Built some DIY Bass traps. Measurements are now worse. - AVS Forum
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post #1 of 52 Old 02-24-2013, 05:56 PM - Thread Starter
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Hello. I posted this in the REW thread but haven't had a response. I'm crossposting here as this seems like the forum where more REW types hang out.

I built some DIY bass traps and put them in last night. They measure 4'x2' and 6" deep, using Safe n Sound sound deadening material (It was praised on several forums with measurements similar to rockwool)

I installed 2 traps laying down under the screen backs directly against the front wall, pushed out to the sides against the corners. A third trap is in a rear corner at a 45* angle. I have a fourth to install but ran out of material.

Now, I didn't have any issues with the sound before, I just built these traps for fun, hoping maybe I'd hear a difference. The theater room is an open back which goes to the bar room. In the bar room it seems that the bass was reduced a lot, however in the theater room it seems about the same.

Now as far as measurements go, it seems like the traps increased the nulls and didn't really take care of any peaks. TO MY UNTRAINED EYE, the traps made the response worse. In the MDAT file I also did a before/after with the other front speaker, and similar results.

I have uploaded before and after measurements for left and right channels.

This is a 5.0 setup, no subwoofer yet. Audyssey XT 32 is turned off. Smoothing is at 1/24

No traps


With Traps


The room. Traps are 2 against front wall laying down. One is in the rear right of the theater room (That little wall sticking out)
Fourth might go flat against the wall or might go into the bar area in a corner. I'm not sure how much treatments in the bar affect sound in the theater.


Mdat:

https://docs.google.com/file/d/0B4Rhnfc1bgENVlJzazRIU0IyUTA/edit?usp=sharing

Any tips, pointers or general opinions of my measurements would be appreciated. Also, does my decay time seem good ? The traps did seem to decrease decay. I have 4 2x2 wall panels to go up at the early reflection points, perhaps that will improve my upper response.
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post #2 of 52 Old 02-24-2013, 06:34 PM
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Bass Traps should be standing up and running floor to ceiling in the corners. But you really need to experiment with placement and you need more than just one or two 2x4ft panels. You need a lot of them in the right places to make a difference, you can't just put them were ever and expect and improvement.

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post #3 of 52 Old 02-24-2013, 06:42 PM
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Best ever paper I have read on Bass Traps:

http://accucalhd.com/documents/audio/Acoustical%20Concepts%20For%20Home%20Theater.pdf

Very good reading. For good results, you need a lot of absorption in the right places. 3 traps is a drop in the bucket in a room that size. gtp's Superchunk traps pictured above are a terrific step in the right direction.

JSS
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post #4 of 52 Old 02-24-2013, 06:54 PM
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Well. Atleast that 120hz peak is damped abit and everything above it looks better. But it has also gotten abit worse between 80 and 110hz (ish) and below 80 you probably dont have bass traps that can affect that much.
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post #5 of 52 Old 02-24-2013, 07:22 PM - Thread Starter
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The problem with the room is that I have very limited space on the front wall due to the size of the screen. The other corners are 25' behind the listening area. I went out and got more material so I will be building 3 more traps for 6 in total. After that I don't think I would have anywhere else to put them. The traps must be doing something though, as it removed a lot of bass from the rear bar area. So perhaps with the 6 traps and some experimentation, I can get some better looking measurements.

I don't have the subwoofers built yet either. Meaning any setup I have now might be irrelevant once the subs are in place.
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post #6 of 52 Old 02-24-2013, 09:27 PM
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Yes, measurements now aren't totally meaningful without subs in place.

Also, your waterfalls have too much frequency resolution. You can have time resolution or frequency resolution, but you can't have both. The good news is that in the bass, frequency response gives you a really good idea of time decay.
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post #7 of 52 Old 02-24-2013, 10:49 PM
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Quote:
Originally Posted by gtpsuper24 View Post

Bass Traps should be standing up and running floor to ceiling in the corners.

Why's that?

The best orientation is whatever works best with acceptable aesthetics.

Noah
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post #8 of 52 Old 02-25-2013, 07:26 AM
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The problem with the traps is that they are not placed in the right positions. You want to place them floor to ceiling, as someone else mentioned, and preferably in the corners. Having them lay flat on the floor and in the middle of the wall is not what I would call a good position. You should re-arrange them to go in the corners, floor to ceiling. Then, once you cover floor to ceiling in the 4 corners, (note that you might have to stack two of your 4' x 2' traps on top of each other in each corner in order to reach floor to ceiling) you can them start by placing them along with mid-way point of the side walls, then along the back wall. Hope this helps!
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post #9 of 52 Old 02-25-2013, 07:41 AM
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Quote:
Originally Posted by noah katz View Post

Why's that?

The best orientation is whatever works best with acceptable aesthetics.

Sorry room acoustic treatments and aesthetics DO NOT go together. You can't have both looks and good treatment (although I for one think a well treated room with nicely done treatments looks nice). For a persons goal that is looks above treatments then yes the way you described it that would be the best way. If you want the best acoustic treatment then your going to have to give up the nice aesthetics. Bass "gathers" in corners so you need to work on filling in the corners top to bottom (fronts most important) with either superchunk style (17x17x24" triangles stacked) rolled insulation stacked or thick panels with an air gap behind them.

Having thick 6" panels on the back wall can help with decay times which can possible tame some boomyness and give the user the tight bass response in the midbass.

With room treatments either do it right or don't bother. Throwing them randomly around the room for aesthetics is just a waste of time.
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post #10 of 52 Old 02-25-2013, 09:59 AM - Thread Starter
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So would the rear two corners be the corners of the other room behind then ? Will traps 25' away in another room make a difference ?

Traps along the front will be a big problem. There simply isnt much room on the sides of the screen.
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post #11 of 52 Old 02-25-2013, 10:11 AM
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Quote:
Originally Posted by jonathanc View Post

So would the rear two corners be the corners of the other room behind then ? Will traps 25' away in another room make a difference ?

Traps along the front will be a big problem. There simply isnt much room on the sides of the screen.
Traps are only as effective as their placement. Find out how to place them here:
http://www.realtraps.com/articles.htm

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post #12 of 52 Old 02-25-2013, 10:42 AM
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From everything I've read, bass likes to collect in corners. You may notice that if you stand in a corner of your room you will notice a lot more bass. This is why traps work most effectively in corners. It's where the bass likes to hang out.
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post #13 of 52 Old 02-25-2013, 11:18 AM
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Quote:
Originally Posted by DanLW View Post

From everything I've read, bass likes to collect in corners. You may notice that if you stand in a corner of your room you will notice a lot more bass. This is why traps work most effectively in corners. It's where the bass likes to hang out.

For the sake of keeping it simple, this is great advice. Bass congregates in corners as room modes terminate in corners. Placing absorbers in the corner aid in dampening modes from multiple boundaries. IOW - its a good place for bass trapping.
Quote:
Originally Posted by tlag View Post

Also, your waterfalls have too much frequency resolution. You can have time resolution or frequency resolution, but you can't have both. The good news is that in the bass, frequency response gives you a really good idea of time decay.

This is not correct. Frequency response is the averaging of the first (however many you choose) milliseconds and ignore the rest of the decay. Frequency response does NOT show you decay time - waterfall graphs do.
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Originally Posted by gtpsuper24 View Post

Bass Traps should be standing up and running floor to ceiling in the corners. But you really need to experiment with placement and you need more than just one or two 2x4ft panels. You need a lot of them in the right places to make a difference, you can't just put them were ever and expect and improvement.

Bingo! OP - he speaks the truth! But I'll comment further:
Quote:
Originally Posted by jonathanc View Post

Now, I didn't have any issues with the sound before, I just built these traps for fun, hoping maybe I'd hear a difference. The theater room is an open back which goes to the bar room. In the bar room it seems that the bass was reduced a lot, however in the theater room it seems about the same.
(...)
I'm not sure how much treatments in the bar affect sound in the theater.

If you're minimally treating the space as you've outlined, traps that far back likely won't make huge differences. You should use the panels you have in the front of the actual room itself. You will likely hear a bigger difference with better placement. I'd suggest placing them at your first reflection points for your mains, and in the front corners wherever possible. See the following video to find your first reflection points: http://gikacoustics.com/video-early-first-reflection-points/
Quote:
Originally Posted by jonathanc View Post

This is a 5.0 setup, no subwoofer yet. Audyssey XT 32 is turned off. Smoothing is at 1/24

No traps


With Traps

A couple of points:
Measurements would be more beneficial with a sub if you plan on having a sub in the room
Audyssey should be turned off when doing these measurements as you've done (you could do Audyssey on vs off measurements as well)
Smoothing doesn't apply to your waterfall graphs. Smoothing only affects Frequency Response graphs in REW.
You're viewing angle on these graphs allows for hardly ANY useful information! This would be a much better view:


Looking at the graph this way, we can actually see your room resonances and the real time scale they occupy (a lot!)
Quote:
Originally Posted by jonathanc View Post

Any tips, pointers or general opinions of my measurements would be appreciated. Also, does my decay time seem good ? The traps did seem to decrease decay. I have 4 2x2 wall panels to go up at the early reflection points, perhaps that will improve my upper response.

Yes, these will improve your high end response. If you make them thick you might get some reduction at the lower frequencies. (Sorry I didn't read this part when I referenced early reflection points above, but i'll keep that in the post for others)

Your null that increases in depth is just likely either a testing anomaly or you moved the microphone between tests (The mic needs to stay in the exact same spot!). Your signal to noise ratio is also not great in the tests either, make sure you're running your speakers LOUD and you can further improve the S-to-N ratio by using more sweeps per measurement (8 is better than 4 is better than 2). For low end, you can test with both speakers on (it will drive your modes harder so you have a better view of the low end).

Alexander Reynolds
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post #14 of 52 Old 02-25-2013, 11:53 AM
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the kind of bass traps that you are using rely on air moving.

as you get close to a boundary, there is very little air movement, so placing said traps on a boundary will do little if anything.

air is moving with greatest velocity at 1/4 wavelength from a boundary. try placing your trap in the air and that distance from the boundary.

the only way that i know of to absorb bass at a boundary is to make it very limp so that the boundary itself will actually flex, absorbing the sound energy and turning it into heat.

Listen. It's All Good.
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post #15 of 52 Old 02-25-2013, 11:57 AM
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jonathan, a couple things....

First off, nice job of starting with a measured problem. Granted, you will need a very good subwoofer to show all the sonic issues, but there is one thing you've learned:
Your DIY "bass traps" don't trap much bass. That doesn't mean you made them wrong as placement is also critical.

Your data shows a drop in output in the 75-150Hz range, but nothing below 75Hz, as expected from a 2" absorber mounted on a surface. You have DIY absorption panels, they do work, but how they work depends on where you put them.
- these are resistive panels, so they work best where air velocity is highest.
- boundaries don't move, so air velocity will always be low against a wall or in a corner. These are pressure maxima.
- to get a resistive panel to work on a surface, space it away from the sorface.
- Unless you get very thick, and spaced quite far away, low frequency absorption will be limited.

Not only will it not trap bass, it will trap lots of higher frequencies, leaving you with a dead room that's boomy.

In addition, without a sub, your waterfalls are only showing harmonics. If I take the room at 30x11x8 (guess for ceiling):
- your peak in the low 40's is (200) (2nd harmonic, long direction)
- your peak at 55Hz is a combination of (300) and (010)
- your mid-70Hz peak is (400) and (001)
- and the 100Hz peak you actually pushed down is (500) and (600) with some (020) thrown in, plus contributions from the stub wall.

Note that I didn't mention (100), your room fundamental mode at 18Hz as you haven't excited it yet. Get a room mode calculator and try it for yourself.

If you want to lower sound levels overall, broad band absorption panesl are great. If you want to absorb upper bass, panels can be configured (thickness, spacing from walls) so they are very effective, but they will not go deep enough to "trap" low bass without greatly deadening the room.

Panels can be modified, or more accurately, their parts used to make effective bass traps.

The corner placement is not bad, but if you cover the face with a skin of masonite, and seal the air space in the corner behind it, you get a real bass trap, a diaphragmatic trap that absorbs bass but not upper frequncies. Perforating this board allows you to tune beyond a basic mass density dependence, but now you're getting into types of sharply tuned absorbers.

Lots of options for real bass energy supression, but you have to look. Your data is a great first step! Here's a link that'll tell you lots more about these devices, starting about page 185,
http://andrealbino.wikispaces.com/file/view/Master+Handbook+of+Acoustics+-+5th+Edition+-+F.+Alton+Everest,+Ken+C.+Pohlmann.pdf

Have fun,
Frank
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post #16 of 52 Old 02-25-2013, 01:07 PM
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Quote:
Originally Posted by GIK Acoustics View Post

This is not correct. Frequency response is the averaging of the first (however many you choose) milliseconds and ignore the rest of the decay. Frequency response does NOT show you decay time - waterfall graphs do.

Ok, so I typed loosely. Not exactly incorrect. What I meant was, given a linear system, which is a decent representation of bass response in a room, frequency response gives a good idea of time response. Peaks in frequency response show ringing in temporal response. To sum up, given a linear system, if you have either the frequency or time response, you can infer the other. Is this really debatable?

Also, waterfalls with high frequency resolution lose time resolution. They still look pretty, but do not show ringing properly. Good waterfalls (meaning good time resolution) have crappy, over-smoothed-looking frequency responses at time zero.

EDIT: By the way, below is what the author of REW has to say about waterfalls vs. decay time...

Waterfall plots cannot be used to read decay times, unfortunately. The plot is produced by moving a window through the response and plotting the frequency content within that window. Using longer windows makes decays appear to take longer, which is at its heart a manifestation of the time/frequency trade-off of these kinds of analysis. Longer windows give increased frequency resolution, but correspondingly reduced time resolution.
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post #17 of 52 Old 02-25-2013, 01:16 PM
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Bass congregating in corners is sorta true, but using that to justify resistive damping is sorta wrong. Bass pressure occurs in corners, but that's zero velocity. The two are 90deg apart, so one's peak is the other's minimum. As it turns out, resistive trapping in corners still works, but only because the traps have non-zero depth, meaning they pick off some of the velocity component. Ideally, one would use pressure-type absorbers in corners.

EDIT: by "resistive" above, I mean flow-resistive, of course...
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post #18 of 52 Old 02-25-2013, 02:02 PM - Thread Starter
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Thanks a bunch for the responses so far. Just to stress, the bass traps are 6" thick. Someone mentioned 2" which is incorrect. My understanding is a 6" thick panel is more ideal than thinner stuff. I finished off the last trap. I will have to play around with measurements.

The first measurement to the treated measurement were several days apart, so the measurements could be invalid.

The biggest issue so far is placing traps on the front wall. I only have about 1' between screen and wall on the sides, and 2' at the bottom.

The early reflections will be addressed with 3" thick 2'x2' panels.

One thing I didn't think of. I used thin cardboard as backing for the traps. If they're spaced away from the wall, will the cardboard cause decreased performance ?

What I may consider is using the traps in other locations around the room and building some thin corner traps. Buying traps and shipping them to very northern Canada is very expensive. The only product I can get locally is auralex, and I don't know if their corner traps are any good.
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post #19 of 52 Old 02-25-2013, 03:53 PM
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Quote:
Originally Posted by jonathanc View Post

Thanks a bunch for the responses so far. Just to stress, the bass traps are 6" thick. Someone mentioned 2" which is incorrect. My understanding is a 6" thick panel is more ideal than thinner stuff. I finished off the last trap. I will have to play around with measurements.

The first measurement to the treated measurement were several days apart, so the measurements could be invalid.

The biggest issue so far is placing traps on the front wall. I only have about 1' between screen and wall on the sides, and 2' at the bottom.

The early reflections will be addressed with 3" thick 2'x2' panels.

One thing I didn't think of. I used thin cardboard as backing for the traps. If they're spaced away from the wall, will the cardboard cause decreased performance ?

What I may consider is using the traps in other locations around the room and building some thin corner traps. Buying traps and shipping them to very northern Canada is very expensive. The only product I can get locally is auralex, and I don't know if their corner traps are any good.

Keep in mind, 6" is one wavelength at 2.2 KHz.

Not exactly gonna trap any "bass" with something that is such a small fraction of a low frequency's wavelength.
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post #20 of 52 Old 02-25-2013, 05:47 PM
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Quote:
Originally Posted by lilmike View Post

Keep in mind, 6" is one wavelength at 2.2 KHz.

Not exactly gonna trap any "bass" with something that is such a small fraction of a low frequency's wavelength.
Even the thickest traps don't trap bass, they trap midbass. Since midbass is what causes boomy response that's not a problem. But you don't need for the trap to be a significant portion of a wavelength thick either. If you did even midbass traps wouldn't work.

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post #21 of 52 Old 02-25-2013, 06:43 PM
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These are easy to make and get the job done
http://www.avsforum.com/t/255432/acoustical-treatments-master-thread/9150#post_22555253
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This will show you one pink fluffy build method, from http://www.avsforum.com/t/1312693/diy-construction-methods-of-hang-able-acoustic-panels-not-fixed-frames/120#post_22131618
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Hi guys,

I've been following a number of the acoustics threads here for a while, though I haven't been very active on the AVS forums. Mike suggested that I share some of my work on room acoustics with everyone here, so here are my corner bass traps:

I followed a similar construction technique that Mike posted for his corner bass traps, but made mine from pink fluffy R-19 instead and wrapped the fabric around the outside of the threaded rod. Here is the photo journey:

My traps are 24x24x34 inch triangles, are about 3.5 feet tall, and probably weigh less than 10lbs each. Since I need access to one of the corners for a doorway, I made them stackable and moveable. I started by cutting triangles, drilling holes for the threaded rod, and inserting T-Nuts for the sides that stack on one another:
287

Then I cut 23" squares of insulation, cut them diagonally for triangles, and clipped off the corners so they fit snugly between the threaded rods:
390


A wire mesh made from separated Cat5 wire goes between each layer to keep the insulation from sagging over time. The numbered arrows indicate the direction of winding the wire to support the insulation. A bead of solder keeps the wire from unwinding. Each layer of insulation is about 5" thick.
400

Here is the first one all stacked up. You can see the supportive wire mesh wrapped around the threaded rod on each side:
516

And then with the Kraft paper glued to the front with spray adhesive:
529

Then, turn the trap upside down to affix the cloth wrapping - a two-pack of curtains from the giant W for $15. Each pack is enough for two traps.
400

The cloth is stapled to the underside of the top plate to prevent sagging over time:
291

When the fabric is fully secured to the top panel, turn the trap right side up again and pull the fabric around to the back. Trim off the excess and then just pull it tight and use a desk stapler to hold the fabric together in the back:
550

The staples produce a few ripples in the sides, but you won't see those once you put it in the corner. The front looks nice and clean:
528

Here is a closer shot of two of them stacked together:
600

And finally, the entire back half of the theater. With a few bean bag chairs up front, we can comfortably seat 10-12 people. The colors are a little off from a combination of CFL and flash lighting:
326

Below are the before and after REW plots. The purple trace is the original measurement with no traps and no EQ. The yellow trace is the difference made by the bass traps alone. The traps took 5dB off of the room-induced peak at 45Hz without sacrificing anything else in the audible range:
329

This is the original waterfall plot made by REW - no traps, no other corrections - just a mess with room modes at 45Hz and 90Hz:
443

And here is the waterfall plot after ONLY the traps are put in place. I was surprised by how much of a difference the traps made in the decay:
447

After several days of tweaking the parametric equalizer with the traps in place and a first order high-pass filter in place, here is my "final" room response curve - flat from 7Hz to 100Hz, plus/minus 3dB :'( The peak at 105Hz won't ever really happen because the preamp crosses the LFE channel at 60Hz.
449

And the "final" waterfall plot - nice and smooth. If I adjust the waterfall graphing limits in REW, the entire response curve is down by 20dB within the first 100-120ms with the exception of a 2-3dB narrow bump at about 23Hz. I am really pleased with this result:
443

It sounds great! The EQ and high-pass filter reduce (but don't eliminate) the incidence of amp clipping, but still provide enough punch to cause visitors to literally jump up from the couch :P

All of my other projects are on my web page, which I think is linked in my signature.

Eric
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post #22 of 52 Old 02-25-2013, 07:51 PM - Thread Starter
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Damn well at least now I know what a GOOD waterfall looks like. I finished off the 4 acoustic panels today. So that makes 4 traps and 4 wall panels. I took some measurements and to build corner traps like above, they would only be 1/2 the size. 1'x1' x whatever the long side of the triangle. The traps are now propped at an angle along the floor at the front. Not sure what else I can do with them at the front.

If you guys had to out 4 traps in places other than the front, where would you put them ? Keeping in mind I have no immediate back wall.
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post #23 of 52 Old 02-25-2013, 11:35 PM
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Quote:
Originally Posted by tlag View Post

Ok, so I typed loosely. Not exactly incorrect. What I meant was (...)

tlag,

Didn't mean to step on toes. I'm not as familiar with the members on this subforum as I need to be it seems. With "loose typing" (I like that term...might use it myself) such a short post, I mistook it for heresay instead of an original comment.

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post #24 of 52 Old 02-25-2013, 11:49 PM
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Quote:
Originally Posted by lilmike View Post

Keep in mind, 6" is one wavelength at 2.2 KHz.

Not exactly gonna trap any "bass" with something that is such a small fraction of a low frequency's wavelength.

6" of Safe'n'Sound will work to a much lower frequency than 2.2kHz!

http://www.stanleyhallstudios.co.uk/pacalc/pacalc.php?e=h&r=r&m=4&l1=6%20inches%20Safe%20n%20Sound&s11=2&d11=152.4&v11=8000

Though I think you're stating that you aren't going to get much action sub-100 Hz, which I agree. But 100 Hz -> 200 Hz Safe'N'Sound should do 'alright' and above 200 Hz should work great!
Quote:
Originally Posted by Bill Fitzmaurice View Post

Even the thickest traps don't trap bass, they trap midbass. Since midbass is what causes boomy response that's not a problem. But you don't need for the trap to be a significant portion of a wavelength thick either. If you did even midbass traps wouldn't work.

I agree with the end of your post, but will disagree with the first portion a bit. Thick velocity based porous absorption in the corners can still give useful absorption down to 50 Hz. Not sure I'd quality <100 Hz as "Mid-bass"

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post #25 of 52 Old 02-26-2013, 05:59 AM
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Great thread we got going here, guys! Lots of good information, debate, and knowledge to be had in this thread!
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post #26 of 52 Old 02-26-2013, 07:02 AM
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mtbdudex,

I noticed that your frequency response is rising considerably going from 20Hz to 10Hz in your non-EQed graphs. Is that unusual to see?
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post #27 of 52 Old 02-26-2013, 07:20 AM
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Quote:
Originally Posted by GIK Acoustics View Post

I agree with the end of your post, but will disagree with the first portion a bit. Thick velocity based porous absorption in the corners can still give useful absorption down to 50 Hz. Not sure I'd quality <100 Hz as "Mid-bass"
That depends on the thickness of the material used, and the absorption coefficient. There are charts out there you can use to calculate what a particular trap material and thickness is capable of.
Quote:
Below are the before and after REW plots. The purple trace is the original measurement with no traps and no EQ. The yellow trace is the difference made by the bass traps alone. The traps took 5dB off of the room-induced peak at 45Hz without sacrificing anything else in the audible range:
I'm not seeing that. The yellow and purple traces should be virtually identical in the lowermost frequencies, so close that they would overlay and appear as one. What I see tells me that the mic positioning and/or drive level wasn't identical for the two measurements, leading to erroneous conclusions. A deviation of even a few inches in the mic position can make a big difference, as Ethan Winer explains:
http://www.ethanwiner.com/believe.html

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post #28 of 52 Old 02-26-2013, 09:59 AM
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Also when analyzing decay times, modal ringing, waterfalls etc, a higher spl measurement is more useful. Shoot for something close to 90db assuming a 40-50db ambient floor.

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post #29 of 52 Old 02-26-2013, 10:57 AM
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For anyone interested in what a good (and bad) waterfall plot looks like, I highly recommend studying Figure 13.23 here (from Toole's "Sound Reproduction"). The description is quite helpful as well.

Basically, you want to see some undulations vs. time, and not just smooth vertical ridges that seem to decay monotonically vs. time.
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post #30 of 52 Old 02-26-2013, 11:39 AM
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Quote:
Originally Posted by lilmike View Post

Keep in mind, 6" is one wavelength at 2.2 KHz.

Not exactly gonna trap any "bass" with something that is such a small fraction of a low frequency's wavelength.


Up against the boundary yes.
However, you place that same 6" of insulation ... well away from the boundary and voilà, it becomes a tremendous effective absorber of the problematic modal range of typical residential rooms. A bass trap.


Some good advice given above, but lets examine the most important aspects of bass trapping with porous absorbers;
Ideally, place the porous material where it works the best .. at the 1/4 wave point off the boundary ... where the velocity is greatest.

If you chose to absorb, do so effectively. Poorly executed porous absorbtion of sidewalls filters the reflected energy in a manner whereby the resultant sound will be lifeless, muddy, bass heavy mess. Even sidewall panels need as much extension into the LF as possible. Oftentimes, prudent use of coverings can elicit a spectrally balanced effectiveness, however this is best confirmed with measurments.

Porous absorption can be effective quite deep, I've seen measured results down into the 30hz range. The thicker the better, if you go very thick, you use the less dense material. If you opt for less thickness, utilizing an air gap multiplies the effectiveness greatly.

As a rule of thumb, based on how much space off the surface you can use;
up to 4", use 3 lb fiberglass equivalent (4lb) mineral wool
4-6", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap
6"-12", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap off the surface, or fill the space with loose/fluffy
12" and more, fill the space with pink fluffy

For corners;
One can either straddle a corner with a 4"-6" thick panel, or employ stacked fluffy, stacked rigid 703, superchunk style.
But be mindful that filling the corner is better than using a 4"-6" thick panel, however only somewhat better. Instead of filling the entire area, one may be better off utilizing the same material elsewhere, covering more corners.

Best case, fill all the corners as deep as you can spare. Remember, the thicker the porous absorber, the less dense (gas flow resistivity) the material is optimum. For example, if one could spare 18" across the entire back wall of a studio, pink fluffy will out perform 703 rigid fiberglass.


Acoustics is anything but intuitive. And from everything I've learned, the lowest range of frequency effectiveness extends much lower than typical 1/4 wave calculations would suggest. When including the effects of the porous material's speed of sound, the low end effectiveness becomes significantly better, and lower in frequency then previously understood. It's simple; the material is acoustically thicker than it's physical dimensions would suggest.

The 1/4 wave distance is shorter in pourous absorption, because the speed of sound in porous absorbent material is slower than it otherwise would be in air. Slowing the speed down, shortens the wavelength, thus shortening gap needed off the wall/boundary needed to affect any one frequency.
I've read individuals studying such things suggesting the speed/wavelength shortening can be as much a 1/3, correlating with a measured results showing that if the thickness of an effective porous absorber is at least 7% of the wavelength, then one could achive significant absorption approaching 100%.

I don't know for sure, but I seen measured results that show effectiveness much deeper than one would intuitively think would be possible ... using the typical 1/4 wave effectiveness mindset. So what I've since read about the speed of sound within the material, certainly is interesting and makes sense.


Largely, in the typical small rooms that comprise our HTs and listening rooms, one can hardly have too much bass trapping. However, enormous benefits are had with when one simply achieves an adequate level of bass damping/trapping. And the cool thing is, the cheapest stuff (pink fluffy) is the most effective. Any large surfaces, ceiling included need treatment. One needn't be concerned about over doing it either, as the traps can be faced with diffusion, plastic, wood slats, anything that's invisible to the deep stuff. That way any amount of energy in the MF/HF can be retained, if one desires.


Thanks


Here's a nice example.

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