Diffusion vs. Sound Absorption views


I have Spatial Audio M4 Triode Master open baffle speaker in my listening room.  I have sound treatment behind the top of the speakers with sound absorption panels hung on the wall and an Aurelex foam panel down to the floor. 

It was recently brought to my attention that sound diffusion panels behind the speakers might yield an improvement in SQ over the sound absorption panels.  The advice I got is that OB speakers benefit more from rear wall sound treatment with diffusion rather than absorption.  I put the issue to you learned folks for advice.  Thanks. 
whitestix
Lately I've been incrementally adding more acoustic treatments to my room and have been discussing my objectives with GIK.  I'd suggest giving them a call (or probably other similar outfits) who really are the professionals and are very helpful in real time.  For me, some of their most effective single products treat through both absorption as well as diffusion.
Agreed, in my experience both absorption AND diffusion was needed.  However, I would look at people who have specific experience with open baffle speakers.  You can try use diffussion panels behind the speaker, but make sure they are generic diffusion, such as an array of pyramids or a convex diffuser such as the GIK Evolution PolyFusor.  I would highly recommend NOT using a quadratic/QRD diffusor right behind the speaker.  The QRD diffusors can do weird things to the phasing of sound waves if they are so close to the speaker or the listener.  A QRD diffusor could work well on the front wall in between the speakers (as long as there is good distance away from the speakers) or on the back wall.  However, I have found that each room is different and I had to experiment a LOT with placing QRD diffusors.  Many locations of the QRD diffusor made the sound worse. 

Diffusors behind dipole/planar loudspeakers are preferred to absorbers by many, though not all, owners of such speakers, for a couple of reasons. Too much absorption can result in a room overly "dead", but that’s true no matter what the speaker design. But since dipoles put out as much sound from the rear as from the front, diffusors behind them creates more depth (detractors claim it is false depth added to recordings containing no such depth), and a "bigger" sound, which is one thing planar lovers like about them.

@auxinput makes a good point about the distance required for diffusors. Not only between the absorber and the speaker, but between it and the listener. A couple feet to the speaker and 6’ to the listener, minimum, is a good rule-of-thumb. Do call GIK, they will provide you with excellent advice. I’ll soon be installing a set of their Gridfusers on the wall behind my planars.

I have found GIK as recommended above, to be a good reasonably priced source for acoustic panels.  I installed them in our previous house and plan on using them in our new one.

If the application allows, I prefer the aspect of free-standing/movable panels not only for initial trial placement purposes, but also for present and future flexibility.  In fact, GIK just sent a flyer out last week mentioning their new "Panel Foot Kit" that will turn almost any of their products into floor standing/movable ones: http://https//www.gikacoustics.com/product/panel-foot-kit/
On a basic level - high frequencies add sparkle and life to music even to those listeners with diminished high frequency hearing. With that in mind I suggest diffusion should be your first priority and when you feel you’ve done a decent job of it then limited absorption behind the speakers as you find provide enhanced detail and depth to your sound. What types of music do you listen to most?
I am keen to check out this diffusion aspect of sound treatments, as I have a remarkably dampened room (two layers of 5/8" sheetrock with Green Glue in between and several sound panels behind the speakers, and thick carpet in the room.)   I have a long narrow room (13x26x9) with the speakers on the long wall, about 3' from the wall, and I sit about 10' away on a sofa against the rear wall.  Not an ideal setup for sure, but I am working around the edges with what I have.  
If diffusion enhances the spatiality of the sound, then I am interested to explore this aspect of my system.

I listen to about 98% jazz, of the straight-ahead variety.  Miles, Coltrane, MJQ, Brubeck, Desmond and most significantly Stan Getz. 
Since you have Spatials, why not contact Clayton Shaw, owner,
and ask the source. He is a person very willing to share. Overall
I understand absorption be best used to stop early reflections,
ie in front of speakers on side walls and ceiling. Diffusion best on front and back walls.
Essentially 
Diffusion everywhere, particularly front and back walls and ceiling in your case. 
Absorption only where benefits are obvious through experimentation 
How do you know when panels are needed? I'm interested in the topic, but don't notice a problem with Golden Ear Triton 3+ about a foot from front wall and 6" from side walls. If it already sounds good don't mess with it? Tinker for better sound? Or is there some measuring equipment for this? 
Conventionally, diffusion improves perceived ambiance while absorption improves imaging.


I suspect that the answer is somewhat dependent on the distance from the rear of the OB loudspeaker to the rear wall.  Why, if the speaker is too close, then there is much more of a chance of the out of phase wave interfering with the sound coming off the front...so absorption should help.Here is a brief but interesting article:

http://www.gikacoustics.com/speaker-boundary-interference-response-sbir/


I think that you can "simulate" the effect of diffusion by moving the panels out from the wall another 2-3' (not that you are going to leave them that way) which will then give you a different combination of direct sound, reflected sound and absorbed sound.
What’s really needed is a test tone such as found on test CDs, say 315 Hz, which works fine, and an SPL meter. Then, determine locations in the room where sound pressure peaks 6 dB or higher than the average sound pressure in the room at moderately high loudness. Those locations are generally where acoustic devices - panels, resonators, etc. - should be placed. Without a method for placing acoustic treatments it’s like shooting blanks in the dark. As the system evolves you should find that the locations of acoustic devices can change along with the system changes. Ditto speaker locations. It’s a fluid situation.
I will repeat my warning and caution about using QRD quadratic diffusors.  snapsc had a point about diffusors " out of phase wave interfering with the sound".  The QRD diffusors can definitely cause problems here if they are placed in certain spots.  I have had situations where some of the midrange frequencies were boosted and others were cut, making the sound completely terrible.  The QRD diffusors can also boost high frequencies to the point where it's too bright/harsh sounding.  These side affects are entirely dependant on where you put the QRD diffusor and the specific room you're operating in, so it becomes much of a test and listen exercise.
@whitestix. 
The drywall you used does not “damp” the room.   In fact by reducing spurious noise from thinner, flexing drywall it provides much enhanced clarity. IMHO virtually every listening room would benefit significantly from your approach. It also significantly enhances accurate bass response :)
Thanks for the comments and suggestions, a particular solid from from GK.  
whitestix, what you have are dipoles. Dipoles radiate in a figure 8 pattern. Very little energy goes to the sides. This actually makes them easier to deal with acoustically because there is only one primary reflection and that is off the front wall. For every foot your speakers are away from the front wall you can add 0.2 millisec to the delay. If your speakers are 3 feet from the wall that would be a delay of 0.6 millisec. This is to fast to be perceived as an echo. All frequencies are not reflected equally. This and the slight delay ruin your image. Diffusing the sound just makes things worse creating more reflections. You want to diminish the volume of the first reflection by absorbing as much as you can in the mid range and treble. Using double sided carpet tape attach two rows of these tiles from floor to ceiling directly behind the speaker. You can alternate the tiles making a nice pattern.
https://www.parts-express.com/sonic-barrier-fwp122-studio-acoustic-foam-wedge-panel-12-x-12-x-2-black-12-pack--260-547
It is an inexpensive thing to do and you will notice that your image sharpens right up. In most cases that is all you need to do. You might notice a slight change in frequency respond with less treble. In most cases this sounds more natural but the big improvement will be in the imaging.
Sir,
Great advice, which I think is pretty much where I am now -- all absorption.  Maybe I could send you a PM and somehow get you a photo of my living room for your opinion.  Thanks, Mark