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 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

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 . . .

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 . . .

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.

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.