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Front wall room treatment for dipoles, Maggies, others

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Posted on November 25, 2009 at 14:21:07
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
Just curious if anyone has actually done any experimenting with different room treatments on the wall directly behind dipoles such as Maggies. I'm not talking about bass trapping in the corners or the middle of the wall, but the part directly behind the speakers. And forget about only posting theory, lets talk about actual experiences comparing absorption versus diffusion since these are probably the two most likely treatment types to be used.

I have found the following so far in my limited experience. In my case the front wall is pretty much one big window with horizontal metallic venetian blinds covering most of glass except up near the ceiling. If I put bass traps (not HF reflection controlling absorbers unfortunately) on the front wall behind the speakers I lose some detail and some soundstage dimension and some air. If I used HF absorbers I suspect I'd lose even more detail and air. The absorption I used did have some benefit too. There was less glare and hardness and it was smooth, but a bit dull.

Going back to no absorption, the binds do not really do any proper diffusion to speak of. Maybe they do more reflecting which does highlight the dipole soundstaging ambiance which is nice in a sense. Angling the blinds allows me to bounce the energy down to the carpet and attenuate some HF, which is nice. I really hope to try some real QRD diffusion behind the speakers one day soon. And I'm not sure what to expect from diffusers other than soundstage ambiance. I've heard that diffusers can reduce combing (my limited understanding of combing is it like acute noise rather than big frequency response peaks and valleys) but I don't know what that will sound like as an improvement. More detail maybe?

I'd be interested hearing other peoples experiences treating that front wall.

Bryan

 

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RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 14:26:31
Neuro
Audiophile

Posts: 668
Location: Bay area, California
Joined: June 29, 2001


That's my front wall. I found pure diffusion to be too, well, diffuse, and pure absorption too be too dead. I found GIK D1 panels to perform better than RPG Skyline diffusors (it seems that 1-dimensional diffusors play better with the planar radiation pattern than 2D diffusors). I also enjoy the ASC StudioTraps, which have diffusive properties at higher frequencies.

Remember that repeating a single diffusion panel over and over will create lobing problems.

-- Nils

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 14:35:52
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
I imagine the picture isn't really good at showing how all that lines up on the front wall. But it looks like your speakers are quite close to the side walls, and toed in, and their back wave is firing directly in to the corners where the Tri Traps are located. I was told that dipoles are direct radiators out the back and it really looks like your D1's are not directly behind the speakers. Is it just the picture that makes it look like that?

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 14:43:26
Neuro
Audiophile

Posts: 668
Location: Bay area, California
Joined: June 29, 2001
They radiate towards the back corners, but remember that dipoles tend to radiate in a figure-8 pattern (with the rear wave 180° out of phase), and that you should be more interested in the first reflection points off the front wall.

-- Nils

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 14:52:08
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
Do you know of a picture or diagram that shows this figure 8 pattern? I'm having a hard time picturing what that looks like and how your diffusers are even catching part of the back wave so they can do their thing. When I spoke with GIK about the best location for the D1s, I was told definitely directly in line with the speakers. Not to the inside of them.

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 15:40:25
Neuro
Audiophile

Posts: 668
Location: Bay area, California
Joined: June 29, 2001
The animated GIFs and descriptions on this page describe the radiation patterns nicely.

-- Nils

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 16:03:51
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
Thanks a lot. I like the visuals. I see the dipole radiation pattern on the right side that looks like a sideways figure 8. But isn't the half of the 8 indicating the patter than fires one direction out of the speaker (e.g. the front), and the other half is what goes out the other direction (e.g. the back). If true, that would mean the back wave to the front wall would not be a big figure 8 that spreads out towards the front wall, but rather it's a tighter radiation pattern. Which you mean that you want diffusion directly behind the speaker. Maybe I'm not comprehending the pattern right.

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 16:14:08
Neuro
Audiophile

Posts: 668
Location: Bay area, California
Joined: June 29, 2001
You need to worry most about the first reflection points in a room because not treating these points will have the most degrading effect on sound quality. See the image below (taken from http://geekwithfamily.com/2006/08/23/home-theater/room-setup/why-i-need-to-build-jon-rischs-diy-acoustic-panels/



-- Nils

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 16:40:54
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
I know first reflection points are key, but with my ET's the side wall reflection point is not that critical as there is little sideways radiation. I'm more concerned with frequency response of the room which I have mostly addressed with bass traps, and the front wall reflections which I hope to address with diffusion and some absorption. And the ceiling reflection points are a concern too.

I'm going to contact Bruce Thigpen and find out more about the front/back radiation and where diffusers should be located on the front wall.

 

Experiment is right, posted on November 25, 2009 at 17:07:43
Barry
Audiophile

Posts: 364
Joined: November 24, 1999
Contributor
  Since:
January 18, 2009



A little bit of everything (you can't see the homemade 7 foot cylindrical absorbers that are placed ~ 2 feet offcenter behind the tweeters - thankfully. They're ugly.

Venetian blinds (hard?) & glass sound like a bad combination - even worse then my concrete. I'd get your wife to make some full length adjustable drapes if absorption's too much.

Have fun!
Barry

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 18:15:18
RickeyM
Audiophile

Posts: 2061
Location: East Coast
Joined: March 15, 2003
I agree with Neuro about the first reflection points on the side wall. Try this to illustrate. Stand at the first reflection points and if you can hear whatever your speakers are playing, that untreated wall is bouncing all that you're hearing, right back to your listening position. Walls make lousy speakers and most assuredly are mucking up the sound of your real speakers.

If you want a cheap way to try diffusion, hang some bubble wrap (yes, bubble wrap) behind your speakers. The large bubble variety makes an excellent diffusor and can show you where "real" diffusors will work best & how much diffusion you may need.

 

I started with, posted on November 25, 2009 at 18:33:11
gymwear5@hotmail.com
Manufacturer

Posts: 2522
Joined: April 10, 2002
4 inch wedge absorbers - It worked well with my mini monitors - also used fabric absorbers at first wall reflection. Sound was not very dynamic. my MMG's sounded weak. Pulled off the side and rear treatements and all is better. FYI I have a 55" rear projection between the MMG's that are spread about 6 feet apart at their inside bases, Room is 15 feet wide sor the direct rear reflection hits the back wall and caroms around the corner back towards the listening position Side wall software and hardware racks may diffuse the reflected sound a bit however.


"The hardest thing of all is to find a black cat in a dark room, especially if there is no cat" - Confucius

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 19:26:24
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
I'm sure I'll hear the speaker anyway. That test doesn't make any sense to me. I can hear my speakers outside the room so I'll hear them standing on the side wall. The designer of my speakers, which are planar magnetic, stated that there is very little dispersion or radiation to the side walls. He said they radiate more vertically, and it is more important to treat the reflection off the ceiling.

However I do have some furniture along the side walls and that is a bit of an acoustic concern but I can't do much about it. If I could, I would put some absorption up for sure because every bit of acoustics improvement is worth going for.

Thanks for the tip on the bubble wrap, but the acoustic guys would probably say it isn't much of a diffuser. If you haven't seen this video, grab some good headphones and check it out. It does a good job of letting you hear how bad a bare wall is for diffusion, how a bookshelf isn't very good either, and also shows how a couple of real diffusers sound too.

http://www.realtraps.com/video_diffusors.htm

Bryan

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 25, 2009 at 21:54:46
GStew
Audiophile

Posts: 608
Location: NE Mississippi
Joined: September 21, 2001
My most sophisticated room treatment (realize this was back in the late 80's) was with homemade tube traps and a pair of highly modified Acoustat 1+1's.

The main traps were 12" diameter 7' tall tubes of 1" polyfoam with foam core board circles top, bottom, and middle. They also had 1/2 covered with plastic (heavy trash bags) to do the absorb/reflect thing the tube traps do.

I had one in each corner behind each speaker, one on the wall directly behind each speaker, one on each sidewall directly beside each speaker, and one at the 1st reflection point on each wall. But then I also had one directly behind each speaker leaning up against it. I would not say these made the biggest difference... it was definitely an ensemble thing, the whole was greater than the sum of the parts. But these made a significant improvement in clarity and detailing and it was very noticable when they were removed.

YMMV

Greg in Mississippi

Everything matters!

 

I use these DIY Jon Risch-designed "Room Lens" copies ..., posted on November 26, 2009 at 02:18:07
andyr
Audiophile

Posts: 10756
Location: Melbourne
Joined: September 2, 2000
to diffuse the direct back-reflection from the ribbon to my ears.



(This pic deliberately shows the LHS Room Lens without its cover, so you can see how it is constructed.)

Amazingly enough, this deepens the soundstage!!?? :-))


Regards,

Andy

 

RE: I use these DIY Jon Risch-designed "Room Lens" copies ..., posted on November 26, 2009 at 06:05:13
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
Andy, I don't believe those room lenses are diffusers. I know they are technically HelmHoltz resonators, but I don't know if they diffuse or not. I had the Argent version years ago when I had different, direct radiator speakers. I would have liked to try them now with my planar speakers.

Diffusers are supposed to increase soundstage depth like you experienced with your DIY resonators so that is one thing they seem to have in common. The resonators do sound like a really interesting option for the front wall reflection point.

Your room looks like a great space for adding bass traps. The front wall corners are naked and just begging for some bass traps! Bass trapping could provide you with a lot of benefit that the resonators do not. Better frequency response in the bass and mids giving you a lot more detail. If you are interested in exploring this, I recommend measuring the bass build up in your room using a filtered LF pink noise tone and an SPL meter or even your ears. Knowing there is bass build up, and where it is, is a good start. If you are getting bass build up you can DIY some tubes or chunks, or just buy some of the affordable corner bass traps like the GIK Tri Traps.

You can burn the MP3 file on this page.
http://www.realtraps.com/lf-noise.htm

And play it back on repeat and use an SPL meter and measure the db's in the corners of the room from the floor to the ceiling and along the ceilings and wall boundaries (use a ladder) as well. You should notice higher levels in these places which indicates there is bass build up. You can also measure at your listening position to confirm it is not in a place where there is bass buildup. If you don't have an SPL meter but have an iPhone or Touch (with a mic) there are a couple of SPL apps that will give you relative measurements which is all you need for this test. If you don't have any devices for measuring you can use your ears. You will hear the bass build up (e.g. in corners) sound like rumbling. You will hear the jet engine like pink noise from the test tone all over the room, but in places where there is bass build up you will also hear the rumbling. That is my experience.

Bryan

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 26, 2009 at 07:47:03
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
Greg, do you know the back wave radiation pattern of the LFT 8s? I have an email to Bruce asking him but figured I'd ask you too. I want to treat that back wave with diffusion, or at least try it out, and I really want to know where the energy direction goes so I don't have to guess where to put the diffusers. I have different theories right now. One is directly back from the speaker factoring in toe in, so it would head towards the front wall corners. The other is an angle that is inwards of the speakers so it heads to the middle of the front wall as seen on this page.
http://geekwithfamily.com/2006/08/23/home-theater/room-setup/why-i-need-to-build-jon-rischs-diy-acoustic-panels/

Bryan

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 26, 2009 at 09:32:26
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
I heard from Bruce Thigpen. He says the radiation pattern of the dipole is a figure 8. When looking from the top of the speaker, one half of the 8 radiates forward in an oval shape, and the back half is the same. He says the link you provided with the reflection point directed inwards to the center of the front wall is for a cone, direct radiator and is not applicable to dipoles. Furthermore, he says diffusers need to placed directly perpendicular to the speaker, on the front wall. Not in the center of the wall. So I'm not sure your D1's are really in the best location if you were really wanting to diffuse the back wave. Just an FYI for you.

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 26, 2009 at 10:03:36
GStew
Audiophile

Posts: 608
Location: NE Mississippi
Joined: September 21, 2001
One of the things that is inherent to dipole speakers (the LFT-VIIIs from about 100hz up) is that they radiate mostly front and back and the sound is largely cancelled to the sides. I've seen diagrams of dipole radiation patterns that show lobes going directly forward and aft from the panel 90 degrees to the panel face. So that's where your main back-radiation will be.

There will be a significant amount of sound to about 45 degrees off-axis, but it will diminish rapidly from there.

Bass is a different thing with the LFT-VIIIs and it will be close to omni-directional due to the small size of the bass driver and cabinet. There you want to treat the room modes (especially the corner modes with bass absorbers like tube traps. This should be true from about 200hz on down.

Note that there is significant overlap in the two radiation patterns due to the slow-rolloff (6-db/octave, 1st order) crossovers.

Greg in Mississippi
Everything matters!

 

RE: I use these DIY Jon Risch-designed "Room Lens" copies ..., posted on November 26, 2009 at 11:45:07
andyr
Audiophile

Posts: 10756
Location: Melbourne
Joined: September 2, 2000
Hi Bryan,

According to Jon Risch's analysis (alas, now unfortunately vanished), the Argent Room Lenses have a dual effect:

1. they are a (not very effective in my room!) Helmhotz resonator for an 8'-ceilinged room. As you can see from the pic, my room has a pitched ceiling ... which pretty much curtials their "resonator" properties.

2. they diffuse. If you take the first reflection point from the ribbon to the front wall and thence to my ears, the pipes are smack bang in the way. The curved surface of the pipes and the fact that the 3 pipes are carefully spaced apart different distances top & bottom, results in them acting as a brilliant diffusor.

Thanks for the info on bass traps and how to tell if you need them; I will do the measuring. :-))

Regards,

Andy

 

Some ideas., posted on November 26, 2009 at 12:21:39
Al Sekela
Audiophile

Posts: 9169
Location: Northern California
Joined: February 18, 2002
Experiment with small area treatments located where you would see your image if the wall were covered with mirrors. Too much area or too much absorption will detract from the dipole behavior, so start with a folded blanket covering a couple of square feet. Wool would be the blanket material of choice, since synthetic fibers have strong acoustic tonal colorations.

Glass is a problem in a listening room. Try rapping the pane with a knuckle and listen to the response. Realize that the glass is adding some of this response tonality to the reflected sound from your speakers.

There are expensive devices to treat glass panes, but a cheap way to experiment is with gel decorations. These cling to glass without adhesive and come off easily. They are in stores now for holiday decoration purposes. Try different sizes and shapes and in different locations on the glass. I would put them near the corners and at the major nodal points on the major axes.

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 26, 2009 at 14:41:45
Jim Treanor
Audiophile

Posts: 1501
Location: Pacific Northwest
Joined: June 1, 2003
You already know some of my thoughts on this, Bryan, but I'll reiterate what has been the subject of a private discussion about actual experience rather than theory.

I've found center-of-front-wall diffusion to be more effective than diffusion directly behind my 1.6QR's (at whatever distance). With respect to perpendicular-to-panel-plane placement, that's what ASC recommended when I inquired regarding Studio Trap positioning, but soundstage presentation didn't sound natural with the trap diffusor sections oriented that way; I got much better results with diffusors oriented as diagrammed at jimtranr.com/ListeningRoom.html, including sidewall diffusion pointed directly at the rear of the room rather than facing out into it, whether perpendicular to the radiating plane or at any angle other than straight toward room rear.

I don't cite any of this as prescriptive, but rather as descriptive of my own listening situation in a 26'3"x14'3"x7'8" rectangular room with the panels positioned in a Cardas dipole configuration and facing the long dimension. As you've probably concluded, it doesn't correspond with the theory propounded by manufacturers you've consulted and suggests that you may want to experiment a bit more, given the particular acoustic characteristics of your listening space.


Jim
http://jimtranr.com

 

RE: Front wall room treatment for dipoles, Maggies, others, posted on November 26, 2009 at 16:07:22
bryan
Audiophile

Posts: 1170
Joined: June 6, 2001
Jim, I think I am going to do lots of experimenting on that front wall. Not only where to put the diffusers, but whether I use absorption instead. If I do use diffusers on that wall, it will be in conjunction with absorption. Should be interesting.

Bryan

 

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