Diffusion on front wall. Where.

[o2so]

Hi there, I was hoping someone could help me with this. I'm considering putting some diffusion panels (vicoustic DC2) on the front wall. I have dipolar speakers and apparently this is what you are supposed to do.

 

At the moment my front wall is completely untreated, and there is a TV on the wall in between the speakers.

 

The question is:

 

Should I cover the full wall (apart from the TV) or would that be an overkill, and putting the panels just behind each speakers would achieve the same goal?

 

Thanks!

 

 

 

 

[frankn]

@o2so 

What are you trying to achieve? improve imaging, improve depth/soundstage, clarity, reduce reverberations? 

What are the make/model are your speakers and how far away from the front wall are they?  Are your speakers aligned facing straight towards you or are they toed-in towards the listening position. 

Are the changes you are contemplating only for 2ch listening or also for HT?

For 2ch I would place a thick towel or a  cover over the tv. Then perhaps the diffusers around the tv. 

How many DC2 panels have you purchased?  I’ve got a few. 

 

[o2so]

[mention=149395]o2so[/mention] 
What are you trying to achieve? improve imaging, improve depth/soundstage, clarity, reduce reverberations? 
What are the make/model are your speakers and how far away from the front wall are they?  Are your speakers aligned facing straight towards you or are they toed-in towards the listening position. 
Are the changes you are contemplating only for 2ch listening or also for HT?
For 2ch I would place a thick towel or a  cover over the tv. Then perhaps the diffusers around the tv. 
How many DC2 panels have you purchased?  I’ve got a few. 
 

Thank you mate.
I'm using Maggies 1.7, about 90 cm from back wall, toed in.


I'm not sure what I'm trying to achieve, I was hoping for a general focus and soundstage improvement.

Have bought the dc2 yet. I need to decide between putting them on the side and bottom of tv or also on top. Perhaps on top is not necessary? They would be right against the ceiling.

[frankn]

Can you bring the speakers further into the room?  Have you tried it?  The current round trip of 1.8m for the back wave is just about 6msec which could be adding to sound colouration / time-smear at your listening position.   

I think if you are looking for focus/soundstage you should 1st cover the tv or put a mattress in front of it and listen - cheap test to give you some idea. 

Diffusers can assist in apparent soundstage/presence/depth. 

@andyr is a Maggie aficionado and will probably have some tips. 

@Red Spade Audio is another very good source of information for room acoustics. 

[andyr]

Paul is the guru - but, for best sound from Maggies, I would say:

1. 90cm is too little for optimal sound.  I am forced to have mine out only this distance and while I think my side-to-side imaging is sensational ... I don't get much in the way of sound stage depth.
2. So, yes, diffusion is required.  IMO, a panel about the height and width of the 1.7s is required - located behind each speaker so that the 'sound rays' bouncing off the front wall and hitting your ears, hit the diffusion panels.

 

Good luck.

Andy

 

[o2so]

Paul is the guru - but, for best sound from Maggies, I would say:

1. 90cm is too little for optimal sound.  I am forced to have mine out only this distance and while I think my side-to-side imaging is sensational ... I don't get much in the way of sound stage depth.
2. So, yes, diffusion is required.  IMO, a panel about the height and width of the 1.7s is required - located behind each speaker so that the 'sound rays' bouncing off the front wall and hitting your ears, hit the diffusion panels.

 
Good luck.
Andy
 

Thank you mate. So just behind the speakers, not all over the front wall. Right?

[aechmea]

I have DC2s in 2 single columns on the front wall where the primary reflection point back to the chair is.  They break up the tendency to beam from that bit of otherwise blank wall.  I think that they have made a positive contribution.

 

I have the feeling that covering all of the front wall, TV etc. is overkill.  Just get one box and try them at the first reflection point.

 

I have never found the TV to be a problem.  Its hard to work out how (in my room ) a reflection can get from the speaker to the TV and back to the chair without having bounced many times and/or be severely attenuated due to the distance traveled and/or the fibreglass absorption (largely) covering the back wall.

 

With planars the biggest issue (as you will already know) is the out of phase backwave reflecting off the frontwall, and the first reflection off the backwall.  Geometry, positioning and alignment are the key.  0.9m sounds a bit close and the reflection will be close on the heels (6 milliseconds) of the direct sound and will likely lead to some smearing (as suggested by Frank) and also have the SBIR nulls and peaks higher in frequency ie. more intrusive into the mid range. 

 

Changing the toe-in, even slightly, can change the path of the backwave leading to a different result.  I would experiment with that even to the extent of "crossing" in front of the chair. [This will probably mean that the backwave is aligned to the side wall or corner or...]

 

All good fun.

Edited December 24, 2017 by aechmea
spelling

[andyr]

1 hour ago, o2so said:

2 hours ago, andyr said:

Paul is the guru - but, for best sound from Maggies, I would say:

1. 90cm is too little for optimal sound.  I am forced to have mine out only this distance and while I think my side-to-side imaging is sensational ... I don't get much in the way of sound stage depth.
2. So, yes, diffusion is required.  IMO, a panel about the height and width of the 1.7s is required - located behind each speaker so that the 'sound rays' bouncing off the front wall and hitting your ears, hit the diffusion panels.

 
Good luck.
Andy
 

Read more  

Thank you mate. So just behind the speakers, not all over the front wall. Right?

 

Yes, just behind the speakers but situated so that the reflections from the rear-wave, onto the front wall and thence to your ears, hit the diffusors.

 

1 hour ago, aechmea said:

 

Changing the toe-in, even slightly, can change the path of the backwave leading to a different result.  I would experiment with that even to the extent of "crossing" in front of the chair.

 

 

Agreed!  My ribbons point at my knees. 

 

1 hour ago, aechmea said:

 

[This will probably mean that the backwave is aligned to the side wall or corner or...]

 

 

Except that the 'backwave' is not directional, like a torch beam - it "sprays" all ways.  So even though the perpendicular from the ribbon might be pointing at the corner of the room, you can still get a pathway going from the panels off the front wall to your ears.

 

The diffusion panel should interrupt this pathway.

 

Andy

 

[almikel]

21 hours ago, andyr said:

Except that the 'backwave' is not directional, like a torch beam - it "sprays" all ways. 

 

The diffusion panel should interrupt this pathway.

 

Andy

 

agreed - so diffusion ideally should be larger than 1.7s if you want to diffuse more of the backwave - but you could certainly start with 1 pack of 6 of the DC2s and put 3 behind each speaker positioned to intercept the early reflection paths and see how it sounds. You can always put more in later.

 

Have you got any other treatment in the room? If not then after some DC2s you could look at some broadband absorption to assist below 500Hz.

 

cheers

Mike

[o2so]

agreed - so diffusion ideally should be larger than 1.7s if you want to diffuse more of the backwave - but you could certainly start with 1 pack of 6 of the DC2s and put 3 behind each speaker positioned to intercept the early reflection paths and see how it sounds. You can always put more in later.
 
Have you got any other treatment in the room? If not then after some DC2s you could look at some broadband absorption to assist below 500Hz.
 
cheers
Mike

Thank you mate, will try with 6.
I've got broadband absorbing panels on the wall behind listening position because it's really close

[Primare Knob]

Interesting to see where it all ends up to. I am about to start venturing forth into acoustics soon myself for pair of Martin Logan's.

 

Doing some measurements I found a few impulse response peaks and their relative timings.

 

The first reflecting to arrive is the direct sound back wall reflection, closely followed by the first reflection point on the front wall, and then with a bit of a delay the speaker back wave wall reflection, and then with another delay the back wall reflection of the front wall early reflection and back wave front wall reflection.

 

Placing one absorption panel at the back wall early reflection point is taming 4 impulse response peaks.

 

What I have read sofar, you still want ellimate early reflection points as you do with conventional speakers but not the back wave of the speaker.

 

It seems this back wave reflection isn't coming from an early reflection point, and based on timing measurements in the impulse response graph it actually looks like it is hitting the back wall and travels through the speaker in a linear path. But that might be a wrong presumption on my end as I am still learning to measure and read the graphs.

 

 

[almikel]

On 1/11/2018 at 1:52 PM, Primare Knob said:

What I have read sofar, you still want ellimate early reflection points as you do with conventional speakers but not the back wave of the speaker.

it's a complicated topic.

With a speaker that has good "on and off axis" response (ie generally flat on and off axis, with the off axis reducing in level, but the same FR shape as on axis), Toole recommends not damping sidewall reflections. Geddes similar. This maintains "spaciousness" in the room. Damping 1st reflections from the floor and ceiling is considered a good idea,

On 1/11/2018 at 1:52 PM, Primare Knob said:

 but not the back wave of the speaker.

I've never owned open panel speakers, so I don't have direct experience in optimising their "in room" response.

One of their key advantages is the null created to the sides, so you don't "spray" the room with off axis energy. This is quite a localised effect, and I wouldn't expect damping the back wave at the perimeter of the room (ie the front wall) would impact the benefit of this side cancellation.

Many open baffle speaker owners deliberately damp the back wave of their speakers on the front wall.

 

IMO, with any room, getting the bass under control, regardless of speaker type, should be the 1st priority. Get the bass right and everything else is icing on the cake.

 

cheers

Mike

[aechmea]

I have planars in a quite heavily treated room.  Some might say, too much.  I have tried most configurations.

 

1.  Side wall absorptive treatment doesn't do anything much, if at all.

2.  Front wall absorptive treatment shrinks the soundstage but it becomes incredibly precise.  Swings and roundabouts.

3.  No absorption at the front is more vibrant, but much of the very strong backwave can bounce directly back to the seat ie. the first backwave reflection is a bit too strong.

4.  Bass response is greatly affected by speaker to frontwall and chair to backwall distances due to SBIR.  Probably greater than box speakers because all of the energy of planars only goes in 2 general directions rather than being sprayed.

5.  Linkwitz's idea of a dead backwall is effective.  Well, I like it.

6.  Neither absorption nor diffusion in front my TV screen makes any noticeable difference.

7.  Ribbons on the outside widens the soundstage into the proper shape.  When on the inside the image is stretched vertically.

8.  Big wobbly speakers really do need to be braced and then coupled through any carpet to the floor.

9.  I think that the 'waterfall' graph in REW is pretty good for seeing what is happening re treatment.

10.  I think that taming reflections/reverb time is at least as important as a 'flat' freq response.

11.  Get the bass right.

 

So what I have at the moment is ...

1. Diffusion panels at the front in a column where the first front reflection aligns with the chair.
2. Stacked tube traps in corners
3. Back wall almost covered with 100-200mm of full semi rigid fibreglass sheets
4. Lounges near back wall as absorbers
5. 'Spare' fibreglass sheets propped against bits of furniture out in the room. [Away from the walls (in velocity regions) is more effective for absorption than against walls (pressure regions).]
6. One sub positioned on the back wall and another on the side to fill the SBIR nulls of the other bass sources at the front.
7. Triangular buttress stands spiked through to the concrete.

You can tell something has been 'done'.  Walk into the room (with no music) and even the ambient 'silence' sounds different.

 

The overall effect is of sitting in front of a stage about half way back with the sound coming from that front direction.  Sort of a concert hall effect rather than being 'on-stage'.

 

[If you are doing a bit of theoretical planning of nulls, peaks, modes, SBIR, room geometry etc. such as the programs that proliferate on the web, then they will almost certainly be wrong.  They don't take into consideration that the backwave of a planar is 180º out of phase.]

[Blk plastic]

@o2so What did you end up doing and did it help?

[o2so]

[mention=149395]o2so[/mention] What did you end up doing and did it help?

Hey, so far I did nothing on front wall. However I covered side wall, which is a whole window, with a DIY velvety curtain , for which I purchased the fabric at spotlight for cheap. Very significant difference on SQ.

 

[o2so]

Hi everyone,

 

I haven't made a move yet and would like your advice on the right panels to go behind the maggies on the from wall. 

Ok one option is a set of DC2. But what about a set of wave wood instead?

 

https://www.vicoustic.com/product/wavewood

https://www.vicoustic.com/product/multifuser-dc2

The wave wood seems to absorb less on the high frequencies, which are mostly diffracted.

 

Which one do you think would allow me to:

- clean up the high frequencies

- increase sound stage depth

?

 

thanks!

Edited February 26, 2018 by o2so

[frankn]

It is a bit hard to compare apples-to-apples with the graphs as the Wavewood absorption profile is normalised to 1 (no absorption) @ 630Hz but the DC2 values appear to be absolute.  I'd say from 1500Hz - 5000Hz the Wavewood is absorbing more energy.  But the DC2 is diffracting a LOT more energy.  IMO it is the specular (diffused) energy that assists in the illusion of spaciousness.

I'd suggest DC2's on the front wall and the ceiling (1st reflection point) and then listen to the results.  Only then if you have a problem with too much brightness/hf then look at Wavewood or some other absorption panel on the sidewalls at 1st reflection.  

[o2so]

thanks mate

[frankn]

Another place to try the DC2 (or diffusion panels) is on side walls behind the speakers - especially as you have them toed-in.  

[o2so]

1 minute ago, frankn said:

Another place to try the DC2 (or diffusion panels) is on side walls behind the speakers - especially as you have them toed-in.  

Not possible unfortunately, one side of the room is all glass (covered with thick velvety curtain), and the other side opens to the dining room. Not the ideal room I know.

[frankn]

Not many people have ideal rooms - it is often about the compromises you make but that get you the best sound.  It is all relative eh.... 

[o2so]

So I finally made the move and purchased the dc2s. My first thoughts are that the soundstage has deepened a fair bit when the Maggies are at 50 cm from front wall (I used to keep them there for casual listening).

The difference is more subtle when I put the speakers in the "proper" listening spot, at a distance of 120cm from front wall. Will do more listening and report back.

 

 

[HdB]

I understand there's a pattern that these units are raised out from the wall - apparently, there's a mathematically derived ratio that ensures a minimum comb filtering - I don't know much about multiple Skylines and often see them just 'wall-to-wall' as you have done, so I don't know how significant it is. 

 

It seems to be an essential part of the design of multiple 1D Shroedder diffusers, so I'd expect a similar advantage if you used it with these 2D ones too. 

 

 Mike may be able to add some details here ...  

[almikel]

1 hour ago, HdB said:

I understand there's a pattern that these units are raised out from the wall

Hi HdB

I'm bit confused by the "pattern that these units are raised out from the wall" comment, especially the "raised out from the wall" bit.

 

There's a minimum seating distance criteria for 1D and 2D QRD diffusors, which won't be an issue in this case (since they're deployed on the front wall).

 

There's also an issue with multiple identical panels of small "order" say N5 of N7 panels being repeated, and getting "peroidicity" issues. With a long sequence of the same panel the diffusion has stronger peaks/lobing - Cox and D'Antonio recommend using an "inverse" panel in the mix, or just turning one of the panels 90 degrees.

This is more an issue with a long sequence of 1D low order QRD panels - less an issue with 2D.

In this case, the number of panels in a row is low, and they'e also 2D.

If @o2so was worried, then rotating 1 of those panels 90 degrees would interrupt any periodicity, but I doubt any difference would be heard.

 

@o2so - nice work 

 

cheers

Mike

 

[o2so]

Wife is not overly happy though