Asymmetrical Room Treatment

Chrisar - I feel your pain. ;)

I would tend to agree with your thinking; as the door way behind your left speaker is for all intents and purposes a very effective and broadband absorber then so too must the wall behind the right speaker to match. How far out from the front wall is your speakers? Using a resistive-type trap with OC fiberglass means you will need to make it very thick and should have a decent air space (e.g. 6") so as to allow absorption down to 100Hz (or below?).

I think that even with a very thick broadband absorber behind your right speaker you will never get it to match the left as glancing reflections off the absorbers won't get absorbed so you will / should always hear a tad more indirect reflections coming from the right side which may pull the image rightwards a tad. But I suspect you already know this . . .

Can you add an exterior door behind the left speaker?

The reflections on the back wall can be just as damaging so have you treated the back wall yet? Back wall treatment may pay more sonic dividends as you usually sit closer to the back wall than the front wall. Try thick bass traps in the floor-to-wall corners and diffusion (2Dimensional like a Skyline Diffuser due to the close sitting proximity) in the centre of your back wall.

good luck! We'll keep our fingers, toes, and eyes crossed for 'ya. ;-)

Hi Kevinzoe, thanks for the response! I'll try and address your points to provide a bit more info:

"How far out from the front wall is your speakers?"

The speakers are about 45" out from the front wall measured from the woofer (or 30" from the back of the speaker). I do have some leeway here and could move them out a bit more- the main reason I have them where they are is to allow enough distance to my seating position, as per Thiel recommendations to allow the drivers to integrate.
As a side note, my ears in the seating position are about 5' out from the back wall, which I have found seems to avoid a heavy mid-bass peak which occurs when I sit closer to the rear wall. This position also gives me just about 8' from the midpoint between the speakers.

"Using a resistive-type trap with OC fiberglass means you will need to make it very thick and should have a decent air space (e.g. 6")"

The traps I just made are 4" thick 2'x4' OC-703. One of them that is across the diagonal corner has about 9" airspace from the corner, and the other that is sitting right next to it on the front wall has about 7" airspace at the bottom, but it is learning against the wall (not mounted- sitting on the floor), so the airspace decreases across the 4' height of the trap. This is definitely a place I could improve on I think, and just double-up each of these traps to make their thickness' 8". Only issue would be the traps would be getting pretty close to the back of the speaker, though this may not be a problem.

"Can you add an exterior door behind the left speaker?"

This is something I had considered (in theory at least!). I can't add a door (one of the many downside of renting...can't add doors or drill holes in walls to mount my traps...not to mention one of the walls is cinderblock!), but I did think about having some sort of removable partition that I could just place when listening, and set aside otherwise. I would assume it would need to cover most of the opening, from floor to ceiling? And also be fairly thick? Perhaps I could find some sort of big wooden board and then mount a bass trap onto that, then put that in place behind the left speaker during listening?

"The reflections on the back wall can be just as damaging so have you treated the back wall yet? "

Unfortunately my back wall has a large window centered in the middle of it, so essentially the whole thing is glass aside from near the top/bottom and left/right sides. I did recently put up some drapes to help with high and mid-frequency reflections, but I am not sure if it would be feasible to treat any additional way. I did consider also having some sort of removable partition with treatment on it that I could place over the window in the area behind my head, but I am not sure how this would work... I do like your idea for diffusion back there though- I've heard this can make a big difference in a small/medium room, I've just never tried one in here.

So I am thinking now that the best option may be a combination of more broadband absorbers behind the right speaker, and maybe some sort of partition to block the opening behind the left speaker. One of the pro's of being single and living alone: aesthetics are not a huge concern, especially if I keep things removable when I have guests over :). Thanks again!

The drapes will only dampen the very high frequencies at best. (20khz+)
Get some absorbers behind you and infront of the window.
I would also pull the bass trap out of the corner thats behind you.

You may not realize it because you don't have your first reflections dampened in the room. But trapping the rear corners will kill the soundstage of a room.
I would take the rear bass trap and put it in the same corner behind the right speaker with the other bass trap and that will kill any bass node that is behind your speakers.

The open wall to the left is allowing the bass to be dampened by escaping the room.

This will help in the bass department, but the imaging and soundstage will only be good with treatments in at least the first reflection points through out the room.
It's only then that you will be able to realize the potentual of your soundstage and imaging and ballance out the system.

My best freind has the same problem as you.
He's treated the room on the sides and behind his seating position and the soundstage is very ballanced.

Hope this helps.
John

Hi John. Thanks for the feedback! I actually do have hi/mid absorbers (foam, not 703) at my first reflection points on the side walls. I will move out the bass trap I have in the rear-right corner and place it with the others in the front behind the right speaker. This will mean I will have my 3 bass traps all in the front right, and the corners in the back will be untreated.

I have 2 additional foam absorbers along with the ones at the side-wall first reflection points, which I currently have against the front wall in the center and near the opening. Perhaps it would be good to use these against the back wall in front of the window? Or better yet, I guess I should probably just get another broadband absorber if not a diffusor, as suggested by Kevin, for back there. Good to know about drapes- I had assumed they would be effective a little further down, but I suppose it makes sense that they would only tame the very high frequencies...

Chisar, while the foam based devices/absorbers are not as "good", assuming they have a shaped face, you may find the following somewhat helpful for the wall behind you.

Glue a strip of very thin plywood (even just a veneer board) to the top of the foam's back. Get a suction cup clamp with hook (ie. Lowes, HD, Michaels) and stick this to the top of the window behind you. Since the foam is so light, even with the strip of wood, you can hang it directly on the window. Easy to remove when guests come over. I do this as my side window is a 1st reflection point and it is much more productive than just drapes (but not as good as a full fledged absorber).

While there is limited absorbption in doing so, assuming again that it has pyramids or something on its face, it will also act as a diffusor.

Another thing to consider for the open doorway is to use a tension based curtain/shower rod and install drapes in the doorway. With the right material and lots of folds in it even in the closed position, this will provide some support to what you are seeking to balance the two sides.

I recognize that there are better solutions, but based on limitations of renting, higher costs, etc. . . mine are only intended to offer easily changeable, relatively cheap and not to require drilling - as opposed to being the perfect solution that allows us to ignore these restrictions.

Hi Chris.
Putting some 4 inch absorbers in fromt of the window will definatly be a good choise.

They will act as bass traps and controle the mid and high reflections.

I found when you treat the rear corners of a room (behind the listening chair) it seems to kill the sound stage.
You must keep the rear corners lively and work on the first reflection points.

The foam you are useing is not doing much to absorb lower highs and lower.

The compressed fiberglass in the two inch size does a much better job.

Go to my system and read how to get the room treatment really cheap.

It's a do DIY set and you can do a room very well with minimum yen.

Hope this helps.
John

Hi Chris,

Thanks for your additional info. I have a different perspective and/or interpretation than one of the other replies so let me explain.

Back Wall Treatments: (1) A window on your back wall will act as the perfect bass trap – the long frequencies won’t even ‘see’ the window and will escape the room easily. So, I would redeploy your back wall bass traps elsewhere, not because it will damage the soundstage because that is more of a side wall reflections phenonmenum. (2) Drape absorbency is dependent upon the type and weight of material, degree of fold, and distance out from the wall. The heavier the fabric is the greater the absorption. From F. Alton Everest’s book, the absorption effect for velour drapes is concentrated in the 500-1KHz region using 18oz/yd2 material, but for 10-14oz/yd2 fabric is more within the 2kHz-4kHz range where the absorption coefficient tops out at 0.4 (40%). The deeper the folds within the drapes the greater the surface area for absorption – taking a drape pulled straight and then introducing folds deep enough that the drape is now only 50% as wide as it was when pulled straight increased the absorption coefficient from about 0.2 to 0.8 for 1kHz. A 1kHz wavelength is 13.6” long so placing the drapes at the 25% point of 13.6” means placing them out from the wall 3.4” to maximize the absorption effectiveness. I would suggest keeping your drapes or upgrading their density/thickness and creating deeper ‘folds’ in the material as you likely can’t move them farther away from the window/wall. (3) Making a diffuser for the back wall should be easy enough, but making it easily movable won’t be, so I’d stick with the thicker drapes for now. You can still place bass traps in the back wall corners from ceiling to floor should you decide to make or buy more.

Side Wall Treatments: (1) Your open-cell foam WILL absorb mid to higher frequencies depending on its thickness. A 2” Sonex foam with the anechoic-type wedges has an absorption coefficient of about 0.6 at 500Hz and the same coefficient at 250Hz when the thickness is doubled to 4”. The wedges look cool but actually reduce the absorption abilities due to the missing material in creating the wedges, and the surface isn’t hard enough to effectively diffuse frequencies. Your better off using OC701 or 703 material for increased absorption capabilities. (2) Soundstage width is a more a function of side wall reflections than back wall corner reflections. Back wall reflections create “listener envelopment” as per Toole in helping the trick the mind into thinking the room is larger than it is. Absorbing or diffusing/reflecting the side wall reflections, especially at the 1st reflection points, is a matter of taste, and the former will make the soundstage narrower with instruments as pinpoints in space whereas the latter will broaden the soundstage/apparent source width and size of instruments to being more life-like. In my latest incarnation of room treatments I’ve used planks of ¾” thick oak angled upwards to preserve the mid/high frequency energy so as to prevent over dampening the room and redirecting the reflections upwards to the RPG Skylines on my ceiling where they are diffused. It is also very effective at eliminating slap echoes between the two side walls.

Front Wall Treatment: If you were to put up a temporary hard surface in the doorway behind your left speaker, then that should keep some mid/high frequency energy in the room – the low frequencies will escape as it’s neither air tight nor thick enough. I think I’d be inclined to keep the opening as is and add broadband absorption to the middle and right side of the front wall to emulate the open doorway situation by the left speaker.

h
Having said all this, have you a SPL meter and test tones where you can get some rudimentary Freq VS SPL measurements to see how flat the frequency response is?

Good luck

Kevin.
The bass wave doesn't go right through the glass because it's see through.
It reflects back into the room because of solidity of the window.
Now, if the window was open then it would escape into the outdoors.
It's obviouse you have spoken above my comprehention, but I know for a fact, not just assuming, that if you dampen the rear corners it will kill the soundstage if the first reflections are treated.

Been there done that.

I guess if the glass window was several inches thick then maybe the bass waves will reflect off it . . . The size of a low freq wave is huge in comparison to a thin pane of glass so as I said before it will easily pass right through it. You need only to listen to the boom-boom of car subwoofers a block away despite their windows being rolled up. Smaller freq wavelenghts will "see" the glass as a hard reflective surface and but bass won't - it's physics.

You have a nice small listening room so I might imagine that with your seat so close to the back wall and hence close to your back wall corners that absorption in the back corners - on top of all the other absorption you have in place - will reduce the reflections further and prevent any kind of envelopment which is not soundstage. Soundstage width is side wall dependant, depth is front wall dependant and height is well ceiling dependnat. Make sense? Maybe we're defining 'soundstage' differently . . .

Kevin.

I've often wondered about soundstage depth and width.
Since it all happens at the ear, how does the width of a room and depth of a room affect what happens at your ear ?

I can hear or perhaps preceive that a kettle drum is thirty feet behind my speakers when in fact, the wall is only three feet behind my speakers.

Is it the illusion of depth from reverberation in the mixing of the music ?

I have some Mapleshade CD's that doesn't go through the mixer, but goes from the mike strait to two track and the depth goes well beyond my wall.

If you can explain this to me it would help me understand the three dim. soundstage much better.

John

John,
Great question you ask. What’s happening at the ear is the sum of direct and indirect reflections which are attenuated and delayed versions of the original direct reflections – in other words reflections of reflections. The reason sound reproduction sounds better in a room than outside is due to the reflections from the room’s surfaces. The surfaces of your room are in their own right sound sources, not of direct sound but of indirect reflected sound. The variables that come into play include reflection strength, direction, and time delays from the direct sound source.
As your mind assimilates direct and reflected sounds, the direction, intensity and time lag from which the reflection is coming helps tell the brain how wide, high, or deep a room is. Think about how music sounds in a large cathedral verses a small residential room – with your eyes closed you can tell if you’re listening in a large or small room. It’s the indirect reflections that provide the mind with the answer.
Reflections from the first reflection points between the speakers and your listening position (side walls, ceiling, floor) tend to pull the perceived sound towards the adjacent surface as your ear hears both the direct sound and reflected but delayed and attenuated versions of itself. Apparent source width, image broadening, impressions of height and depth are the result of reflections occurring at delays of less than 80ms and include the reflections that are included within the recordings.
The sound reflections from the recorded venue which makes up its natural reverb, or electronic reverb introduced into the mix, is what you’re hearing when your kettle drums appear far away.
Hope this helps . . .

Hey everyone, thanks for the responses; they've been very helpful! At the moment I have treated the front wall behind the right speaker pretty heavily, and just with this I can tell that there is a pretty large improvement in imaging detail; instruments seem more defined into specific spatial locations. I will experiment with treating or not treating the rear corners and see what results I get.

On a related note- I currently have two, 4' tall (4" thick) bass traps in the front, right corner (one on top of the other). My ceiling is only about 7'8" tall, so the bottom trap is at a pretty tilted angle, which at the bottom floor/wall/wall corner leaves a substantial airspace, probably about 2' at the furthest point. While I know that having a decent amount of airspace between a trap and wall is beneficial, is there an upper limit on this? My guess would be that about 1 or 2 times the thickness of the trap would be ideal, but I am just guessing here.

Chrisar wrote:
"While I know that having a decent amount of airspace between a trap and wall is beneficial, is there an upper limit on this? My guess would be that about 1 or 2 times the thickness of the trap would be ideal, but I am just guessing here"

If you're using fiberglass filled absorbers (which you are) then you simple change the frequency at which the absorber maximizes its effectiveness at by changing the air space distance. I don't believe there is anything intrincically good or bad about how far you pull the trap out, it's a matter of using that flexibility to your advance to attenuate a problem frequency. Remember that the absorber works best when the particle air speed is fastest which is at the 25% mark of a frequency's wavelength. It's easy enough to calculate what that is as follows using a 440Hz frequency for example:

Speed of sound 1130 / 440Hz *12inches = 30.8" then multiply that by 25% and you get 7.7" which is the airspace needed to max the absorber placement.

I would think other practical issues like asthetics or not having the absorber blocking a walking path through the room etc might factor into it too.

I think Kevin has some very good answers, though I am not sure if the OP can take advantage of all of them due to his restrictions in renting.

My question is regarding a 4" diameter (I am assuming it round) or is it a panel type? If its round, doesn't the 4" diameter preclude it from acting as a bass trap? I was under the impression that for the round tubes, one need really about 16" to get into the bass notes?

FYI - I spent my evening installing some new Real Traps - Corners panels (mondos), a couple of PFZ (?) on the ceiling (suspctd about 4"). On one wall, left, I have four 2 X 2 X 4" thick Real Trap Panel sound panels (tilted, out 6" at the top and flush at the bottom) in a 2 X 2 pattern with 2" between each panel. On the opposite side where my window is, I glued laminate thickness playwood onto the auralux backing. They are 4 feet tall, 2 feet wide and 2" thick. The window is about 3 feet tall by about 5 feet wide. I basically bent the laminate so that panels are wedged between the top and bottom window framing - bowing outwards.

My back wall (behind me) is about 15 feet behind where I sit and am justing letting the physical aspects of what behind me defuse the sound (I have nothing specifically addressing the soundwaves behind me due to the distances involved and it doesn't sound like I need anything).

Finally, I have two of the auralex panels between the speakers (in front of the fireplace opening, positioned at about a 15 degree angle - with a point in the center).

I need to put some decent heavier drapes up over the window and auralex bowed panels, but already the room is sounding much better.

Bass is much tighter and I can play a much high SPLs without the sound breaking up, conjecting and just becoming over energized.

Anybody want to make any additional comments it would be fine. I think I need still some corner (wall/wall/ceiling triangles and possible raise higher my front wall corner traps which are pretty wide frequency ranging).

Ckoffend.

I'm sure he's talking about a 4" panel.

Also, it sounds like you're putting together a really nice room.
I would stress that the 15 ft. space behind you should also be treated in some mannor.
Stand at your listening position and clap your hands, do you here any echoe ? If not you are good to go.
If so, you need to dampen the area behind you.

15 ft. is a long distance and the reflections at 15 ft really turnes into 30 ft. by the time it arrives at your chair. That's a huge timing gap.

Anouther point I'd like to make is that if you treat a room one wall at a time and you sit to listen to the results, if you make a mistake and over dampen the room, you will immediatly know that you went to far. Your sound stage will collaps.

It sounds like the combo of absorbtion and deflection, your going to have a great sounding room.

There's nothing in the electronics world that can give the upgrade that a well treated room can.
If someone is reading this and never heard a treated room, then you need to treat your room.
It is the most bang for your buck in audio.
Have fun.
John

Ckoffend - Are you asking if the original poster (Chrisar) is using 4" thick absorbing panels? I think he said he is. Whether it's a 4" thick panel or 4" diameter circular trap or a 4" radius semi-circular trap, 4" is still only 4" and is the bare minimum considered for a broadband absorber. The thicker the better because you don't want the trap to act like a low-pass filter which would only absorb mid/high frequencies as it'd skew the tonal balance of the reflections and the reflections of those reflections from the speakers you paid dearly for. So, go thick or go home . . . so to speak. My front wall corner hemi-cylindrical diffusers/bass traps have a total thickness of 37" which includes an 8" air space, as my 21' * 14' room can acommodate it.

Are you using the room for 2channel music or HT as the back wall treatment varies? Check out Dr Floyd Toole's book - it'll teach you many useful and pragmatic things to help you decide what your room needs and what kinds of treatments go where.