 Old 02-06-2016, 03:09 PM - Thread Starter
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When you say 1/4 do you mean 1/4th of 62 feet? The mlp is 15' to the front wall and 15' from each of the side walls. So are you telling me I'm sitting in the worst possible spot in my room?


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post #30 of 34 Old 02-06-2016, 04:28 PM
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Dude, just contact GIK acoustics. They are professionals and the owner even posts here. I was going to buy $1500 worth of stuff and they only recommended $700.

The difference after is night and day. You can fill out the questionnaire on their website and they will fix your problems in no time. Probably one of the best investments I have made in audio other than building my Clearwave Dynmamics.
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