[Morespam]

Hi,


I have been reading a bit about room treatment (as it relates to HT in small rooms ~14x16) and the general consensus seems to be the use of absorption on the front wall(100%), side walls (~50%), rear wall (~50%), and ceiling (~50%) along with some base trapping in corners.


My question is, wouldn’t this absorption kill the diffuse field the rear speakers are trying to create?


Also does room treatment differ when using dipoles as surrounds?

 

[Kensmith48]

I've been asking this question about bi-poles (Def. Tech.) for a few years now. Everyone seems to avoid answering. I've sent letters into a couple of magazines also with no answer. Good luck.

 

[Morespam]

GREATTT!! why are the questions I ask always the ones with no bloody answer!


Bump



Anyone? even an opinion, speculation anything.....

 

[Terry Montlick]

Here's an answer, though you might not like it.


IMHO, surround-sound dipoles were a compromise by THX which exploited the relatively live home listening rooms of earlier days. Instead of adding more surround speakers, as is done in professional cinema rooms to decrease surround localization, why not just use dipoles to get a similar effect by using the liveness that you are stuck with anyway? I think it was a good idea by Tom Holman at the time.


I believe that the superior solution in today's dedicated home theater rooms, which (hopefully!) have proper treatment for acoustical accuracy, is to use additional side surrounds. If you're already running a 7.1 system, what's a couple of extra side surrounds?


Regards,

Terry

 

[John Robert]

Terry:


Would you just split out and feed these "extra side surrounds" the same signal as the existing 7.1 surrounds???


John

 

[Terry Montlick]

Quote:

Originally posted by John Robert Terry: Would you just split out and feed these "extra side surrounds" the same signal as the existing 7.1 surrounds??? John

Yes. You can wire extra surrounds in parallel or in series, depending on your amplifier's output impedance rating.


- Terry

 

[John Robert]

Would those extra surrounds also be dipoles???


TIA,


John

 

[Terry Montlick]

Well, you certainly want to match existing side surrounds, whatever they are.

- Terry

 

[W4ZOO]

Quote:

Originally posted by Morespam Hi, I have been reading a bit about room treatment (as it relates to HT in small rooms ~14x16) and the general consensus seems to be the use of absorption on the front wall(100%), side walls (~50%), rear wall (~50%), and ceiling (~50%) along with some base trapping in corners. My question is, wouldn’t this absorption kill the diffuse field the rear speakers are trying to create? Also does room treatment differ when using dipoles as surrounds?

My impression on dipoles is to use diffusion in the rear and maybe the side back parts to maintain a good sound field.

 

[Morespam]

Awesome, thanks for the feedback.


I thought just as much (about dipoles as rears) considering the theory they are based on.


I am currently in the buying phase of the project so I haven’t bought any rears. The room will be properly treated (~40% absorption) so I might likely go with 4 surrounds rather then 2 dipoles then.

 

[Jim McDougall]

Go to the Sigfried Linkwitz web site. ( www.linkwitzlab.com ). The section on room acoustics is an excellent primer on how dipoles interact with a room. Using dipoles is not alot different than the general rule above. However, placement is more important. You want to kill the early reflection from the radiation pattern adjacent to the dipole speaker i.e. The wall surface immediately on either side of the dipole. Also, the speaker(s if 7.1) should be positioned so that the main listening position is in the null of the dipole. This means that the speaker should be in a straight line with the prime listening position. Since surrounds are also mounted higher. The ceiling area above the speakers (half circle) should also be treated as absorbent as this will be another source of early reflections. The point is to kill as much early reflection as possible. Early reflections from any speaker in a HT setup cause one of two problems. Early reflections frelating to front speakers muddy the sound and can cause voice to be blurred, indistinct and hard to hear/understand. From the surrounds, it allows the ears to pinpoint the surround sound location, killing the purpose fo the surrounds which is to create a distributed sound field. THX did not spec dipoles as a cheap way around a problem. Mounting two monopole surrounds being fed the same signal creates an illusion of a single monopole located between them (virtual center). In a theater, the array of surround sound speakers are located waayyy above people's heads. There are no early reflections that can reach the audience at any level to create localization!

 

[BasementBob]

Jim McDougall

With a dipole there is much less energy radiated above and below the speaker than along the wall. The null is a plane. So a "ceiling area above the speakers (half circle) " may not be the most cost effective shape. Perhaps two absorbers on the ceiling, out a bit from the speaker, assuming of course that absorbing surround energy is a good idea in the first place which I'm not so sure. Would not letting a dipole surround build up a complex sound field using early reflections be a good idea ?

 

[Morespam]

"Would not letting a dipole surround build up a complex sound field using early reflections be a good idea?"


Bob you summed up my initial question well here, and as far as I have been able to deduce this seems to be true. You want a dipole to diffuse sound and create this spacious non-distinct "surround" field.


What I am interested in is how this relates to treatment of the room, going on what has been said here already and what I have read, absorption (or to much of it) when using dipoles could possibly impede their functionality...


In which case you are left in a tricky position because absorption required for the front three speakers is going to interfere with how well the surrounds (dipoles) will function.


What Terry said further up makes a lot of sense then and that is dipoles are better suited in a non dedicated room, and monopoles or direct radiating surrounds are better suited for a dedicated room with proper treatment (mostly absorption).

 

[BasementBob]

Morespam:

Quote:

In which case you are left in a tricky position because absorption required for the front three speakers is going to interfere with how well the surrounds (dipoles) will function.

The absolute quality of the sound and imaging from the front 3 speakers is paramount, more important than modes, and much more important than surrounds.

I note that many DE designs have the front wall covered with insulation, and the bottom half of the remaining 3 walls with insulation.


I believe that a possible problem with corner traps in HT is that the dipoles are aimed at them. Admitidly the corner isn't the whole wall, nor even more than half of it, so perhaps I'm worried about nothing. Nevertheless one possibility is to put something reflective over the top half of a corner absorber.


I think reflections are good for surrounds.

But you don't want that sound bouncing around your room for 8 seconds. Lower wall absoption, and furniture and people and stuff, will keep the RT60 down to a reasonable range.

 

[Jim McDougall]

BasementBob

"With a dipole there is much less energy radiated above and below the speaker than along the wall. The null is a plane.'


From a total energy output, that is a true statment, but it varies over freq. and it dependent on the width of the dipole baffle. The midrange on a typical baffle 300hz --- 2000hz does act as a true dipole. However, the higher freq range does not as the wavelength is higher than the baffle width factor and acts much more like a bipolar. This gives the higher freq much more directionality and that is the reason for doing some absorption in the immediate vicinity both side and top.


"Would not letting a dipole surround build up a complex sound field using early reflections be a good idea?"


Early reflections have by defintion the lowest db loss, 1st incident energy loss. Also 1st relections have the lowest time domain loss. That is why for the fronts, most designs try to absorb these early reflections from the immediate back and side walls, because they will arrive at the listen area with a very short time delay and at a level only slightly below the original signal. Because our hearing is not very time acute, we hear these waves in sequence with the original wave and when our brain try to process, we hear a sound that is 'muddy'. We canot resolve the clarity of the origial wave. In HT this is most noticeable as difficulty with soft spoken speech. With surrounds, it causes localization rather than imaging a diffuse sound field. The reference on the Linkwitz website explains it in detail although it is pointed more towards front speaker music listening the same priciples apply. As most of you are aware, Sigfried Linkwitz is the Linkwitz in Linkwitz-Reily crossover designs.

I run Dipoles all the way around. see www.jim-mcdougall.com/id3.htm and look through the photo's section (these are about 4 years old now). The final pictures of the Phoenix Grand HT's were two apts ago. They now sit beside a 80"wide 16x9 screen fed from a Marquee 8500.

 

[LarryChanin]

Quote:

Originally posted by Terry Montlick Here's an answer, though you might not like it. IMHO, surround-sound dipoles were a compromise by THX which exploited the relatively live home listening rooms of earlier days. Instead of adding more surround speakers, as is done in professional cinema rooms to decrease surround localization, why not just use dipoles to get a similar effect by using the liveness that you are stuck with anyway? I think it was a good idea by Tom Holman at the time. I believe that the superior solution in today's dedicated home theater rooms, which (hopefully!) have proper treatment for acoustical accuracy, is to use additional side surrounds. If you're already running a 7.1 system, what's a couple of extra side surrounds? Regards, Terry

Hi Terry,


Some folks, including you in an other thread, seem to have some concerns about comb filtering distortion when identical speakers are playing the same signal. Here's your response to a question I had about stacked dual center channel speakers.

Quote:

Originally posted by Terry Montlick Yes Larry, comb filtering is a manifestation of the phase issue that concerns me. For a 1 kHz sound, a 1/2 foot difference in the distance from the two speakers results in 100% cancellation. So this arrangement is not very good for those who are not sitting at exactly equal effective source distances from the two center speakers. - Terry

Don't you think the likelihood of individual listeners sitting at exactly equal effective source distances from multiple side surround speakers is even less likely than from two stacked center speakers?


I notice that an occasional professional designer will use multiple side surrounds in some of their upscale theaters, but these are huge rooms that more closely approximate commercial theaters with multiple surrounds.


Didn't THX originally recommend dipoles, not so much due to liveliness of typical home theaters, but more importantly due to the relatively small size of home theaters? In such close quarters wouldn't having direct firing monopole surrounds literally "in the faces" of some of members of the audience more likely result in localization problems than positioning the home audience in the null of a dipole?


Thanks.


Larry

 

[Terry Montlick]

Hi Larry,


Yes, there will be some random phase cancellation due to comb filtering, but this is reduced by placing the surrounds high, far apart, and not aiming them directly at the nearest seats. Aiming home monopole surrounds at the ceiling is a good strategy.


Commercial theaters overcome this difficulty by spacing the surrounds no closer than about 9 feet, and by aiming at the seats at the opposite wall. Fortunately, our ability to accurately localize side and rear sound is inherently much poorer than for front sound, so we don't have to take extreme measures to minimize such localization.


I'm not suggesting that you throw away your dipole surrounds and replace them with monopoles! The original question concerned the apparently problematic requirements of combining dipoles with acoustically "dead" walls. In any engineering solution, there will be compromises. In the original decisions about home THX, a home theater room which had aggressive acoustical wall treatment was probably not a major consideration.


In any case, sound will still get reflected from the absorbing walls to create a diffuse surround sound field. The downside is that one has to supply more power to dipoles, since more sound gets absorbed by these walls.


Regards,

Terry

 

[LarryChanin]

Quote:

Originally posted by Terry Montlick Hi Larry, I'm not suggesting that you throw away your dipole surrounds and replace them with monopoles! The original question concerned the apparently problematic requirements of combining dipoles with acoustically "dead" walls. In any engineering solution, there will be compromises. In the original decisions about home THX, a home theater room which had aggressive acoustical wall treatment was probably not a major consideration. In any case, sound will still get reflected from the absorbing walls to create a diffuse surround sound field. The downside is that one has to supply more power to dipoles, since more sound gets absorbed by these walls. Regards, Terry

Hi Terry,


Thanks very much for the response.


Getting back to the original question, I'd like to pick up on Basement Bob's comment regarding Dennis Erskine's (DE) designs. As Bob mentions, in Dennis' designs generally only the bottom portions of the side and back walls have insulation. Since dipoles, for the most part, rely on the immediate side wall reflections, for these acoustic designs, even though overall the room is fairly dead, wouldn't the "dead walls" be a non-issue for dipoles mounted on the reflective upper "half" of a side wall?


Thanks.


Larry

 

[Terry Montlick]

Quote:

Originally posted by LarryChanin As Bob mentions, in Dennis' designs generally only the bottom portions of the side and back walls have insulation. Since dipoles, for the most part, rely on the immediate side wall reflections, for these acoustic designs, even though overall the room is fairly dead, wouldn't the "dead walls" be a non-issue for dipoles mounted on the reflective upper "half" of a side wall?

Well, Dennis and I disagree about his claim that polyester batting is a diffusion material and not an absorption material. The absorption coefficients of 1-inch polyester are reasonably high (though not as high as 1-inch 3 pcf fiberglass). So the upper walls are still relatively dead. We're also dealing with large angles of incidence with the side walls, and under these circumstances, lower density will actually increase absorption.


- Terry

 

[outlier2]

Best thread I've seen in a long time and very much overdue. So, are we okay going with absorption on the front wall and partial side wall absorption with Dipoles located in the back third of a small room? I had tried monopoles once and the sounds quickly localized and were distracting to my ears. Nice for MC music though. Overall I like the dipole effects better and I don't want to ruin it by over absorbing the reflections. I plan to have no absorption on the back wall, full absorption on the front wall and I'm still contemplating whether or not to do much to the side walls. Any thoughts on what might be best?