Diffusor panels behind the speakers?

[DocG]

A room treatment question.

Giving speakers enough room to breathe is often a challenge in real life (even more so for dipoles). I learnt that diffusors can 'make the room bigger', and that diffusors behind panel speakers work very well, solving the 'sound cancelling' while preserving (even enhancing) the depth of the soundstage.

My question is: how far behind the speaker should the diffusor be to make it work? When too close, is it just less effective? Or does it then do more harm than good?

I hope any one in the know could help me out here...

Thanks

 

[matt49]

I can't answer the specifics of your questions about distance. I can only describe my set-up. My MLs are about 1 m from the rear wall. Behind each speaker is a GIK 244 Bass Trap Panel. 'Bass trap' is something of a misnomer, as the panels are specified to work over the whole frequency range. Their dimensions are: 600mm x 1200mm x 118mm.

The theory is that the panels simply absorb rear-firing sound from the panels, and that this results in crisper transients and better imaging. It certainly seems to work very nicely. The potential downside is that you suck a bit of life out of the sound, though in my medium-sized room that's not a problem.

Of course, these are absorbent panels and not diffusors, so I guess it's a pretty useless answer to your question! (I went for absorbency rather than diffusion because I prefer the look. I needed large panels and I think large diffusors look weird.)

Matt

 

[Electro]

I can't answer you question directly but I have been toying with the idea of using diffusing panels rather than the absorbtion panels that I am using at the moment .

This company was recommended to me as having some of the best designs and they should be able to give advice on the correct application .

Hope this helps .

http://www.rpgeurope.com/products/product/skyline.html

 

[AlmaataKZ]

There is some basic info in the Genelec's speaker set up guides here: http://www.genelec.com/catalogues-brochures/

Their FAQ on room acoustics are also worth checking.

 

[DocG]

How to Acoustically Treat a Room - Tip 6
A very common question is: how do I acoustically treat my room so that I get the best from my monitors? Well, this is a very large subject area covering room geometry, reverberation and decay time, sound reflection and refraction, material properties, etc, so we will just give here a checklist of the most important features that a listening room should have.

• First, choose or build a listening room that does not display any parallel walls. Parallel walls should be angled at a minimum of 15 degrees each to be considered then non-parallel at all frequencies.
• Ensure that the reverberation time is both low and smooth with frequency. RT60 usually increases towards low frequencies, but make sure it rises slowly without any peaks.
• Primary sources of reflection should be treated so that reflected levels are at least 20 dB down from the direct sound pressure level.
• In general, if diffusers are used they should be placed to the rear part of the room.
• The front wall should be a hard, heavy and smooth surface if flush-mounting of the monitors is to be used.
• The front wall can be absorptive if free-standing monitors only are being used.

I like the "Build a listening room that does not [...]" part!

Diffusors in the rear part of the room and absorptive front wall: that's more like what your do, Matt! But somewhere I read otherwise for dipoles...

 

[DocG]

I can't answer you question directly but I have been toying with the idea of using diffusing panels rather than the absorbtion panels that I am using at the moment .

This company was recommended to me as having some of the best designs and they should be able to give advice on the correct application .

Hope this helps .

http://www.rpgeurope.com/products/product/skyline.html

Hi Electro,

Thanks for the link. I'm not sure my wife will allow some of these in the living room though... :? I guess they're some kind of foam. Probably very effective, but the looks... I suppose that's what Matt is talking about...

But there's more elegant solutions too. I like these less intrusive diffusors, though they work less well, I'm sure (diffuse in space, not in time):

 

[ellisdj]

I dont think diffusion works very close range -- so unless the speakers are a long way from the wall then it will likely have no effect.

Behind the speakers Real Traps recommend nothing - however I tried that and removing my DIY panel from the front wall made me centre speaker bass worse and the imaging of my main speakers worse.

You have to treat the front wall corners and behind the speakers with absoprtion to the best of my knowledge same as the ceiling if its not a mega high room

What I found to works well is GIK scatter plates fitted in 244 panels on my back wall I have 3 of them.

Even better would have been 3 Monster panels but they stick out a bit too far in my small room.

I sit just over 2 metres from the back wall and that is the minumum distance recoemmended from memory for the diffusion to take effect.

 

[ellisdj]

I dont think diffusion works very close range -- so unless the speakers are a long way from the wall then it will likely have no effect.

Behind the speakers Real Traps recommend nothing - however I tried that and removing my DIY panel from the front wall made me centre speaker bass worse and the imaging of my main speakers worse.

You have to treat the front wall corners and behind the speakers with absoprtion to the best of my knowledge same as the ceiling if its not a mega high room

What I found to works well is GIK scatter plates fitted in 244 panels on my back wall I have 3 of them.

Even better would have been 3 Monster panels but they stick out a bit too far in my small room.

I sit just over 2 metres from the back wall and that is the minumum distance recoemmended from memory for the diffusion to take effect.

 

[Electro]

I can't answer you question directly but I have been toying with the idea of using diffusing panels rather than the absorbtion panels that I am using at the moment .

This company was recommended to me as having some of the best designs and they should be able to give advice on the correct application .

Hope this helps .

http://www.rpgeurope.com/products/product/skyline.html

Hi Electro,

Thanks for the link. I'm not sure my wife will allow some of these in the living room though... :? I guess they're some kind of foam. Probably very effective, but the looks... I suppose that's what Matt is talking about...

But there's more elegant solutions too. I like these less intrusive diffusors, though they work less well, I'm sure (diffuse in space, not in time):

They do many different types that have a less striking appearance

http://www.rpgeurope.com/products/product/allproducts.html

 

[RobinKidderminster]

Room dependant. For me, bass trap behind fronts is most important first step.

 

[sheggs]

A few points that should help you with regards to diffusion -

First thing to state is that different diffusors have different diffusion patterns and therefore the effectiveness of their diffusion or scatter is dependent upon the route of that design. This means that some diffusors have different optimum distances

Also for a diffusor to work it is not how close it is to the source that is the thje most improtatn but how far away it is from your ears.

There are many different places that diffusion can be used, on the ceiling, on the side walls and on the back walls. They achieve different things.

The most common is behind your ear.

The best material for most front walls (behind the speakers is aboisprtion) unless you have elctro statics and then diffusion is great

Wanna know more? - We have a large articel called How Diffusion Works on our website -

http://gikacoustics.co.uk/how-diffusion-works/

 

[DocG]

Thanks for that, Sheggs. I was hoping you would pop in too! Very informative link, certainly the FAQ-part.

So it would make sense to put diffusers, say 30 cm behind the speakers, with the listening position at 3-3.5 m?

Let me tell you what it's actually about. I'm very impressed with panel speakers (like the Maggie 1.7 or Quad 2812). But I need a pair of standmounters, to be put on a low cupboard (4m wide, 60 cm deep and 70 cm high). Which leaves no more than 30 cm between speaker and front wall. Now I really like the sound of the AudioVector Si1 Avantgarde. IMO the sound (and the soundstage) is in great part due to its ribbon tweeter, whose rearward output is fired out of a hole in the back of the cabinet. Getting the speaker too close to the wall hardens the sound and compresses the soundstage. So I hoped a diffusion panel could help here. I'm guessing that diffusion of the trebbles would be most important here, so a rather shallow diffuser could do...

Can you enlighten me some more? :cheers:

 

[matt49]

Thanks for that, Sheggs. I was hoping you would pop in too! Very informative link, certainly the FAQ-part.

So it would make sense to put diffusers, say 30 cm behind the speakers, with the listening position at 3-3.5 m?

Let me tell you what it's actually about. I'm very impressed with panel speakers (like the Maggie 1.7 or Quad 2812). But I need a pair of standmounters, to be put on a low cupboard (4m wide, 60 cm deep and 70 cm high). Which leaves no more than 30 cm between speaker and front wall. Now I really like the sound of the AudioVector Si1 Avantgarde. IMO the sound (and the soundstage) is in great part due to its ribbon tweeter, whose rearward output is fired out of a hole in the back of the cabinet. Getting the speaker too close to the wall hardens the sound and compresses the soundstage. So I hoped a diffusion panel could help here. I'm guessing that diffusion of the trebbles would be most important here, so a rather shallow diffuser could do...

Can you enlighten me some more? :cheers:

Yes, I'd also be interested in knowing a bit more. ATM I have the GIK 244s behind my Martin Logans, but I actually bought these to go with my previous (non-electrostatic) speakers. Now that I have ESLs, moving to diffusers might be of interest.

Doc, as far as your question's concerned, this seems to be the key passage (from the description of the Q7d Diffusor): "The Q7d Diffusor is a 7-root well diffusor design that begins scattering at approx 350 Hz and offers effective and even diffusion up through 3 kHz. The Q7d Diffusor offers additional scattering to the upper limit of approximately 7 kHz." The Si1 Avantgarde's X-over is at 2.9KHz. So maybe you need to find a diffusor that's "tuned" to a higher frequency band.

:cheers:

Matt

 

[DocG]

Doc, as far as your question's concerned, this seems to be the key passage (from the description of the Q7d Diffusor): "The Q7d Diffusor is a 7-root well diffusor design that begins scattering at approx 350 Hz and offers effective and even diffusion up through 3 kHz. The Q7d Diffusor offers additional scattering to the upper limit of approximately 7 kHz." The Si1 Avantgarde's X-over is at 2.9KHz. So maybe you need to find a diffusor that's "tuned" to a higher frequency band.

I might have found something already: the fractal diffuser by Tim Perry (website), a DIY design (well, a "have-it-done-by-your-carpenter design" in my case).

These are the plots up to 3 kHz.

And this graph (for some reason I can't paste the graph in my post; I'm talking about image 22 of 26 on this web page) seems to suggest an effective scattering up to 8 kHz (and probably higher).

And it's not too deep, so looks practically feasible too (for a carpenter with a CNC machine, that is).

And I could have them finished like the cupboard veneer, which might make them acceptable for Doc's wife(!)

 

[matt49]

I might have found something already: the fractal diffuser by Tim Perry (website), a DIY design (well, a "have-it-done-by-your-carpenter design" in my case).

Good find. DIY potential and and a 'fractal' design philosophy too! 8)

My dad likes messing about with Mandelbrot Sets. He's often tried to explain their uses to me, but I've never quite got it.

Matt

 

[sheggs]

Thanks for that, Sheggs. I was hoping you would pop in too! Very informative link, certainly the FAQ-part.

So it would make sense to put diffusers, say 30 cm behind the speakers, with the listening position at 3-3.5 m?

Let me tell you what it's actually about. I'm very impressed with panel speakers (like the Maggie 1.7 or Quad 2812). But I need a pair of standmounters, to be put on a low cupboard (4m wide, 60 cm deep and 70 cm high). Which leaves no more than 30 cm between speaker and front wall. Now I really like the sound of the AudioVector Si1 Avantgarde. IMO the sound (and the soundstage) is in great part due to its ribbon tweeter, whose rearward output is fired out of a hole in the back of the cabinet. Getting the speaker too close to the wall hardens the sound and compresses the soundstage. So I hoped a diffusion panel could help here. I'm guessing that diffusion of the trebbles would be most important here, so a rather shallow diffuser could do...

Can you enlighten me some more? :cheers:

Yes, I'd also be interested in knowing a bit more. ATM I have the GIK 244s behind my Martin Logans, but I actually bought these to go with my previous (non-electrostatic) speakers. Now that I have ESLs, moving to diffusers might be of interest.

Doc, as far as your question's concerned, this seems to be the key passage (from the description of the Q7d Diffusor): "The Q7d Diffusor is a 7-root well diffusor design that begins scattering at approx 350 Hz and offers effective and even diffusion up through 3 kHz. The Q7d Diffusor offers additional scattering to the upper limit of approximately 7 kHz." The Si1 Avantgarde's X-over is at 2.9KHz. So maybe you need to find a diffusor that's "tuned" to a higher frequency band.

:cheers:

Matt

Spot on you need either a polycyndrical design or something else that would catch the higher frequencies

 

[matt49]

Spot on you need either a polycyndrical design or something else that would catch the higher frequencies

Cheers!

Something like this?

If so, Doc was on the right lines earlier in the thread.

Matt

 

[sheggs]

Yes but your DIY design might work too. Just need something that is going to catch his crossover 2.9khz

 

[DocG]

Spot on you need either a polycyndrical design or something else that would catch the higher frequencies

Cheers!

Something like this?

If so, Doc was on the right lines earlier in the thread.

Matt

Well yes, but it won't impact the time domain. And I'm not sure what radius it will need to diffuse the +3000 Hz frequencies.

@ David: in the plots I refer to, one makes a distinction between diffusion and scattering. Can you explain this in layman's terms?

 

[Vladimir]

Wool throw rugs (sound waves transform into heat), tilting back speakers on their back legs by a small angle and moving your listening position and speakers back and forward to go dodge the first floor reflection IME solves a lot of bass boom. Covering glass surfaces and tiles with textile also solves harshness. Althought I am all for diffusors and panels, I find them too extreme for the home. If one has a dedicate listening room, then it's a different story.

The human mind has an amazing built in room correction DSP. It takes microseconds to adapt to any room for optimal listening. I gues those that didn't have it were eaten by the bear in the cave. Point being, getting used to your room is good enough and there is no need to tile your living room with panels and corner bass traps.

I also have a very special method of solving room acoustics. I own a jewelers expo stand as my HiFi rack and it is uterly distracting together with the shiny Roksan Kandy glass face and B&W CM1 shiny alu rings. I may have been a baracuda/raven/female in my past life.

 

[lejockey]

I think this might be repeating, but in general, when designing recording studio control rooms, we put absorbtion I front of the listening position, dispersion behind. Bass traps are a pretty misleading title, they can control maybe at the very lowest, 250hz, but the wavelengths of low frequencies are just far too big to do much about. Bitumen sheet can tame a bit, but normaly just have to try and make the control room asymetrixal.