Built some bass traps that do very little. - AVS Forum
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post #1 of 15 Old 03-19-2013, 07:30 PM - Thread Starter
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I thought bass traps sounded like a fun, cheap project, so I built four of them. They are 2'x4' with 6" framing on the short sides and 1" on the long sides. The interior is 6" of Roxul Safe'N'Sound. The backs are covered in a cheap muslin and the remainder with flannel.

My room is laid out diagonally, and I've placed the traps diagonally across the corners behind the speakers and behind the listening position, stacked on top of each other. I have ten foot ceilings, so they leave a two foot gap.

I don't see much effect in the response. In the 100-300Hz region where you would expect them to act most heavily I see some shuffling of peaks and nulls, but nothing I would classify particularly as an improvement.



Below 80Hz, there is predictably little to no effect.



Do I simply need more traps to see improvements, or are the traps I built fundamentally flawed in some fixable way?
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post #2 of 15 Old 03-20-2013, 11:00 AM
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So, the panels are made of 2x6's at the top and bottom, with 2x1's on the sides? Why muslin only on the back? What keeps the Roxul in? You have then placed them across the front/rear corners of a diagonal room?

You're getting what your design should deliver.
- you started with a good, broad band absorber - the Roxul
- you covered it with muslin and flannel, which reduce absorbance at higher frequencies
- you placed it close to room surfaces, which limits its low frequency absorption

You're getting quite a bit of effect in the 80-400Hz range, right where this design should be absorbing. The high frequency absorbance is likely better with just muslin around the Roxul (flannel's not that porous in my experience), and this type of panel is very effective placed behind the L and R speakers. Hard to predict with diagonal walls, but this placement usually helps flatten the 100-300Hz range where front wall bounce (sometimes mis-termed the Allison effect) has a large effect. I would imagine a corner placement, mounted flat on diagonal surfaces, would make these even more effective.

But I bet you were hoping for some movement on that 52Hz dip...

First, confirm the dip.
Measure repeatedly around that location (your listening position perhaps?) to see the local interference pattern, not just one point. I just did my room, and moving the mic was critical to seeing the whole picture. A dip like this "appeared" at one point in my measurements, just due to a mic position change; no dip in 3 spots nearby so non-issue. I'll move my head.

Next, level match your sub to the rest of your system. I find even a 5dB rise is noticable, and you're a good 10dB higher at 80-100Hz. Once you've got a fairly constant bass SPL level over a region of space larger than your chair, we can talk about residual issues, and how to address those specific issues.

And I'm serious about confirming the dip. Litteral from one shoulder to the other, I get a completely different pattern, and yet another pattern behind and in front of me - 4 locations within 3 ft of each other, 4 curves with different bumps from 40-100Hz... and this in a nearly resonance-free room.

Have fun,
Frank
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post #3 of 15 Old 03-20-2013, 12:24 PM
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Quote:
Originally Posted by intangiblefatman View Post

Do I simply need more traps to see improvements

Probably, but:

* Was the measuring microphone in precisely the same place for your Before and After graphs? Moving even an inch or three can render the comparison useless.

* How large is your room? Four 2x4 traps will make a real improvement in an 8 by 10 foot bedroom. Not so much in a 20 by 30 foot great room.

* A better way to assess the effectiveness of bass traps is with a waterfall graph. That shows both the change in frequency response and also the change in ringing decay times. Since you're using REW, generate a waterfall using parameters similar to those shown here:

Room Measuring Primer

--Ethan

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post #4 of 15 Old 03-20-2013, 02:25 PM - Thread Starter
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Thanks for your thoughts, guys.

I used muslin on the back, because it was cheap; it is what physically holds the rockwool in. I used flannel on the front, because I wanted to attenuate the high frequency absorption. I am moving shortly to a room where I cannot position things diagonally and was planning on building first reflection point panels at that point. I didn't want to accidentally deaden the room too much.

I wasn't actually worried about the 52Hz dip. It is an artifact of that particular measuring position, as you suggest, as I don't see it in the neighboring seats.

I did take care not to accidentally bump the microphone while moving the traps into the room. The two measurements are to the best of my knowledge at an identical location.

The current room is 15'x13'x10' but opens up through a double archway to the right-rear of the listening position to the rest of the apartment.

The waterfall plots are very similar.






I'm actually not really sure what effect I was expecting these to have. There's clearly an effect in the region between 100Hz and 400Hz, but plot does not appear, to me at least, to be superior than the other. I read a lot about the benefits of bass traps that made me expect a lot more noticeable change.

I may save two to place behind the main speakers as suggested and recover the other pair to use as first reflection absorbers.
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post #5 of 15 Old 03-20-2013, 02:54 PM
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Do you have much of an opportunity to move your speakers and seating? I would think pulling speakers and the LP away from boundaries would have a larger effect than bass traps. And below 80hz, a multi-sub setup would be better than bass traps.
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(sometimes mis-termed the Allison effect)

Thank you! This is kind of a pet peeve of mine when people say that. Good advice in the rest of your post also.
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post #6 of 15 Old 03-20-2013, 03:31 PM - Thread Starter
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I am moving in about a month, so I will take the opportunity to try to better optimize the speaker and listening position placements when I do. I have two subs at the moment, but they are rather constrained in where they can go at the moment by the rest of the room and are thus perhaps in suboptimal positions.
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post #7 of 15 Old 03-21-2013, 08:41 PM
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Safe'n'Sound is a very porous absorber. While it works down to 100 Hz at 6" thick, you won't have a huge absorption coefficient at those frequencies. Safe'n'sound works really well for really thick absorbers, like a foot thick. Similar with pink fluffy insulation (I believe Safe'n'Sound gives a better absorption coefficient than fluffy at low frequencies). Straddling the corner likely won't yield as good of results as you see a typical panel straddling. The absorption would be spread lower in frequency, but also lower in efficiency. So in other words, for corner absorbers with safe'n'sound or fluffy, its best to make them very thick (like superchunks or soffit style traps).

Alexander Reynolds
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post #8 of 15 Old 03-22-2013, 06:10 AM
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Quote:
Originally Posted by intangiblefatman View Post

I thought bass traps sounded like a fun, cheap project, so I built four of them. They are 2'x4' with 6" framing on the short sides and 1" on the long sides. The interior is 6" of Roxul Safe'N'Sound. The backs are covered in a cheap muslin and the remainder with flannel.

My room is laid out diagonally, and I've placed the traps diagonally across the corners behind the speakers and behind the listening position, stacked on top of each other. I have ten foot ceilings, so they leave a two foot gap.

I don't see much effect in the response. In the 100-300Hz region where you would expect them to act most heavily I see some shuffling of peaks and nulls, but nothing I would classify particularly as an improvement.



Below 80Hz, there is predictably little to no effect.



Do I simply need more traps to see improvements, or are the traps I built fundamentally flawed in some fixable way?

Plug your absorber design into Chris Whealy's Porous Absorber Calculator:

http://www.whealy.com/acoustics/Porous.html

I can tell you the results in advance - less than sparkling amounts of absorption at the frequencies of interest.

What you are observing is a simple matter of cause and effect. In this case the effect and cause were not so much!

Better luck next time, but to reduce the effects of luck, use the available design tools before you cut wood. ;-)
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post #9 of 15 Old 03-22-2013, 11:38 AM - Thread Starter
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It sounds like I do not have space in my room to put in effective bass traps, and that I would be better off repurposing the traps for other uses.

Thanks for all of the advice.
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post #10 of 15 Old 03-23-2013, 05:48 PM
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Not true, something like a Metal plate resonator (only commercially available as the RPG modex plate) can have great absorption below 100Hz in a package only 4" thick.


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post #11 of 15 Old 03-23-2013, 07:34 PM - Thread Starter
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Those run a little over my $100 budget. smile.gif
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post #12 of 15 Old 03-25-2013, 10:12 AM
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You could build a mass loaded vinyl fronted panel absorber that works well down to 60Hz in a 4" deep package if the metal plate absorber is too rich.


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post #13 of 15 Old 03-26-2013, 09:26 AM - Thread Starter
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Like these?

http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html

They look very doable, although I am still changing apartments every couple of years and thought it would be best to wait until I had a more permanent room to work with before trying anything tuned to a specific frequency.
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post #14 of 15 Old 03-26-2013, 05:26 PM
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Quote:
Originally Posted by intangiblefatman View Post

Like these?

http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/743040-tims-limp-mass-bass-absorbers.html

They look very doable, although I am still changing apartments every couple of years and thought it would be best to wait until I had a more permanent room to work with before trying anything tuned to a specific frequency.

I would build a few with center frequencies 20Hz apart. Most of the effective very low frequency absorbers are tuned devices unfortunately. You know if you have only one seating position you really care about I would just use parametric EQ to dial out the peaks. Either that or add some more subs in mode canceling arrangements.


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post #15 of 15 Old 03-27-2013, 09:16 AM - Thread Starter
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Alright, thanks a lot for all of the advice. Audyssey actually does a pretty miraculous job at the two seating positions that get the most use right now, so I will probably wait a while before trying to build anything else, but when I do I have a much better idea now of which designs will actually be effective at which portions of the spectrum.
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