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PostPosted: Tue Dec 06, 2011 3:18 am 
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Hi Guys!
I am dealing with acoustical problems in my room.
I treated all the walls (front left right behind) with absorbers and diffusers to my liking and I am pretty happy with my results.
However I noticed that I still had some noticeable bumps in my mid range. Especially between 140 and 180hz.

I fired these frequency with a tone generator and I am now pretty sure that it comes from ceiling reflections (which is untreated yet).
This is pretty fun as when I kneel down are climb up the ladder I can clearly here the nulls of these frequencies.

It also makes sense since my room is is 2.40 meters high/ (this matches with my mid range problem).

At this point I am torn between 2 options:
- Building absorbent rockwool pannels at about 60 cm from the ceiling (1/4 a wavelength of 140hz)
- Building diffusers that work precisely at this frequency => PVC tube cut in half along the length. As large as possible so that it treats again the 1/4 wave lenght.

Since my room is a "little too dead" I would go with the PVC tube that will reflect the mid and high frequencies. Or maybe I could wrap up the rockwool with a plastic bag to prevent absorption from the high frequencies?

What do you guys think?
Thanks a bunch!!!

FYI: I already have tons of bass traps in all my corners.


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PostPosted: Tue Dec 06, 2011 3:19 pm 
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What would help a lot to determine what the solution might be, is:

1) All of the dimensions of your room.
2) A diagram of your room, in SketchUp, showing what treatment you already have.
3) Photos!
4) A complete analysis of the room with REW.
5) Details of the test equipment you used to measure the room (amp, speakers, mic, interface, and anything else in the signal chain.)

- Stuart -

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PostPosted: Tue Dec 06, 2011 5:38 pm 
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The nulls and peaks are going to be there regardless. Unless you do a lot of praying that the mix will come together or you'll be finished on time, you won't do much mixing on your knees - and I don't know anyone who mixes up a ladder :P

You fail to mention what the problem is at the Listening position. I would assume from the dimensions 2.4 with a typical head height of 1.2 you're near dead centre, and if you've got nulls above and below, then perhaps you have a peak at the LP so this frequency dominates.

Not sure if you really mean a diffusor or a trap, but your description seems to infer a poly. Again, because of the dimensions I would'nt suggest diffusion at that proximity, I think it will cause you more problems.

I would suggest hanging a cloud above (much as your 1st option), angled at about 12° or more (downward) toward the front, and with a hard back (ply, osb, mdf or gyproc). This should have a modifying effect on the local modal propagation at the LP.

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PostPosted: Thu Dec 08, 2011 12:13 am 
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I have attached 2 pictures of my room.

Dimensions are 4.2 meters long 4 meters large
2.42 meters high.

Almost a square I know...
But I really feel like the diffusers that I put are doing a really good job at breaking the standing waves. They are supposed to diffuse pretty low as they are 2D and 30 cm deep.
I also have bass traps in every corners.

As you can see, only the ceiling is left to be treated properly.

Although I only have absorbing foams in the front, I feel like the room is already pretty dead. That s why I felt like another diffuser as a cloud would to a good job instead of some more absorbing material.

I already ran a lot of test to find the sweet spot and I am confident that I found it. (I used softwares (fuzz measure) then solely relied on my ears and It led me to the same conclusion. i.e where you see the chair)

I really need to kill this 140 / 160 hz standing wave from the ceiling. what can I do?
Thanks!


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PostPosted: Thu Dec 08, 2011 4:28 am 
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There are many, many things wrong with that room!

First: "Almost a square I know...". Yup. That's a problem.
Second: There is no symmetry! Your speakers, desk and listening position are not on the central axis of the room. This is critical. You absolutely must have symmetry, at least for the first half of the room. To fix that, get rid of the sofa and move the desk / speakers / chair onto the center line of the room.
Third: Get rid of those diffusers: They are doing more harm than good right now, since the room is way too small to be able to benefit from QRD diffusers. Also, you seem to have them at your first reflection points! :shock:
Fourth: "I also have bass traps in every corners". Yes. but they are way too small to be of any use. Superchunks need to be about 80cm across the front: Yours don't seem to be anywhere near that big.
Fifth: The absorption you have on the front walls is too thin: It should be at least 4 inches thick (10cm) for a room that small.
Sixth: "Although I only have absorbing foams in the front, " Then you need it on the back, too! On the rear wall you will need at least 4 inches of absorption, spaced at least 4 inches away from the wall, and covering the entire wall if possible, or as much as you possibly can. Please post a photo of the rear wall too.
Seventh: Room Ratio. Your is 1 : 1.65 : 1.73, which is not near to any known good ratio.

Now, on to your basic questions and comments:

Quote:
However I noticed that I still had some noticeable bumps in my mid range. Especially between 140 and 180hz.
That's normal for any small room, and the reason you still have those issues is because you don't have anywhere near enough treatment in there.

In any event, those are not your biggest problems. In reality, your issues start at around 82 Hz, where you have FOUR modes practically on top of each other:

82.0 hz 0,0,2 (Axial)
82.1 hz 1,0,1 (Tangential)
83.2 hz 1,1,0 (Tangential)
86.1 hz 0,2,0 (Axial)

Next, at 92 Hz you have another two right on top of each other:
92.6 hz 0,1,2 (Tangential)
92.7 hz 1,1,1 (Oblique)

So your problems at around 160 to 180 Hz are most likely the second harmonics of those modes. And your problem at 140 Hz is most likely the overlap of two other modes:
142.1 hz 1,0,3 (Tangential)
142.3 hz 2,0,0 (Axial)

As you can see, several of those do not even involve your ceiling (vertical axis), so just putting treatment on the ceiling is not going to solve your issues.

Quote:
I fired these frequency with a tone generator and I am now pretty sure that it comes from ceiling reflections
I doubt that this is a problem with reflections! Much more likely to be a modal problem, especially since the frequencies you mention do seem to coincide with predicted modes for that room.

Quote:
It also makes sense since my room is is 2.40 meters high/ (this matches with my mid range problem).
Only for 1 of your 7 modes! (Your ceiling is involved in other modes, yes, but only one mode is directly related to the ceiling height).

Quote:
Building absorbent rockwool pannels at about 60 cm from the ceiling (1/4 a wavelength of 140hz)
:shock: That would bring the headroom down to less than 170cm! I hope you are not very tall... In any event, that probably would not be very effective.

Quote:
Building diffusers that work precisely at this frequency => PVC tube cut in half along the length. As large as possible so that it treats again the 1/4 wave lenght.
Most types of diffuser are no use in small rooms, due to the specularity of the reflections and lobing. Diffusers only really make sense in large rooms, where there really is a reverberant field. In a small room, there is no reverberant field for diffusers to act on. The problems in small rooms are modal, nor reverberant.

Quote:
Since my room is a "little too dead"
Cover some of your absorption with thick plastic, to keep the high frequencies in the room while still treating the lows.

Quote:
Or maybe I could wrap up the rockwool with a plastic bag to prevent absorption from the high frequencies?
Yes.

Quote:
But I really feel like the diffusers that I put are doing a really good job at breaking the standing waves.
Sorry, but they aren't: They are nowhere near big enough to be having any real effect on room modes. They might be breaking up some of the reflections, so that's probably what you are hearing, but they certainly are not doing much to the modes, especially not the axial ones.

Quote:
They are supposed to diffuse pretty low as they are 2D and 30 cm deep.
Your problem wavelengths are in the range 2m to 4m... Much, much larger than those diffusers.

Quote:
I really need to kill this 140 / 160 hz standing wave from the ceiling. what can I do?
Nothing! :) It is impossible to "kill" a standing wave. Like Brian already said, the modes in any room are ALWAYS there. You cannot destroy them. They are a simple mathematical function of the room shape and dimensions. You cannot get rid of modes. all you can do is to damp them somewhat, by absorbing problematic frequencies.

I also agree with Brian about the cloud: Build a large frame, with a hard top on it (thick wood), angle it down at least 12° (probably more: you need to ray trace to find out for sure), and fill it with at least 4 inches (10cm) or mineral wool or fiberglass.

But you also need to fix your symmetry problem (this is a BIG issue!), as well as put in more, larger, deeper bass trapping in the corners (Superchunk).

Also, please post the graphs from the analysis you did, and explain how it was done: (what mic and speakers you used, where you positioned the mic, etc.)


- Stuart -

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PostPosted: Thu Dec 08, 2011 5:05 am 
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Thank you so much Stuart for the taking the time to respond to this.
And what a response!!!! :-)

Yes I am not DEAD center. but wait! I can explain!

I am not that "far off" center. Indeed, I am only 20 cm to the left of the symmetry axis. Maybe the angled picture suggests otherwise. I did this because as you said, due to the square shape of this room... being on the symmetry axis is a modal humpf humpf festival. I calculated and verified by ear that being 20 cm slightly to the left prevents me from being in the nod or a null of the first 3 or 4 axis mode.
I don t know about the tangential modes... :oops:

I will certainly follow up on your suggestions and replace these way too thin absorbers at the front. I bought them from a reputable company whose name will be withheld...

Now I know how to build proper thick absorbers.

However, can you please elaborate on the 12 degrees angle and the reflective board at the back of my cloud? why 12? why a reflective board behind? Why behind? to break the mode maybe?

Also: what works best in order to preserve the high frequencies? plastic bags? kraft paper? perforated plywood?
Should I wrap up all my absorbers then? Or the highs need to be treated at the first reflections points? the cloud too?

Thanks a bunch!


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PostPosted: Thu Dec 08, 2011 5:16 pm 
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Quote:
I am not that "far off" center. Indeed, I am only 20 cm to the left of the symmetry axis. Maybe the angled picture suggests otherwise. I did this because as you said, due to the square shape of this room... being on the symmetry axis is a modal humpf humpf festival. I calculated and verified by ear that being 20 cm slightly to the left prevents me from being in the nod or a null of the first 3 or 4 axis mode.
You are using the wrong solution for the wrong problem, and making things worse, not better! In other words, instead of treating correctly to damp the modes, you destroyed the symmetry of the room to stay away from some of the modes, but in fact by doing that you are placing your head in a different set of modes that are much harder to calculate, since you are off center. But by being off center you have totally destroyed any possibility of having a properly balanced tight sound stage. If your room is not symmetrical (the front half of it, at least). then you CANNOT get an accurate stereo image with identical equalization on both sides. Sorry, but you just cant. It is not possible, no matter how good your ears are, because it isn't an "ear" problem but rather a position problem. If your room is not symmetrical then early reflections take different path lengths to get to your ears: you have different sets of comb filtering and other phasing artifacts on each side, which absolutely kills all possibility of getting a precise, well-centered stereo image. I doubt that you can even get a clear, tight phantom center with your current setup.

Then to make matters even worse, you have those QRD diffusers at your first reflection points! That messes up the whole issue even more.

Yes, I know that you think you made things better by going off center and using those QRDs, but in reality you just made things worse. If you want an accurate, precise, clean, tight stereo image and clear, broad soundstage, then you have to get your room symmetrical again, and replace those QRDs with thick absorption.

All that you accomplish by going off center is to replace one set of modal issues with another. There are modes everywhere in the room, not just on the center lines! No matter where you go in the room, you will always be in the middle of one set of modes or another. It is impossible to get away from all of the modes of a room. That would be like trying to not get wet in the shower by moving over 2 cm: all that you accomplish is that you get wet with a different part of the spray.

So once again, you first need to fix your symmetry issue. Symmetry is critical. Once you have your mix position correctly set up, on the center line at 38% of room depth and with the speakers angled such that the acoustic axes intersect about 20 cm behind your head at an angle of 60° then you can take a new set of readings with REW or Fuzz Measure, and find out what modes are REALLY giving you problems.

Quote:
However, can you please elaborate on the 12 degrees angle and the reflective board at the back of my cloud?
It's just a large frame, perhaps 1.5 m x 3 m, or some such, with a thick piece of MDF or OSB on the top (15mm at least) and 10cm of rockwool or fiberglass below, hung from the ceiling on chains or wire. Hang it roughly over the area between your speakers and the desk, at an angle of at least 12°, with the part over the speakers being lower and the part over your head being higher. The hard back helps to break up vertical modal issues by creating an extra hard, reflective surface that redirects and retunes the modes to different angles and different frequencies. You need at least 12° in order to ensure that you are re-directing the waves sufficiently, and this also helps to break up flutter echo. You can also put some rockwool on top of the hard back, for additional effect in the gap between the ceiling and the top of the cloud. This type of cloud does several things at once: it treats your first reflections, it move the modes around, it kills flutter echo, it absorbs mids and highs, and it even diffuses to a certain extent.

Quote:
why 12?
Minimum 12°. You can go more, if you want, especially if you are trying to create an RFZ room (not your situation). There's no theory that I'm aware of on why it needs to be at least 12°, but John (and others) have determined that this is the minimum effective angle, from years of experience in building studios. It is an empirical figure, not one determined through theory.

Quote:
why a reflective board behind? Why behind? to break the mode maybe?
It doesn't really "break" the mode: it just changes it. Since it changes the path length, it moves the mode to a different frequency. And since it is angled, in reality there will be several new modes at several frequencies, mostly tangential and oblique (which have less energy in them anyway). It also makes it much harder to predict the modal response, since angles mean that the normal simple equations for rectangular rooms no longer apply, and now you need FEM/FEA to predict. But predicting is not necessary; Its much better to just measure reality!

Anyway, please post photos of the back part of your room, and also post the graphs from your initial analyses.

Quote:
Also: what works best in order to preserve the high frequencies? plastic bags? kraft paper? perforated plywood?
Depends on the response you are getting right now. If you post the curves, then we can suggest what would be best. Probably just get some plastic sheeting from your hardware store.

Quote:
Should I wrap up all my absorbers then? Or the highs need to be treated at the first reflections points? the cloud too?
No, just the bass traps and maybe some of the rear wall. Do not wrap the first reflection points or the cloud, as they need to treat all frequencies, especially the mids and highs.


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PostPosted: Thu Dec 08, 2011 8:17 pm 
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I must admit upon seeing your photos I was quite taken aback seeing all those diffusors.
From an acoustical standpoint (if you could see an acoustic hologram of your room) it must be utter chaos.
I'm not surprised when you might think it sounds good, but in fact it just sounds different than a normal room, and because the brain can't make head or tail of what it's hearing you figure the diffusors are working. Well. of course they are, but not in a constructive way, and this is the problem with diffusors in a small environment. I'm not totally opposed to using diffusors in a small room not at first reflection points of the listening position however, and provided their dimensions are congruent with the rooms dimensions - small space-arrays, even polys on the rear wall if the room length is sufficient.

Regarding the cloud being discussed, one often speaks of this modifying or altering the local modes. The truth is however, that it in fact only attenuates the existing modes and in turn sets up additional modes - the original modes are still there but hopefully (depending on the density of the material) subdued to the point they are no longer a problem. This same effect is true of room in room - the modes of the original outer shell are still there, but can be taken as secondary or even ignored due to their reduced effect. Normally, we can say the modes are altered or modified, because this is less confusing, and in terms of effect may be considered true, but I only mention this because one may wonder when testing why the original modes might still show some telltale signs of their presence in the results. This is especially in regard to a cloud as it's "modifying" effect will be dependent upon the materials used, and it's size while it's influence will only be local.

Regarding the absorption, If you like the look of your existing panels, you can keep them as facing for new thicker panels. The corner absorbers might be allright as they are flanked by additional panels. Provided you beef the up existing wall panels you can probably leave them as is - testing will show.

Plastic sheeting as Stuart says would probably be sufficient. Paper, carton, foil, masonite, or thin ply are all candidates but should only be decided upon after testing. Plastic foil will have the minimum impact dependent on it's thickness. The thinner the foil, the higher the frequencies that will be reflected. Perforated ply gets into another area of treatment, so I'd avoid that unless you do some preliminary studying.
The cloud, although being a first reflection point, you might also want to sheath in very thin plastic, not for acoustic reasons this time but rather against mineral or glass dust falling down over time onto your workspace assuming typical wools will be used in the cloud.

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PostPosted: Fri Dec 09, 2011 12:58 am 
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Quote:
From an acoustical standpoint (if you could see an acoustic hologram of your room) it must be utter chaos.
Absolutely agree! What the man said... :)

I also just noticed that the QRD on the left wall is oriented vertically, while the one on the right is oriented horizontally. That too must be damaging the stereo imaging.

Quote:
The cloud, although being a first reflection point, you might also want to sheath in very thin plastic, not for acoustic reasons this time but rather against mineral or glass dust falling down over time...
Good point! I had neglected to mention that important detail.

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