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In Reply to: RE: Front wall room treatment for dipoles, Maggies, others posted by bryan on November 25, 2009 at 14:52:08
The animated GIFs and descriptions on this page describe the radiation patterns nicely.
-- Nils
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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.
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
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.
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/ListeningRoom.html
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
Edits: 11/26/09
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.
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'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
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