Room Acoustics - Absorption Panels Behind ML's

[Stefan_DR3]

I did exactly that. Put a huge blanket up on my wall. Just tucked it in w the suspended ceiling. This gave me a feel for the direction absorption would take it. Now if someone could give me an idea for diffusion like that I would appreciate it. I have no fake plants to try out.

The old “blanket on wall” trick is a common mistake most diy’ers make - too much, too thin. Sucks only your ultra high frequencies and doesn’t help with image smearing or midrange glare. You need fibreglass panels or thick foam.

For diy diffusion try a bookcase or media tower with unevenly spaced/sized media.

 

[kach22i]

My speakers used to be against a staircase with open balusters. I built a homosote/plywood wall with perforated metal diffusion and oak grid/trim will with batt insulation with weep holes to trap sound waves behind the speakers. That was in addition to treating all the obstructions in between the speakers that destroy sound stage with foam and wool blankets.

Look at how they set up the rooms in very high-end stereo shops, that's the way to do it.

 

[Andrew Perry]

Here are the frequency responses and the linear interpolations calculated with Wolfram Mathematica (red and green lines): both lines are -1.3 dB/octave.
Sub 250 Hz range with Home Audio Fidelity convolution files (DRC); 1600 Hz - 20 kHz with slight (±3dB) 3-band parametric equalization (ROON eq. function).
All responses taken in the listening spot with UMIK-1 microphone in vertical position (90°).

 

[Andrew Perry]

Small adjustments...

 

[johnyjax]

View attachment 19941
View attachment 19940

Here are my ML with Oudimmo Akupan panels (5 cm thickness, 60x120 cm - 2"x 23.6"x 47.2").

Distance Akupan panels - wall: 5 cm (2").
Distance electrostatic panels - front wall: 1 m (about 39").

The graphic shows the difference between before/after panels positioning (blue = with panels).
The system is equalized with DIRAC LIVE digital room correction, and target curves are different (above 1 kHz) in red/blue measurements.
Green line: interpolating line, showing average 1 dB/octave slope of blue graphic response.

Hi. Sorry this might me off the topic. Just wondering, if I toe in the speaker 30 degree, do the absorption location still the same as the one you have or should the absortion panel also be move 30 degree wider to the left and right from the one you have now? thanks.

 

[JonFo]

Hi. Sorry this might me off the topic. Just wondering, if I toe in the speaker 30 degree, do the absorption location still the same as the one you have or should the absortion panel also be move 30 degree wider to the left and right from the one you have now? thanks.

Very on topic, no worries.

Absorption of the rear wave needs to be placed in as direct a line of aim from the panel as possible, so if you toe them in, the absorptive panel would likewise need to slide outwards along the front wall. If the angle is such that it's aiming at a corner, then you need to treat both the front wall and the sidewall.

 

[johnyjax]

Very on topic, no worries.

Absorption of the rear wave needs to be placed in as direct a line of aim from the panel as possible, so if you toe them in, the absorptive panel would likewise need to slide outwards along the front wall. If the angle is such that it's aiming at a corner, then you need to treat both the front wall and the sidewall.

Thank you for the info. Basically the absorption panel is in line with which way the rear panel facing. Also, 2" panel with 2" air gap, would be enough?

 

[JonFo]

Thank you for the info. Basically the absorption panel is in line with which way the rear panel facing. Also, 2" panel with 2" air gap, would be enough?

Correct, in-line with where the rear of the panel is facing.

A 2" thick absorber might not go low enough. Ideally, a 4" thickness, offset from the wall 2 to 3" is the best, as that will absorb evenly down to the 300Hz, and we want to help reduce the dipole cancelation at those lower frequencies, not just mitigate the high-frequency reflections.

 

[johnyjax]

Correct, in-line with where the rear of the panel is facing.

A 2" thick absorber might not go low enough. Ideally, a 4" thickness, offset from the wall 2 to 3" is the best, as that will absorb evenly down to the 300Hz, and we want to help reduce the dipole cancelation at those lower frequencies, not just mitigate the high-frequency reflections.

I see. Thank you.

 

[Andrew Perry]

Very on topic, no worries.

Absorption of the rear wave needs to be placed in as direct a line of aim from the panel as possible, so if you toe them in, the absorptive panel would likewise need to slide outwards along the front wall. If the angle is such that it's aiming at a corner, then you need to treat both the front wall and the sidewall.

Absolutely 100% agree. :bowdown:

 

[Soundtechguy]

Sorry, but I disagree. Did you happen to notice the pictures and other info that he posted? Dipolar speaker panels less than three feet from a front wall that consists entirely of hard, reflective surfaces, in a 13'x15' listening room. I can't speak to all the possible room interactions, but I can guarantee you this: the rear wave of those speakers (sounds from about 500 hz. on up) will reflect off that front wall and arrive at the listener's ear very shortly after the front wave, and this reflection will muddy the imaging and soundstaging the listener would normally get from that front wave. This is simple physics and psycho-acoustics. If the waves arrive too closely in time and energy levels, the brain can't separate them and properly interpret the spatial cues inherent in the audio signal.

There are two ways to fix this particular issue: decrease the energy of the rear wave and/or increase the distance to the front wall, thereby increasing the latency of the rear wave. If the speakers stay in place, absorption of the rear wave is the only viable option to get proper imaging and soundstaging from these speakers. If the speakers can be moved another foot or two out from the wall, that will help tremendously with the issue by further delaying the arrival of the rear wave, but diffusion panels behind them would help even more to get the ultimate performance the speaker panels are capable of.

You seem to be caught up in the notion of predicting overall room response, bass response, etc. That's not what I am doing. I am not talking about or giving recommendations on bass performance or other possible room interaction issues. That's a whole 'nother ball of wax and very complex. I am simply referring to the effect of the rear wave of a dipolar speaker, which is a specific property of Martin Logan and similar panel speakers. Your suggestion that "it's impossible to say what is right" in the situation presented by O.P. is poppycock. Again, the physics are very simple, straight-forward, and long-proven. Anyone that's seriously experimented with ML speaker placement is going to understand the need to ameliorate the deleterious effects of the rear wave. You have to get the speakers far enough from the front wall to adequately delay the rear wave for best imaging and soundstage presentation. If you can't do that, your next best option is to absorb the rear wave. If you do neither, I guarantee you will not get the imaging and soundstaging performance these speakers are capable of. Period.

Hi Rich,

I just came across this thread looking for insight on this very topic. Thanks for the information on the rear wave issue. It gave me some direction.

 

[Soundtechguy]

After having experimented with absorption, diffusion, and nothing at all… I figured I’d chime in on this topic with my own personal findings. Upfront, I’m not looking to start any dispute, or discredit anyone else’s findings, but simply adding some helpful input based on my own personal experiences in my listening environment.

My listening room is basically my living room which shares a small dining room. The room is 20ft wide x 15ft deep and has a few jogs in the sides and rear wall for closets, windows, doors, hallway etc. The wall behind the Martin Logan Quest speakers is flat Sheetrock with no interruptions and the distance between the speakers and wall is 26 inches. The room is quite acoustically stable with the only acoustical treatment being three 2’ x 3’ broadband panels on the wall behind the listening position to eliminate a problem I had with a near-field reflection.

To be honest, I am very happy with the sound reproduction that my system offers in the room. If it was a perfect world, my only change would be moving the system to the left about three feet to get the right speaker further away from the wall. Unfortunately, there is a dining set to the left that does not allow for this so it is what it is. I do have some thick and heavy drapes on the wall close to the right speaker, and its close proximity to the wall does not seem to present a concern.

The reason for this post is that I have often seen pictures on the internet where Martin Logan owners have placed sound absorbing panels (as well as diffusion panels) behind the speakers. I also have read some conflicting information between forums and what is written in my owner’s manual to the Quest speakers. The owner’s manual goes into detail regarding room acoustics, bipolar speakers, dispersion, placement, anomalies, experimentation, etc. All this information lead me to contact Martin Logan in which I shared a pleasant conversation with someone there who reinstated what was indicated in the manual.

According to ML, the Quest speakers are designed to integrate the wall behind the speakers as part of the overall listening experience. If the speakers are placed 2.5 – 3ft from the wall, the reflection from the wall brings room ambience into play. They recommend the wall be flat without things like windows, doors, etc and that the surface not be too hard or soft for best results. If things are in your favor, the sound will seem natural (like having a quartet playing in your room vs being beamed at you).

I found this interesting and decided to try a few experiments. I tried absorption behind the speakers and found the results to be less than favorable. As the gentleman from ML expressed, the result was a very sterile music delivery (like wearing noise cancelling headphones). Although it was interesting, there was no room ambience, the sound was dry, lifeless, and unnatural. What I really found interesting was that if you stepped outside of the sweet spot, you were in a null. The speakers had a spotlight effect and the listening experience was limited to an area within the dispersion pattern of the speakers. This phenomenon is generally desired in professional applications (such as concert systems etc) but in a living room it was not my cup of tea. My first pair of premium speakers was a pair of Ohm Walsh F’s which radiated sound 360 degrees and filled the room with sound and made for a pleasant listening experience in virtually any seating position (something I’ve grown to enjoy). I further tried dispersion behind the speakers in which I found that the difference was minimal. If anything, I think the ambient sound was a bit smeared and wasn’t as good as without.

For me… my conclusion was less is more. Having the speakers perform in a stable room environment as instructed in the manual yielded the best results. However, sound can be subjective, and rooms can vary and all can have their own issues and anomalies. As the owner’s manual concludes; experiment and have fun! Placement, tilting, toe in, room treatment, etc all play a role in the outcome. Do what works for you and enjoy!

 

[Rascal]

Dear bikernail,

I think the others are offering useful advice. I especially agree that 2'x2' panels located as you've shown wouldn't be helpful -- it's the ESL membrane rear radiation that matters, not the rear output from the bass cabinet.

I myself use 2" thick acoustic absorbing panels -- wooden frames of size 2' x 6' with acoustic batting within and the covered with a decorative fabric -- behind each of my Montis ESLs. The result is very effective and beneficial (more on this in a second.) My panels are hung from the ceiling and run downward behind each of the Montis catching the bulk of the back-emitting surface waves from the ESLs. The biggest difference I see in applying this approach to your set-up is what that would look like in your room -- you've got an attractive shuttered window behind the speaker (IMHO window surfaces are a problem, but I'm sure you already know that) and covering it up could cause friction with the others in the home. There's always the possibility of drapes, but anything that covers the windows might draw fire from others in the family.

However, I still think you owe it to yourself to do some experiments to better gauge what absorbing the back wave could mean for your setup, including the possibility of placement in another part of the room or home.

Without a whole lot of effort, you could fashion T-shaped ceiling hangers ($20 worth of 2x4s?) to temporarily clamp and hang carpeting or heavy blankets from the ceiling to the floor behind the speakers. It doesn't take an enormous amount of absorption material to achieve a major change. What you should notice is far more even frequency response in the mid and upper ranges and significantly better stereo imaging.

Before I installed the panels, I had a really bitchy mid-range resonance around 3kHz that was ultra annoying. I verified the frequency and the effect with a parametric EQ, but I wasn't thrilled with the noise level and other characteristics of the EQ unit as the solution. So I experimented with hanging treatments, as I've described, and was pleased to discover that the effect could be eliminated with the panels. The imaging was also significantly improved.

Getting back to what I believe to be the goal of treatment, it's doing away with the rear wave. I personally think it's just a nuisance. The ESL doesn't need it to achieve a ruler flat front response and so forth, but I know there are others who might disagree. The rear wave can't be blocked -- that would color the ESL membrane's tone. And, as I've said, I don't believe it shouldn't be reflected or re-enforced -- that interferes with the front wave, makes the overall requency response scalloped and adversely affects the ability of the pair of ESLs to create a stable and solid stereo image.

One more thing: How the ESLs are driven (what amplification) matters a LOT. If you drive the ESLs with tube amps, for example, as I've done in the past, the high end rolls off and the effects of the back wave are less important. If that's the end goal, well OK, but frankly it's a terrible waste of a brilliant acoustic transducer.

In contrast, driven with a cutting edge low distortion high current low output impedance amp (in my case, the Benchmark AHB2), the ESLs achieve astonishing levels of transparency and grace. They disappear and what's left is the music. in astonishing clarity and realism. I've taken down my panels, and then put them back, and without a doubt, the paneled arrangement wins.

My two cents.

 

[kach22i]

I'd like to add to my two older posts in this thread, and the things I've learned since then.

In retrospect the reason I found improvements when I placed a heavy wool blanket over my old CRT tube screen TV, and then again when I relocated the system in front of my fireplace with a diffuser in-between and a low profile open sided equipment rack is that I was moving in the right direction in minimizing front/center acoustic obstructions and reflections.

The best thing in-between your speakers is nothing, or as little as possible for the sake of soundstaging.

Link to that discovery process in my new room:

kach22i's system

I'm still making changes but have it sort of figured out (Back in the Saddle Again - Kind of). Moved after 27 years into a little house on the river (built 1947) with a finished walk-out basement, concrete floor at last. After watching several Dennis Foley (Acoustic Fields) and many other...

whatsbestforum.com

In short, my personal assesment of the original image is that getting the equipment out from between the two speakers will be the largest improvement. Also miminzing reflections off the fireplace will help as it is yet another center stage obstacle.

If you want "open sound", you gotta open up your front/center stage.

That said, the low open equipent racks, amps on low amp stands and even a center located subwoofer if small enough are much better than putting a tall equipment rack or reflective surface like a TV in-beteeen the speakers.

Just my two cents, or is that six cents by now?