Creative bass trap or waste of time and money? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 17 Old 02-09-2016, 06:14 AM - Thread Starter
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Creative bass trap or waste of time and money?

I have a built in, in my basement that happens to be in a corner. It's definitely resonating with bass frequencies. I'm starting to treat the room, and thought I'd ask it was worth turning it into a bass trap. What I mean is that the thing is in large part hollow. 4'x3'x8'. Should I pop out one of the panels on the bottom and start filling it with insulation? If so, what do you recommend? The panels are 1/2" mdf, should I just replace the panel and call it a day, or should I cover the opening with acoustical fabric? Any help would be appreciated, just don't want to waste time and money on something that probably won't do anything.


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post #2 of 17 Old 02-09-2016, 09:32 AM - Thread Starter
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Found these on sale at Menards this week. Currently thinking about just squeezing them in through the drop ceiling above. I'll bet I could fit 15 of them in there. From what I can gather based on what I've read here, the bass waves will penetrate the 1/2" plywood and get absorbed inside of the cavity by the insulation. Definitely playing the ignorant card here. I'm also open to the fabric idea either replacing the bottom panels, or the backers to the shelves, or both. Again, just don't want to be throwing good money after bad. For $100 it would be an incredibly large bass trap.

http://www.menards.com/main/building...m=flyer_hosted
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post #3 of 17 Old 02-09-2016, 10:25 AM
 
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You can fill the cavity to damp reflections, but if the sound waves can't get in there they can't be damped, so the fabric idea is best.
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post #4 of 17 Old 02-09-2016, 12:19 PM
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If you had a known problem mode of narrow bandwidth, you could attempt to turn it into a pressure/resonant/membrane absorber. Perhaps using pegboard covered in fabric. Would have to identify a potential frequency and go through the design calcs. Not a ton of area so would have only a mild effect, but concentrated in a narrow bandwidth vs spread out as a broadband velocity absorber, perhaps that might have a more audible final result?

Not sure. Worth a thought.

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post #5 of 17 Old 02-09-2016, 01:01 PM - Thread Starter
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@Bigus it kind of is a lot of area. In the bottom portion it's 48 x 36x36. There could be 2 sides of exposed panels. In the top portion it's 24x24x60 and again 2 sides exposed. The real problem I have is a big spike at 21hz and a null at 18. Not sure if anything can be tuned that low.


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post #6 of 17 Old 02-09-2016, 01:37 PM
 
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Quote:
Originally Posted by Mike Schacht View Post
The real problem I have is a big spike at 21hz and a null at 18.
It's very unlikely that a trap will affect that.
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post #7 of 17 Old 02-09-2016, 01:52 PM
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I recently built my own bass traps. It took a lot more effort than I had anticipated and the resulting improvements were marginal at best. If you want to improve the bass in your room I would suggest multiple subwoofers instead. For me, that produced a much more noticeable improvement.
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post #8 of 17 Old 02-09-2016, 02:05 PM - Thread Starter
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@Bill Fitzmaurice I agree @buckchester I've got 6 subs in the room as is, so I'm past the point of adding subs, but I agree in normal circumstances that's a great solution.
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post #9 of 17 Old 02-10-2016, 12:53 PM
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Quote:
Originally Posted by Mike Schacht View Post
... I'm starting to treat the room, and thought I'd ask it was worth turning it into a bass trap. ...Any help would be appreciated, just don't want to waste time and money on something that probably won't do anything. ...
Then start by measuring the room acoustic decay, to see what your problems might be, before you decide on corrective action. How do you know: "The real problem I have is a big spike at 21hz and a null at 18." as it's very rare to have a peak/dip only 3 Hz apart, for acoustic reasons. In fact, many rooms get very flat below room modes, where room gain predominates. If there's quantitative data supporting this tight peak/dip pair, please share it.

Also, be careful when someone tells you something is "impossible." as it may be perfectly possible, but beyond that person's capabilities.

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post #10 of 17 Old 02-10-2016, 01:57 PM - Thread Starter
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@fbov that probably wasn't fair of me to say. The peak is the low end boost to extend the range of the sub south a little bit. With 6 subs that adds up and causes a peak because it hits the wall of the null at 18hz. The null at 18ish he is seen below. This is one sub running. The graph looks much better with the rest of them going after EQ. It's flatish until the other null at 125. If you have any suggestions on how to deal with that, I'm all ears. I've got some corner traps coming but I have no expectation of them doing anything for me down that low.

This REW is comparing the frequency response close mic'd vs at mlp.


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post #11 of 17 Old 02-10-2016, 02:58 PM
 
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There's no way that you're going to hear that dip, so stop worrying about it. Change the chart resolution to the 1/6th octave that you are capable of hearing and it won't be there anyway.
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post #12 of 17 Old 02-10-2016, 06:28 PM
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Depending on how high your subs are crossed, and what the response from your mains looks like, I might consider targeting that 125Hz null.
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post #13 of 17 Old 02-10-2016, 08:34 PM - Thread Starter
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@Bill Fitzmaurice I agree with you, time to move on and like I said, I don't anticipate being able to fix that, but if people have suggestions I'm happy to listen. Regarding the graph and whether I can hear it, the problem compounds as more subs are added. The additional subs remain constructive down to 20hz. Less so south of the null so I wind up with a shelf down 15db from 13-18hz. That is noticeable. I haven't ruled out a delay problem and will have more time to play with it on Saturday.
@Bigus I've got some GIK corner traps coming in about a week. The room I'm in has 8 walls, so less than ideal traps add up. I'm going to start with these and see what I can do about that. I've managed to push that null over to 140hz by adding subs and it's not as severe. Crossover is currently set at 100hz. I've had one of my mains out for repair, so I haven't focused too much on the mains integration. I picked up my repaired speaker today so once I get my subs working well together, I'll start focusing on that integration.
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post #14 of 17 Old 02-11-2016, 05:32 AM
 
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@Bill Fitzmaurice I agree with you, time to move on and like I said, I don't anticipate being able to fix that.
The issue is that you're trying to fix something that isn't broken. Measuring gear is far more accurate with magnitudes of order higher resolution of what it can measure versus what we can hear. Do as I suggested, look at the same result with 1/6 octave smoothing, and there will be no dip. 1/6 octave resolution is the best that you can hear, so anything that shows up on a graph with finer than 1/6 octave smoothing simply doesn't matter.
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post #15 of 17 Old 02-11-2016, 08:48 AM - Thread Starter
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@Bill Fitzmaurice I'm going to defer to your expertise here, but I believe that I'm really not doing a great job explaining the problem as it pertains to the constructive combing of subs. None of it really matters without measurements. I'll try and get things aligned this weekend and post back, with 1/6th smoothing and we'll see if it's really not a problem, or if I'm so outside of my element that I just don't have the knowledge required to make my point. Thanks for your input as always. It's greatly appreciated.
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post #16 of 17 Old 02-12-2016, 01:09 PM - Thread Starter
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@Bill Fitzmaurice I had a little time to kill so I started playing with smoothing. I see what you mean. The response looks much better with smoothing. If that's what we can actually hear, then there's no reason to get upset over the small stuff. It's still there, but greatly reduced, and rather than a null, I think I'm going to wind up with a cliff there. I also understand that it's time to just enjoy the speakers and watch some movies. I think I'll be able to get everything sorted out much better this weekend when I have time to spend on it. Also grabbed some insulation to stuff the perimeter of my drop ceiling.

On a separate note that hopefully someone finds interesting. As I was running my sweeps I kept on hearing weird sounds down low. I originally thought it was port noise which really just made me mad. After further inspection, my front height speakers were violently shaking. I took them down to confirm that's what the problem was. When I re-ran the sweep after removing the speakers, my response cleaned up quite a bit. I don't clearly understand the theory behind why that mattered, but I was happy with the results. I'll have to either do without the front height speakers or find a better way to secure them.
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post #17 of 17 Old 02-12-2016, 01:39 PM
 
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If something's vibrating enough so that you can hear it then it will show up on a sweep. A more secure mounting method for the heights should fix that.
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