Behind the speakers: absorption or diffraction?

In my dedicated 16x23 music room the speakers are on the short wall. They are roughly 5 feet from the front wall.

The front wall has two stacks of bass traps in the two corners, a combination of absorption panels and Vicoustic bass traps.

Between these columns there is a 4' wide x 2' tall x 6" thick absorption panel flanked by two 2' wide x 4' tall x 6" thick absorption panels. These are free standing and located 6" away from the front wall.

Bass is very good in the room, much improved, and I know those central panels contribute to this significantly, though they are meant more for mids and highs.

I'm not unhappy with this setup but wonder if those two 4' tall panels, which are directly behind my speakers would be better replaced by diffraction/diffusion panels. Either skyline or quadratic type.

The rest of the room has scattered absorption panels, carpet, cellular shades etc. Including first reflection points on the side walls. Nothing on the 8' ceiling.

The room has basically no slap echo, but doesn't seem "dead" but I wonder if it needs some life pumped in by using diffusion.

Cheers,
Robert

 

If you're room is 23 feet long and you're sitting far enough away from where any QRD, Skyline or other phase grate diffusers will be placed (like at least 6 feet or more), they can be amazing at making a room sound big and airy and alive. But if you're too close to them or they're in the wrong place they can make the sound in the room weird and phasey.

They're always going to be most effective with a direct angle of incidence, so more behind your listening position than behind the speakers, unless the speakers are dipoles. But I've also found it that diffusion high up, above ear level, not just behind the speakers but on the sidewalls behind the listening position, can really open up a room. But again, properly placed relative to one another, and far enough away from you so as not to give the experience sci-fi out of phase weirdness. You can experiement and see. Small, but deep enough to be effective, EPS QRD diffusers are available from the likes of GIK. You can buy 4 2X2 Gridfusors for a couple hundred bucks and play around, get a feel for the dos and don'ts of adding diffusion to your room.

 

I suspect you're right, experimentation is in order. Just really don't want to buy stuff I don't need since resale and shipping of large items is stupid expensive and there is no local audio community.

Cheers,
Robert

 

That's a lot of absorbion, almost the entire wall. My belief is there should be some areas of reflection or diffusion. As an experiment, remove some of the front wall panels; try removing the middle, leaving the 2 vertical. Then try removing the vertical and leaving the middle.
IME, too much absorbion behind the speakers reduces depth of the soundstage, although that may not be a problem since you have distance between speakers and wall. I've found some absorbion is needed to provide a focused image. I may be wrong considering your room is larger than mine, but I think it's worth trying.

 

I have diffuser panels on the front of six of my GIK panels ... https://www.gikacoustics.com/product/impression-4inch-bass-trap-diffusor-absorber/ but the wood panels are removable. I might remove a couple of them sometime ...? Four in front (between speakers on wall) and two behind me on the wall behind my head.

I also have four GIK Acoustics - Tri-Trap Corner Bass Trap - Custom Height Bass Absorber with membrane to increase low bass absorption.

Two FreeStand Acoustic Panel© (gobo)

and one Monster Bass Trap with FlexRange Technology

 

About a year ago I replaced the absorption panels behind my speakers with diffusion panels. This move did open up the sound stage some. I purchased mine from Pi Audio Group. They are relatively inexpensive and very easy to install. I'm definitely going to buy another set to place behind the listening spot, and maybe on the ceiling.

 

temporarily remove the free standing panels and see how it sounds.
generally you want diffusion on the wall between the speakers.
I prefer diffusor / absorbers because I do not have bass traps.
If I had bass traps I would use odd surface diffusion. you may have too much absorbtion.

 

This

taking off the absorption panels will give you a bit of an idea of what a diffuser will sound like.
I also think that 6” absorption on the front wall is too thick. But that’s just me.
I would use more like 3” at most. Will give more depth to the sound I think.

in my room a like a bit of absorption behind my speakers but that’s it. Nothing more to the sides.

of course my room is substantially smaller.

 

Now that I think of it, I would experiment with taking off the panels which are on the front wall which are not behind the speakers first, and leave the panels which are behind the speakers.
Then take the panels off which are behind your speakers and leave the ones on the front wall which are not behind them.


See what happens

 

That front wall behind your mains is critical.
You don't want ANYTHING in the area that isn't part of the projected image.

If you put a cabinet in front for instance---it MAY upset your soundstage with all kinds of reflected bits and pieces of sound bouncing off the rack.
I use a cabinet but either close it tight OR leave it open with the ear level parts empty but for a touch up of Sonex to dry out the cabinet energy.

You WANT to leave some hard flat reflective surface dead center.
This will establish where the middle "phantom speaker" lives.

By varying how large an area this dead center reflector IS you can alter the presentation to get a seamless side to side picture of what is going on.
Make it too big the soundstage gets "splashy" and loose.
Make it too small and the vocals won't have presence.

My cabinet IS breaking up standing waves and flutter echo.
If I didn't use a cabinet I would place two panels directly behind my main speakers if possible and experiment to get these panels to make the front image clearer and drier.

I then would make sure EVERYTHING is precisely symmetrical.
Whichever speaker sounds the best leave it be and go move it's opposite member to exactly match the distance off that back wall.
This will double the "best" placement you have so far and will really cut down on slightly "off" setups where certain notes pop out in the wrong spot.

You want instruments to occupy precise locations and NOT wander.
When everything is perfect you will find that moving a speaker box forward or backward a quarter inch will shift the presence and leading edge of transients making them harder and softer.

This stuff WORKS.
It is PAIN to do set ups.
But they are the ENTIRE BALL GAME.

Good luck.

 

Great write up.

I would also take measurements of the room to find where the reflections are.
Also see which bass freq are more present.

I agree for the center to be a touch reflective to add to the phantom image.
And yeah if the center reflection point is too big things will get wider but more indistinct and splashy.

 

I posted earlier today about using internet test tones off YouTube at certain low bass frequencies to spot problems BEFORE you set up your chair and speakers and discover it SUCKS!
Just play tones below 250hZ for a while and go "walk the room."

In my big room playing 90hZ really really LOUD I sat in my first chosen chair position AND THE NOTE COMPLETELY DISAPPEARED!
That learned me real good to go move that darn subwoofer to get out of the "null."

When I measured my chair I discovered it was 12.4 feet off the front wall.
And from my ears then BACK to the rear wall?
12.4 feet!
The length of a 90hZ wave?
12.4 feet!

So obviously I should have suspected pretty severe cancellation at 90hZ depending on where my boxes were in relation to a full wave length.
So I moved ALL the boxes and put my four subwoofers on the front wall so they would hit at a different spot and fill in that null a bit.

AHA!
I was going insane trying to fix it before I did some more testing and found that null.
Yup.
The great Doctor forgot to measure before cutting!
Oops.

 

You may find these two videos by John Darko valuable.
#1

#2 https://youtu.be/3N-yG_Y7XEc

 

Yes.

 

Ha yeah it’s pretty maddening sometimes.

I took and take measurements of my room with REW.
I pretty much know it’s weaknesses.
I have 3 kinda problematic bass modes. No suck outs at all. Thank god.
One is at around 30hz, one at around 55hz and the worst, and one around 110 hz.

I have tried everything to reduce those as much as I can.
What works is to not let them ring too long. The rest I can deal with.
So the maddening thing for me is to set the center stage correctly, no pulling to the left or right, get the rake angle correct and during that whole time try not and let those modes ring.
The last part is the hardest as a slight change in rake may make one of those modes ring longer than I want.

you learn a lot putzing around and experimenting. Learn the room a lot.
I just figured out today that without closing my curtain on the left, the soundstage pulls a bit left.
Another piece of the puzzle solved.

I know what treatment effects have on the front , rear and side walls. So that part is mostly solved as well.

speakers are at the same distance from the rear wall.. which in my room makes more difference than equal distance from the front wall. If they aren’t the same distance the soundstage pulls more or less to one side. Check.

the last frontier now is to get the speaker rake to be good and not have one of those bass modes ring. That’s the tricky part.

They need to make a flipping robot to do this. Lol.

Hey Alexa, take measurements of my room and find the optimal speaker placement please. I’m going to take a nap.

Tell me when you are done.


lol

 

Sorry but no. Just no.

zero measurements, no reference , took out all the furniture before treatment. Had way too much treatment if you ask me. And no measurements after.
I and no one else has any clue as to what that setup did.
Nothing.

apologies for being so adamant about that.

 

The improvement from a great room setup is virtually hallucinogenic, heh, heh!
At last someone else besides me that realized how important this stuff is.
And realizes that there is a method to take you there.
It's not hard once you grasp the fundamentals of room setup.

Let's start a club with decals and jackets and be the "cool guys that know room tuning".
Or just call me whatever the heck you want...
Like I care?

You don't know what's under my hood, snicker snicker.
You wanna RACE?
Sure.

 

Hmmm...

In my my main HT / listening room, between my F L/R Koss CM/1030 stacks, there's... another Koss CM/1030 stack. Where else would you put the centre channel in a multichannel (music) listening system?!

Now on the front wall between the F L stack and the centre stack there's... an absorber panel. Same on the R side.

Mind you there's also symmetrical stacks (from bottom to top) of Velo sub / Koss CM/1020 / Koss CM/530. And between those stacks and the centre stack, there's some gear to the left, and a big SVS cylindrical sub to the right.

Wouldn't have it any other way. Works great.

Jeff

ps. photos on the last post(s) in the link below.

 

Without knowing where exactly your subs are ( I assume you don't have bass management) and knowing more about your room (measurements), no one can pinpoint anything. Generally, you would only need about 6inch thick or a bit more absorption right behind your speakers and behind you at ear level for a decent stab at SBIR. Everything else on front/side/back walls can be diffusion. It sounds like you may have a case of deadening. Use the Harman room mode calc and place subs at locations that address more than one null. There is a method to every room's madness and it is hard to explain in a comment section like this.

 

God I so dislike coming off as arrogant or that I know everything.

but that video was so strange to me.
And the title . Don’t buy speakers until you try this. Or something like that.

Yeah you don’t need measurements to put in treatment for sure.
But to make a video about it, I would expect something a tad more concise.

no talk on first reflections, or anything.
Just weird

 

Thanks for the input folks, some very helpful. In the end I'll have to experiment, but some good thoughts offered here.

Cheers,

 

Do you know why the existing panels were placed where they are, or what they were trying to fix? I'm curious how they came to be there.

No judgment, just curiosity. There is nothing wrong with broadband absorption, like your 6" thick panels.

 

I consistently prefer absorption behind my speakers. The best systems I’ve heard all had extensive absorption on the forward wall level with the speakers and between them.

Diffusion panels can make for a larger sounding space but I prefer them in corners or higher up the wall because otherwise they can make for unfocused, imprecise imaging.

I find absorption behind the listening position is much less necessary and often detrimental, even when the rear wall is relatively close.

 

I tried both types behind mine and have stayed with absorption because diffraction smeared the center image.

 

I think diffusion is the way to go for the wall behind you speakers, and absorption on the side walls and the wall behind the listening position. If you have a high ceiling, diffusion works wonders there too. The farther you listen to diffusion panels, the better the effect. If closer to the panels, absorption. Just don't overdo either.