Front Wall Diffusion vs. Absorption - A Case Study

Let me preface this thread by acknowledging that everyone's experience will be different based on their own individual room acoustics, system components, and personal tastes and biases. I have been experimenting with different acoustic treatment setups in my dedicated listening room and I wanted to share my experiences in the hopes that others can benefit from what I have learned.

Room and System Info: I have in place RealTraps Corner Mondo bass traps and Tri-Corner bass traps in the four wall-wall corners and eight wall-wall-ceiling corners in the room and RealTraps RFZ panels along the side wall first reflection points. My room is appx. 14' wide by 17' long, with a 2' deep alcove along the rear wall and 9' ceilings. I have Summits for my mains, a Stage center, and Clarities for the rear, fed by RAM-modified Oppo DV-970, Meridian G68 Pre-Pro, and Sunfire Cinema Grand Amp. Due to the room constraints, the Summits' Panels sit about 3' from the front wall, 2.5' from the side walls and are about 7.5' apart. My listening position is about 12' from the Summit panels and about 4' from the rear wall, which has two windows and is heavily draped.

In deciding how to treat the front wall to deal with the rear wave of the Summits, I did a lot of reading and research on the web. I found that many people prefer diffusion behind their Logans, and many people prefer absorption. Ethan Winer, of Real Traps, tends to prefer absorption in general over diffusion in a smaller room and particularly for the front and side walls. Others swear by diffusion. I decided to do some careful A/B listening tests with music I am very familiar with. I tested the front walls: (1) untreated; (2) treated with RealTraps MicroTraps; (3) treated with RealTraps High Frequency Mini Traps; and (4) treated with RPG Systems OmniDiffuser Panels. To make a long story short, the HF Mini Traps blew everything else away. Here is what I heard:

With the walls untreated, there was just too much reflection and the sound was not clear. The volume had to be turned down because of brightness and imaging and soundstaging were poor. Please note that in this room I do not have space to pull the Summits the recommended five feet or so away from the front wall, which may account for some of this. Neither Stereo imaging nor the Meridian Tri-Field processing sounded very good.

Putting the RPG diffusers in place resulted in very similar sound to the untreated wall. It was still bright, confused and had poor imaging and soundstaging. I couldn't really detect any positive contribution from the OmniDiffusers vs. the untreated walls, much to my chagrin. I really expected these to perform well behind the Summits.

Placing the microtraps behind the Summits improved things tremendously. The brightness disappeared and soundstaging and imaging improved. I thought I had found the result I was looking for. Then I put the HF Mini Traps back there . . . Oh My Gawd!

With the HF Mini Traps behind the Summits, the background was suddenly much blacker, instruments jumped out from their precise locations on the soundstage, imaging was spot on and the soundstage deep and wide. Details, transients, everything was much clearer and smoother. I could turn the volume up and the speakers just disappeared and there was a band playing for me right in my living room. This is how it is SUPPOSED to sound! It is just amazing how it all clicked into place with these particular treatments behind the Summits. Meridian tri-field also sounded much better with this setup, although I still preferred two-channel for the most realistic portrayal of the sound.

One of the reasons I believe the HF Mini Traps work so well is that they evenly absorb sound from about 400 Hz. on up. This is pretty much the exact frequency range of the Summit's panels, so these traps evenly absorb the entire rear wave of the Summit. This allows the ear to clearly hear the front wave with minimal reflected frequencies to muddy the sound. After a little more experimentation, the setup I am going to end up with is the HF Mini Traps behind the Summits, with a MicroTrap mounted horizontally behind the Stage, and an OmniDiffuser Panel mounted on the side wall directly beside each Summit. This setup appears to give the best overall sound with the widest soundstage and most precise imaging. I will also mount a couple of the OmniDiffuser panels behind the Clarities to enhance the surround soundfield.

For those of you with Martin Logans in smaller rooms where you can't get them too far away from the front wall, I highly recommend you try the HF Mini Traps from Real Traps behind your speakers. For a few hundred dollars you could have a sonic upgrade to your system easily equal to spending thousands on a new component.

For the record, I have no relationship with Ethan Winer or Real Traps other than being a satisfied customer.

Great write up and timely for me as after a great deal of procrastination, I have finally started the process of taming the acoustics of my 2 channel room. I just purchased a stand for my sub and some bass traps for GIK and was debating on the array of diffusion products but leaning toward the Real Traps as recommended by Jonfo. That said, at $1,000 vs. $400 I might have to retink that if the performance is the same or better. Hard to know what is going to be the best in any given room set up with varying tastes (as you noted) and although the pros are great I don't think there is a try and buy program anywhere so you have to take a leap of faith.

While I contemplate I would love to see some pictures of the set up if you have them. In any event, enjoyed the write up and happy listening.

Rich said:

... One of the reasons I believe the HF Mini Traps work so well is that they evenly absorb sound from about 400 Hz. on up. This is pretty much the exact frequency range of the Summit's panels, so these traps evenly absorb the entire rear wave of the Summit. This allows the ear to clearly hear the front wave with minimal reflected frequencies to muddy the sound. After a little more experimentation, the setup I am going to end up with is the HF Mini Traps behind the Summits, with a MicroTrap mounted horizontally behind the Stage, and an OmniDiffuser Panel mounted on the side wall directly beside each Summit. This setup appears to give the best overall sound with the widest soundstage and most precise imaging. I will also mount a couple of the OmniDiffuser panels behind the Clarities to enhance the surround soundfield.
...

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Rich, great write-up, thanks for sharing the details of how you arrived at this.

I'm not surprised the mini-traps were substantially better than the micro's, as I believe the main problem is not the >1Khz range, but the 200 - 1Khz range.
At upper-bass frequencies (100 - 400), dipole cancelation is a big deal, so absorbing the rear wave at those points really helps the balance of that range. Also, cutting the rear wall bounce of the midrange frequencies really cleans up our perception of instruments and vocals, many of whom have their fundamentals in that range.
Of course damping the highs is helpful to gain the more balanced soundstage you describe.

Since the mini-trap really helped so much, would you consider a Realtraps Diffusor in that location behind the summits?

I’m not saying it would be better a-priori, but some measurements I made with diffusion behind the center channel make me think that >800Hz diffusion + <800Hz absorption is ideal for a dipole speaker.
As the HF is not bounced back through the panel, but instead spread in the horizontal plane. This gives a sense of greater openness to the soundfield.

Although from you descriptions, it sounds like you are pretty close to ideal. Just wanted your (or anyone else’s) views on the combination of diffusion and absorption back there.

For rear channels, I’m quite certain this is best, but for front left/right, I’m still wondering. I'll be ording some RT diffusors in the next few weeks. So I'll be able to post some real measurements and report the experience.

Rich said:

Placing the microtraps behind the Summits improved things tremendously. The brightness disappeared and soundstaging and imaging improved. I thought I had found the result I was looking for. Then I put the HF Mini Traps back there . . . Oh My Gawd!

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Rich - While I do not use the HF Mini, I do use absorption on my front wall and have the same experience and results as you - that is, the right solution. I have tried both absorption and diffusion behind my ML's, and absorption proved the better solution for me and my room.

But like you said, each needs to try both and find what they like best and what works best with their room. And like I have stated before many time, people do not realize what benefits they can get from acoustic treatment of their room.

Good morning Rich, nice write -up ! I do enjoy reading everyones journeys down the "acoustical" highway, escpecially knowing that once done we have all realized very, very nice improvements to our overall systems.

While Dan was over to my place last weekend we discussed some rear wall / corner applications that I plan on doing with my room as well. With Fall upon us these are all good indoor projects !

BTW, I've been off the site for a few days, are we a proud Papa yet ?? !!!

Thanks for the replies, all.

Johnwa, I hope to get some pictures posted soon but I have a few details to finish first. Your dilemma regarding using the HF mini-trap or the RT diffuser is a tough one. There is a steep price difference between the two, and I think both are probably very effective at doing what we need them to do. More thoughts on that below.

Dan, well-said. People have no idea just how proper acoustic treatment can bring their system to that next level of performance. I think a lot of the difference between front wall absorption vs. diffusion performance may depend on the size of the room. I think a large space (20' by 25' or larger with high ceilings) probably benefits more from diffusion, where a smaller room benefits more from absorption.

JonFo, I agree with you that the lower mid-range / upper bass reflections, while not as noticeable to our ears as the HF bounce, have a huge negative impact on soundstage and imaging. They really muddy up the soundfield. I think that is why the MiniTrap bested the Microtrap and Omnidiffuser so soundly in my test.

I'm glad you brought up the RealTraps Diffuser. The RPG diffuser I tested is a different type of diffuser from the RT diffuser and I believe the results would have been very different with the RT Diffuser. I understand that it does a much better job of absorbing and diffusing those lower midrange / upper bass frequencies and this very well may be the key. Unfortunately, I determined that it was not practical to use the RT diffuser in this location due to space and mounting considerations so I didn't bother purchasing and testing it. Also, with the MiniTraps, I saved $600 over the RT Diffusers and got the desired result. So I am satisfied with that. But I do encourage anyone that can to try out both the MiniTraps and the Diffusers to see what works best in their system. I am interested to hear your impressions, because I think particularly in your larger room, the RT Diffuser will work great. I believe you hit the nail on the head: absorption below 800 Hz. and diffusion above is probably the ideal scenario for maintaining soundstage and imaging while not sacrificing liveliness and ambiance.

Dave, you will not regret the time and effort spent on some simple acoustic treatments. Your ears will thank you. Cindy and I still have a couple of weeks until the due date. I will certainly let you all know once she arrives.

Rich, NICE write up!!!

I to find the absorption the choice for me as I have 4 absorption panels on my front wall behind the Odyssey’s. I too experience a bit of glare and some smearing from the bare dry wall. I also have a very heavy curtain which I have between the Odyssey’s which helped in increasing the field of depth perception. I truly believe the room acoustics is half the battle in achieving the sound which we each desire. Again, thanks for the excellent write up.

Rich my friend,

VERY NICE case study.... I think you just saved me a lot of trouble here because here I was thinking that getting a diffusor was the best idea, when you just showed (or proved?) that it may not be the right answer for us Summit (stat) owners.

Thank you very much for the study, I seriously thought it was one of the better bits I've read on the forum for a while.

Joey :cheers:

Joey_V said:

VERY NICE case study.... I think you just saved me a lot of trouble here because here I was thinking that getting a diffusor was the best idea, when you just showed (or proved?) that it may not be the right answer for us Summit (stat) owners.

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Joey....Treatment is not specific to the speaker model, it is specific to the room, setup, and listener preferences. You will need to try out both and see which works best for you, your setup, and room. GIK and Real Traps do have return policies if I am not mistaken.

About the only thing in common with most acoustic treatments is corner treatment for bass trapping. Tri-Corners first then the entire corner. Treating all corners is the key to proper bass trapping, then ceiling/floor wall seams if needed. Floor to ceiling is ideal, but sometimes not practical or financially possible. But one does not have to do it all at once.

You can try out some simple absorption with heavy blankets and/or large pillows behind your speakers to see what it could sound like - never as good as the real thing but gives you an idea. If you know someone with some fake fiscus trees, borrow them and put them behind for some simple diffusion - again not as good as the real diffusion models.

I completely agree with Dan's comments above. Just because this was the right solution for my setup and room doesn't necessarily mean that will hold true for your room and setup. As I mentioned above, even a different style of diffuser may have given me much better results than the one I tested. The point I really want everyone to take away from this is that room treatments make a really big difference and you have to experiment with different combinations to find the one that works best in your room and system according to your preferences. If you put the money, time, and effort into it, you can make a huge difference in the sound of your system that will only magnify the positive effect of any future equipment upgrades.

Hi Rich,

Great write up on acoustics. Can you tell me what the tri corner traps did for you as I was thinking on getting some for the ceiling wall corners as I had heard it was important to treat this area as well.

Thanks, Greg

I am going to check with RT's to see if they have a return policy and if so will probably try the difusor and HT absorbtion products in my room to really make sure whats right for my particular room dynamics.

I placed so form mats that my wife had lying around to use uder our sleeping bags when we go camping just to see if that helped the front room bounce and I was pleasantly surprised by that. Of course I neglected to tell my wife and she had some friends over and was touring the house and lets just say my ad hoc engieering was not a big hit with her.

As everyone has already pointed out though, it is amazing what a few tweaks with treatments can do for the overall sound. That said, knowing the facts and even experiencing the dramitic effects personally, I am still finding it hard to part with money for this critcal (and in the grand scheme very cheap) part of the listening experience. I have no problem dropping thousands on amps etc but ask me to spend about less than $2K on this and I get could sweats.

That said, I am slowly but surely getting there and its these types of experiences and shared information that is pushing me in the right direction. Best money I have spent in a long time but hardest to part with for some reason for me.

On a related note (no pun intended) I went back to using my sub without a stand as it may have added a little clarity but took some of the punch I worked so hard to find in the first place. I guess I prefer a little more boom.

I agree what works with one may not work with another, but what I got from this case study of Rich's was that perhaps it is prudent to try an absorber rather than a diffusor considering the biggest confounding factor between his setup and mine is room size. Speakers are the same, ceiling height same... those are controlled.

I've always wondered about the two, but this is one point for the diffusor. I wil try both someday, but my eyes will be on the absorber for now. It may not work like it did for Rich, but that's ok... trial and error.

johnwa said:

...
I placed so form mats that my wife had lying around to use uder our sleeping bags when we go camping just to see if that helped the front room bounce and I was pleasantly surprised by that. Of course I neglected to tell my wife and she had some friends over and was touring the house and lets just say my ad hoc engieering was not a big hit with her...

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LOL :haha1:

Been there, done that too.

johnwa said:

...As everyone has already pointed out though, it is amazing what a few tweaks with treatments can do for the overall sound. That said, knowing the facts and even experiencing the dramatic effects personally, I am still finding it hard to part with money for this critical (and in the grand scheme, very cheap) part of the listening experience. I have no problem dropping thousands on amps etc but ask me to spend about less than $2K on this and I get cold sweats.

That said, I am slowly but surely getting there and its these types of experiences and shared information that is pushing me in the right direction. Best money I have spent in a long time but hardest to part with for some reason for me.

...

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Johnwa, you make a good point, in the audio world, we've been conditioned that it's ok to part with +$2K in an amp upgrade, or spend 2x more on a source or pre-amp, and the worst, add up what you've spent in cables, and I bet it's a significant portion (or greater than) of $2K. And yet we all seem to have a block at the one class of change that has an immediate, slap you in the face effect. Not closing one's eyes and struggling to hear the 'added depth' from a new pre-amp. Acoustics changes deliver night and day contrast over untreated rooms.

I just added my cable bill and even though I’m engineering/value oriented (vs marketing driven) in my purchases, I still have nearly $4K in cables (and over 100 of them ). So even though I preach and believe firmly in acoustics (I did build the room around the speakers ), I’ve still spent a bit more on cables than on treatments. Shame

What’s needed is to make acoustic treatments part of the Audiophile ‘bling’ thing. People proudly display glowing tubes in shinny chassis, but are ashamed of room treatments.

It needs to get to the point that if we walk into an untreated room we turn up our noses a-priori and make fun of the owner -Just kidding- :devil:

Part of it might be the aesthetics of current treatments, which generally are functional rather than décor centric. Challenge is, there are so many different décor vocabularies that it would drive up costs. But it’s something to think about…

"About the only thing in common with most acoustic treatments is corner treatment for bass trapping. Tri-Corners first then the entire corner. Treating all corners is the key to proper bass trapping, then ceiling/floor wall seams if needed. Floor to ceiling is ideal, but sometimes not practical or financially possible. But one does not have to do it all at once."

Forgot this...remember the first reflection points too when addressing room acoustic treatments.

DTB300 said:

Forgot this...remember the first reflection points too when addressing room acoustic treatments.

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I always thought first reflection points were NOT a significant issue with ML's, given their limited lateral dispersion (even with the curvilinear design).

Perhaps I'm mistaken.

sleepysurf said:

I always thought first reflection points were NOT a significant issue with ML's, given their limited lateral dispersion (even with the curvilinear design).

Perhaps I'm mistaken.

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Actually, ML's disperse laterally just fine with the curvilinear design. They don't disperse vertically too much, so ceiling treatments are not as necessary. I would hazard a guess that first reflection treatments may be more important with MLs than with point source speakers since the db level of the sound wave doesn't drop as much with distance from the speaker.

Rich said:

Actually, ML's disperse laterally just fine with the curvilinear design. They don't disperse vertically too much, so ceiling treatments are not as necessary. I would hazard a guess that first reflection treatments may be more important with MLs than with point source speakers since the db level of the sound wave doesn't drop as much with distance from the speaker.

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Agree. And with the tilt tweak, the ceiling reflections should drop even lower towards negligibility.

Joey

Great thread Rich. And nice comments from the members.

Fairly new owner of CLX Art speakers. And dual 212 subs. BTW, I LOVE IT!

At Axpona I purchased sets of the CIK corner bass traps with scatter plates. I think I said that correctly.

I have been using some DIY absorption panels for my Home Theater and tried them out to place behind the CLX. Granted they are not that big. But the sound did feel more focused and tad less bright. I liked it.

GIK told me in my setup. 26 feet long, about 14 wide. CLX about 4 from front wall...that a combo of diffusion and absorption would be their choice. Other ML owners seem to like one or another.

Kind of hard to demo these where I live.

And Rich is correct, there may not be a wrong answer. One certainly doesn't want to kill their sound. We all bought Martin Logans or at least I did for the big huge sound. So don't want to over dampen.

But clearly some type of room treatment is SO needed. I am thinking of some DIY absorption panels big enough to get a true test. Then go from there.

Thoughts?

Since it's been over 10 years, I can chime in with a decade+ of experience here and say that absorption is the first thing to try, and that with a big speaker like the CLX, you will need a LOT of it. And given @timobi relatively narrow room, rear reflections really do not have the space to 'develop' and come back to the listener in a constructive way. So I'd dampen the front wall and a portion of the side wall up to the side of the speaker.

It is surprising just how much room treatment it takes to tame a large ESL setup, see this thread on my custom side wall treatments, and see the diagram on the last page outlining all the treatment elements in the room.