[Nils]

First, some background: I have Magnepan 3.6r speakers (radiate as dipoles) and am still waiting for my Bryston monoblocks to show up (this week, YAY!).  Therefore, I've been stuck with a 50wpc NAD c340 and consequently I haven't listened at decently high SPLs.  I have an effective listening space of 12' x 17' x 8' in a room that is 17' x 22'.  I'm sure this isn't ideal, but c'est la vie.

I've been endlessly playing around with my oodles of acoustic treatments (2 x GIK tri-traps, 2 x 244 panels, 2 x monster bass traps, 8 x D1 diffusors) and after playing around with arrays of diffusion panels on the front wall (i.e. behind the speakers), I've noticed the following things:

- With diffusors, it feels like my acoustic space has fundamentally changed.  The entire soundstage shifts back and becomes much deeper.  Elements sometimes seem to appear far behind the wall, even.  Imaging isn't razor-sharp and there is some loss of detail, but sometimes I wonder if imaging ever is razor-sharp in real life.

The most serious drawback is that I get this odd "soundstage depth shift" on musical elements to the far left and right.  This odd phenomenon might happen because my room is just too small for diffusion.  I hear treble energy from the diffusion panels as well as the ribbon tweeters, and this really causes my brain to go "WTF?"

- With absorption in the corners using tri-traps in the corners and 244 panels on the front wall's FRPs, I get a much flatter soundstage, but one that extends just as far left and right.  In fact, absorbing the sound that ends up in the corner helps chip away at that boundary of my room.  Imaging and detail is better, but there seems to be a loss of air around elements.

My initial reaction was that there was a loss of shimmer with absorption, but I think I was hearing less brightness and a more natural sound.

I'm still playing around.  Hopefully I can reach a satisfactory configuration of my acoustic space.  Maybe I'll try the diffusion on the rear wall.  Any thoughts or comments are welcome.

-- Nils

[bpape]

Hey Nils.

I'd definitely try the diffusion on the rear wall and on the rear half of the side walls.  Kill the corners with the Tri Traps.  Try the 244's on the front wall but between the Maggies, not behind them.

Also, if you want to try the diffusion up front behind the Maggies, orient them do diffuse vertically instead of horizontally.

Bryan

[woodsyi]

I played around with a pair of Soundlabs.  For me absorption on the front wall (behind the speakers), bass traps on all tri-corners and all wall ceiling corners, diffusers on the back wall behind the listening position presented the best sound. 

[Big Red Machine]

Is it true diffusors are of no real benefit unless they are 6' or more behind the listening position?  I tilt my chair back and I'm only 6 inches from the rear wall, so I use Aurelex absorbers.  I own many diffusers and might try some on the ceiling rear of the first reflection point absorber panels. 

[zybar]

Is it true diffusors are of no real benefit unless they are 6' or more behind the listening position?  I tilt my chair back and I'm only 6 inches from the rear wall, so I use Aurelex absorbers.  I own many diffusers and might try some on the ceiling rear of the first reflection point absorber panels. 

Not sure about 6', but I do know that you need some space.

Six inches certainly isn't enough.

George

[samplesj]

Are your ribbons on the inside or outside?

Inside images MUCH better.

Perhaps if the ribbons are outside you're running off the difusors as it pan over?  My skylines are not directly behind, but rather centered on the ribbon to catch as much of it as possible (doesn't do much for the bass so no real loss there).

[Jason Jones]

In general, absorbers give very precise imaging and placement with small "pinpoint" instruments. Diffusers widen the soundstage (and depth) and make instruments that sound larger and more "natural"...not pinpoint.

The research shows that listeners prefer diffusion to the sides and behind them from 40 to 140 degrees from the speaker (or soundsource if other). Reflections of any kind that are heard from the front wall and the front part of the sidewalls tend to have some adverse effects. However, some dipole owners seem to like front wall diffusion. While I won't argue with them (it's their preference who am I to say), I think in most cases, even with dipoles, rear and side wall diffusion is preferable.

Nils, what are the dimensions of your room? I'll make up a quick graphic to show where the 40-140 degree area is.

Jason

[Jason Jones]

Is it true diffusers are of no real benefit unless they are 6' or more behind the listening position?  I tilt my chair back and I'm only 6 inches from the rear wall, so I use Aurelex absorbers.  I own many diffusers and might try some on the ceiling rear of the first reflection point absorber panels. 

Ideally diffusers should be 6' or more from the listener. At a distance of 4-6 feet there is some compromise - definite, but not disastrous. It depends on the type of diffuser and the location (sidewall, ceiling, backwall) and personal preference. At 6 inches, your head may be IN a diffuser well - not good. 

Jason

[Nils]

Are your ribbons on the inside or outside?

Inside images MUCH better.

Perhaps if the ribbons are outside you're running off the difusors as it pan over?  My skylines are not directly behind, but rather centered on the ribbon to catch as much of it as possible (doesn't do much for the bass so no real loss there).

Inside, my friend.   I'm an "innie" in the maggie crowd.  I found that the ribbon integrates far better in my space.  I'd need much more space between the side walls and my speaker for outboard ribbons to even have a chance.

-- Nils

[Nils]

Ideally diffusers should be 6' or more from the listener. At a distance of 4-6 feet there is some compromise - definite, but not disastrous. It depends on the type of diffuser and the location (sidewall, ceiling, backwall) and personal preference. At 6 inches, your head may be IN a diffuser well - not good. 

Jason

Do certain diffusors perform better for certain size rooms?  I contacted RPG a while ago, and they recommend their QRD734 for large rooms, the Skyline for medium-sized rooms, and the BAD panels (which rely on a crazy array of absorption/reflection dots) for smaller spaces.  Of course, they didn't give exact dimensions for each... doh!

As for my room... well, it's 17' x 22'.  For my livinig room space, I have a 12' x 17' section with a partition of varying height between the right speaker and the kicthen.  A pseudo-sidewall, if you will.  Maggies are 4.5' out and 1.5' from the side walls.  Pictures would be best.. perhaps when I get home.

-- Nils

[Ethan Winer]

Is it true diffusors are of no real benefit unless they are 6' or more behind the listening position?  I tilt my chair back and I'm only 6 inches from the rear wall, so I use Aurelex absorbers.  I own many diffusers and might try some on the ceiling rear of the first reflection point absorber panels.

A coupla thoughts on this:

* Good diffusors on the wall behind you are better than a bare wall even if you're right in front of them. Absorption is probably even better if you're very close, like George's six inches example, but a flat bare wall is the worst surface of all at any distance.

* There are good and bad diffusors. You didn't mention what kind you have, but I've seen cheap stamped plastic diffusors with a "proprietary" shape that sound even worse than a bare wall.

* A diffusor does no good unless sound strikes it. This is one reason the rear wall behind you is useful for diffusion - that's where your speakers are pointing. If you put diffusors on the side walls or ceiling but away from reflection points, they won't do much useful because what little sound reaches them is after bouncing around the rest of the room first.

--Ethan

[Jason Jones]

Do certain diffusors perform better for certain size rooms?  I contacted RPG a while ago, and they recommend their QRD734 for large rooms, the Skyline for medium-sized rooms, and the BAD panels (which rely on a crazy array of absorption/reflection dots) for smaller spaces.  Of course, they didn't give exact dimensions for each... doh!

-- Nils

Yes, but it is not quite that pat. Polys are best for rooms with lots of distance between listener and diffuser and lots of volume. As you get smaller you will probably want to start adding some QRD's to your mix with the polys and as you continue to get smaller drop the polys and use only QRD's. As you get smaller still you will want to use less QRD's and more absorption and eventually just use absorption. That is the general progression from concert hall to practice "closet".

Skylines are good for reflection points that are close to your head because they diffuse in two directions. They, however, have two problems. First, they do not really touch low frequencies. 1-dimensional QRD's can be spaced and increase their effective frequency quite a bit. There is research showing this to aid in low frequency response in smallish rooms.

Second, and this is the more important objection, they repeat too much. Repetition is a sin in the diffuser world. That is why all the fancy math is used, to avoid repetition. The skylines are made from a good sequence - no complaints so far. But, each skyline is made from 4 repeats of the same sequence. i.e. the upper left quadrant is the same as the lower right as the upper right as the lower left. As an individual unit this is fine...but stack a bunch next to eachother and it changes things dramatically.

The repetitions make the polar plot (the image of the reflected sound) very "spiky". The whole point of the 2-D diffuser is for a very subdued reflection. With a  large array this advantage goes away.

I will stop here for now: I prefer to keep my posts short and boring, rather than long and boring. 

Jason

[Jason Jones]

* A diffusor does no good unless sound strikes it. This is one reason the rear wall behind you is useful for diffusion - that's where your speakers are pointing. If you put diffusors on the side walls or ceiling but away from reflection points, they won't do much useful because what little sound reaches them is after bouncing around the rest of the room first.

--Ethan

For mid-high frequency reflections you will get the most bang for your buck at an early reflection point. But, within the limits of the dispersion of your speakers, sound is hitting quite a bit of your sidewalls fairly equally. It just doesn't normally get to you quickly. But, if you diffuse these other reflections, some of that sound will be redirected to you that otherwise wouldn't be.

Jason

[Nils]

Hi Jason,

Thank you for sharing your vast knowledge!  I really like the GIK D1s--I just primed and painted them to match my apartment--and I'm curious what else I can do.  I remember someone saying that I could add mass in the diffusion cavity to improve rigidity and bass absorption.  What are your thoughts on doing so?

-- Nils

[Glenn K]

Nills,

What color did you end up painting them?

Glenn

[Nils]

Hi Glenn, I first primed them and then painted them matte frost (standard apartment color).  I got the paint at the local store.  The entire project took about 3 hours and quite a few glasses of water to offset the combination of primer and warm Santa Cruz weather, but they look great now!  The matte finish really helps them blend in with my interior.  I'll post pictures soon.

-- Nils

[Nils]

Also, if you want to try the diffusion up front behind the Maggies, orient them do diffuse vertically instead of horizontally.

Bryan

How, pray tell, do D1s mount horizontally?  I think I'd have to get some after-market hardware and get creative with hanging picture wire.

[bpape]

Sorry, guess I was kind of unclear.  You would still mount them on the wall but turn them 90 degrees so that the 'ribs' are running side to side instead of vertically. This will keep the horizontal imaging tighter but increase the vertical size.

Bryan

[Nils]

Sorry, guess I was kind of unclear.  You would still mount them on the wall but turn them 90 degrees so that the 'ribs' are running side to side instead of vertically. This will keep the horizontal imaging tighter but increase the vertical size.

Bryan

That's what I understood.  However, there are no screw holes in the non-ribbed side of the D1, so I'd have to figure something other than the suggested mounting procedure.

[Housteau]

I'd definitely try the diffusion on the rear wall and on the rear half of the side walls.  Kill the corners with the Tri Traps.  Try the 244's on the front wall but between the Maggies, not behind them.

I have used dipoles for years and have learned quite a bit about what they like in my room.  As Bryan wrote, I don't recommend adding a lot of absorption directly behind them either, however between them is a very good idea.  Most have liked diffusion behind them in my room and it even stated so in the manufacturers literature.  Since my current speakers were designed with the idea of being able to adjust their dipole nature to each room they would be used in, I took the liberty of experimenting by mixing both absorption and diffusion on the wall behind them.

In my case I chose 6' tall 12 " tube traps that I was not currently using.  They left plenty of open wall on either side.  I then added in some tall artificial plants along side them.  So, the inside back half of each upper range speaker as some diffusion offered by the plants, and the outside back half of that wall behind each speaker has a tube trap.  In my room so far this has proved to be the best method of attack.