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post #91 of 109 Old 04-30-2014, 03:23 PM
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Another question. If ceiling interactions are a concern with waveguide type surrounds, could this issue be resolved by simply mounting them a little higher and tilting them down? And can someone explain how a coaxial would be an improvement over a waveguide for surrounds?

I think a coaxial would have a more dispersed sound while a waveguide is more direct. 

A lot of coaxial speakers have waveguide speakers as the inner drivers:

http://www.usspeaker.com/b%20&%20c%20index-1.htm#COAXIAL SPEAKERS

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post #92 of 109 Old 04-30-2014, 07:46 PM
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I told you that the plots were available, but I think I was mostly wrong. I've been through most of the threads over at DIYSG just now, and only found two designs that had vertical plots. They are enough to illustrate the point, but I'm not sure how informative they are to your decision.

Both of these designs were done my Matt (mtg90 - I hope he doesn't mind my reposting them) about a year ago for Bass Addict. The first is a large monitor for main channel use using the AE TD15M midbass driver, crossed to a compression driver (he developed two crossovers) around 1150 Hz. You can see in the plots I'm posting here that within 10 degrees of horizontal, there is no change in response (maybe 1 dB) but up or down 20 or 30 degrees, the response dips 10dB (above axis) or even 20dB (below axis).

Above axis:


Below axis:


This second design uses smaller midwoofers - 2, 6" woofers side by side - and was intended for use as height channel loudspeakers (maybe surround for Bass Addict, too?). In this case you can see that the response above axis does not dip at the crossover (around 1500-1600Hz), while below it does. I'm not sure what to attribute this better vertical behavior to, but I think it's the closer center-to-center spacing afforded by the smaller midwoofers. The notch around 700Hz is not related to the crossover, but you can see that the vertical axis does play a role, because it has to do with a reflection, and therefore two or more apparent sources combining in space.

Above axis:


Below axis:


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post #93 of 109 Old 04-30-2014, 07:59 PM
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I've posted vertical plots for my early designs. But no one seemed to use the info. If someone could use that info in sure it could be made available with a PM to the designer.

BTW, great posts Fred.
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post #94 of 109 Old 04-30-2014, 08:11 PM - Thread Starter
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I believe I'm beginning to understand the issue a little more. My thought was if the directivity is controlled over all frequencies across say 40 to 50 degrees in the vertical direction, then it would be fairly easy to angle the surrounds so that the LP was still in the sweet spot, but the off-axis nasties would be directed over the LP.

It doesn't look like it works that way though. If I understand what I've been reading correctly, then the directivity is controlled over a beamwidth for a certain range of frequencies, but below that (the crossover region I believe) the beamwidth begins to widen. I think this is what Fred was trying to tell me in his earlier post, but I'm just now catching on.

That said, there doesn't seem like much that can be done with non-coaxial speakers as surrounds other than put treatments on the ceiling. I can't help but wonder if I'm missing something since you rarely see treatments on the ceiling, particularly above the seating where it would be necessary to treat for the surrounds.

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post #95 of 109 Old 04-30-2014, 09:11 PM
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I'm learning as I go here too, but I think I can help you with a couple points from this last post.

The beamwidth does vary - but that's not really relevant to the issue of lobing at the crossover and the ceiling reflection. With the two-way SEOS designs, there will be two patterns of beamwidth variation. The woofer will transition from omni-directional radiation to narrow beaming. This is simply a function of the relationship between driver size and wavelength of sound. It's the same in the vertical as horizontal. It can be influenced by baffle size and shape to some extent (this is part of the purpose of baffle walls). In a well-handled crossover, the beamwidth of the woofer at crossover will match or nearly match the beamwidth of the tweeter at crossover.

The waveguide allows this trend (transition from omni- to narrow beaming) to be changed. SEOS waveguides are 90 degrees horizontal and 60 degrees vertical, so within their bandwidth (which varies based on the size of the SEOS - SEOS10 has a smaller bandwidth than SEOS24), the beam of sound is narrowed (and also spread if I understand correctly) to the nominal 90x60 pattern. SEOS does this quite well. Above some frequency, the tweeter may begin to beam excessively, and the waveguide may be unable to disperse the sound to the normal pattern. All of that can be seen in this plot, stolen from the hey guys... thread:


Getting the patterns to match in the horizontal plane is important to SEOS designs. Some variation is acceptable - a little waistbanding is fairly common and shouldn't be bad to listen to. If the crossover is higher in frequency than ideal, the pattern of the woofer will be narrower than 90 degrees just below the crossover, and then widen back out to 90 degrees once the waveguide takes over. When measured 45 degrees off-axis, the response will show a dip at the crossover frequency in this case. Potentially then, the lateral reflected energy also shows that dip. The dip is small and dips are easier to listen to than spikes, so waistbanding is acceptable to most people. Wayne Parham (Pi Speakers) has a nice description of a lot of this, here.

Now let's start looking at the vertical issues. If the woofer beamwidth was matched to the horizontal pattern of the waveguide at crossover, it can't also be matched to the vertical pattern - since the waveguide is wider than it is tall. If the woofer beam was down to 90 degrees at crossover, now the pattern jumps to 60 degrees in the waveguide. Is that a problem? No, not really.

The real problem is totally independent of changes in the beamwidth. It's an interference pattern due to the fact that there are two acoustic sources at the same frequency. I'm going to copy from rane.com, because the description is as succinct and complete as I can imagine: "The cancellation nodes are not due to the crossover design, they are due to the vertically displaced drivers. (The crossover design controls where cancellation nodes occur, not that they occur.) The fact that the drivers are not coaxial means that any vertical deviation from the on-axis line results in a slight, but very significant difference in path lengths to the listener. This difference in distance traveled is effectively a phase shift between the drivers. And this causes cancellation nodes -- the greater the distance between drivers, the more nodes." and their picture:


This picture shows why there is a thing called a forward lobe. The direction it points can be changed by moving the drivers relative to one another, or by adjusting the crossover - in particular I think it's the phase effects of the crossover that are most significant here. If you go to the thread at DIYSG where Bill Waslo introduced the Cheap Thrill, you can read some discussion relevant to this behavior. Bill found that the best response (and therefore what he defines as "on axis,") was above what would normally be seen as "on axis" and explained that this axis could be adjusted by changing the woofer recess in the baffle ("you do the trig").


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post #96 of 109 Old 04-30-2014, 09:16 PM
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I can't help but wonder if I'm missing something since you rarely see treatments on the ceiling, particularly above the seating where it would be necessary to treat for the surrounds.
You're not missing something - at least in theory we should be treating the ceiling more than most do. I think there's a few things: one is the general lack of awareness of this sort of off-axis behavior; another is a shortage of understanding of the psychoacoustic effects; and three - there's less willingness to treat ceilings because of the combination of WAF and low ceiling heights.


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post #97 of 109 Old 04-30-2014, 09:54 PM
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ceiling treatments for non theater rooms isn't waf. I don't want treatments on ceiling and I'd guess the large majority of guys wouldn't either.

Carpets, good drapes, are realistic solutions
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post #98 of 109 Old 05-01-2014, 05:20 AM
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One of the attractions of a DIY star ceiling on optical is that you can use it and install it into sound treatment panels on the ceiling- so the WAF factor is a non factor and you get a superior sounding ceiling.

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post #99 of 109 Old 05-01-2014, 05:43 AM
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I can't help but wonder if I'm missing something since you rarely see treatments on the ceiling, particularly above the seating where it would be necessary to treat for the surrounds.
You're not missing something - at least in theory we should be treating the ceiling more than most do. I think there's a few things: one is the general lack of awareness of this sort of off-axis behavior; another is a shortage of understanding of the psychoacoustic effects; and three - there's less willingness to treat ceilings because of the combination of WAF and low ceiling heights.

Good points. In a technical sense the ceiling is a wonderful place to put treatments. Treatment bandpass is about thickness. If you put the same thickness treatment on the floor as can be easily accommodated in a room with reasonably high ceiling, nobody would be able to walk across the room.
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post #100 of 109 Old 05-01-2014, 08:14 AM - Thread Starter
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Thanks for the post HF! Again, I need to think about the consequences of that.

In the mean time, here's what I'm trying to reconcile. DE designs use Procella speakers which as I understand it have a radiation pattern similar to the SEOS designs. My DE layout does NOT specify a particular speaker, but he used something that looks remarkably like a Procella in my CAD drawings. Take from that what you will. With that in mind, I have seen lots of DE designed rooms that don't seem to have ceiling treatments for the surrounds. I'm sure his signature coffered ceiling does a great job of providing diffusion for the surrounds, but he has lots of rooms that I don't see that sort of treatment.

If this type speaker design inherently causes issues with ceiling interactions for the surrounds, why not spec coaxials for the surrounds when you can't treat the ceiling either due to WAF, headroom, or personal taste?

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post #101 of 109 Old 05-01-2014, 08:28 AM
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I'm sure there are plenty of compromises in every design, but keep in mind that the proximity to the reflective surface was at the heart of the concern.


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post #102 of 109 Old 05-01-2014, 08:32 AM - Thread Starter
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No question, but I don't recall seeing a DE design that didn't use Procella surrounds, and not all of his rooms have 12' ceilings. That may be one of the design issues that gets ignored for the "cheaper" layouts. I really don't know.

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post #103 of 109 Old 05-01-2014, 02:44 PM
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I've seen his designs with triad speakers quite a few times I think too. His equipment recommendations are usually pretty stagnant and not necessarily concerned with value. He often makes it sound like other products won't perform as well but that's mostly BS.

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post #104 of 109 Old 05-05-2014, 07:59 PM
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J_P_A - how's the selection process going?

Let's see if I can work through the design details. Your room has 9' ceilings (108") and soffits are 8" tall by 26" wide (I think - that's pretty close anyway, right?). An SEOS baffle will vary in size, but I'll measure my Cheap Thrill baffle and we can use it as reference - the SEOS 12 being between the 10 and 15, but probably pretty close to both. The SEOS 12 is 6.5 inches tall, and there is 1 inch between the WG and the top of the cabinet, so the tweeter is 4.25 inches from the top of the cabinet. The 15" midwoofer is centered 15" from the top of the cabinet. We can estimate then that the acoustic center of the system is 8-10 inches below the top of the cabinet for most SEOS designs. The Cheap Thrill is a larger cabinet than most of the others, but let's take the Tempest as an example and say that a full size SEOS cabinet is probably about 12-14 inches deep (Tempest is 14.25, but Alchemy is only 9" - but EOS, not SEOS, and the Alchemy MTM is 12" deep). So the acoustic center is 14" or so from the wall, and 10" below the soffit. The corner of the soffit is 10 inches up and a further 12 inches out from this acoustic center, positioned at an approximate angle of 50 degrees (please draw this out and check my math!) Let's assume these approximations are fairly accurate and generalizable.

The cancellation notch is probably going to be up 30 degrees (per the article I linked the other day, as well as the vertical plots MTG90 made). 30 degrees means that the anomaly will not be reflected on the lower surface of the soffit and instead hit the ceiling. The spot on the ceiling should be (let's see - 30-60-90 triangle... ) twice as far out from the acoustic center as it is above the acoustic center... so 18" up and 36" out, plus the 14" of speaker cabinet... 50" out into the room. The flight path for that soundwave is (18 squared, plus 36 squared - root of answer...) about 40" so far. By the time it gets back down to ear height at say 40" off the ground (that's a further vertical travel of 68") it's moved more than eight feet farther across the room. That's a total displacement of over 13 feet from the wall. SInce your room is nearly 19 feet wide, that will impede the farthest listener (instead of passing over his head) Bummer, I was hoping it would miss and the concern could be rendered moot. Alright, so a total vertical movement of 58" and a total lateral movement of (twice that) 116" means the total flight for the soundwave is (more pythagorean) about 130" compared to the direct sound only having travelled something in the neighborhood of 102". The difference means the delayed reflection should arrive very soon (during the fusion interval), and will indeed need to be dealt with, IMO.

I'd love for someone to disagree. (not for the argument, but because I hope I'm wrong)

Edit:
Wait! This was the wrong way to model a nearby obstruction. I've assumed that the notch emanates from the acoustic center, which of course it does not! The notch only exists because waves from the waveguide exist in the same place as waves from the woofer. If the sound from the tweeter is blocked, there can be no notch. The tweeter center is much closer to the bottom of the soffit and 30 degrees up from the waveguide should be pretty squarely in the underside of the soffit, right?

Somebody tell me this it right - right?


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post #105 of 109 Old 05-06-2014, 05:21 AM
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You can invert them too if mounted high in a column or on wall so the tweeter is on the bottom and woofer on the top.

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post #106 of 109 Old 05-06-2014, 05:42 PM - Thread Starter
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HF, I did a back of the envelope and came up with the same conclusion you did. I decided there would be an issue for some of the seats under most reasonable assumptions for the angle of the interference, so I abandoned the idea of tilting the surrounds down enough to deal with it.

After that I returned to trying to decide how DE specs similar speakers for surrounds without treating the ceiling. It's either not that big of an issue, or there's something to the analysis that Im overlooking.

So to answer your first question, I still haven't decided. I'd hoped this would be a much simpler decision, and I could just jump in. Sadly, it hasn't turned out that way smile.gif

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post #107 of 109 Old 05-06-2014, 05:49 PM
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Can't you ask DE specifically ? Is it just me or does it always seem like getting info from Erksine group is like pulling teeth?

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post #108 of 109 Old 05-06-2014, 05:52 PM - Thread Starter
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I don't know. For the entry level service, I understood that they were not providing equipment recommendations aside from screen size and placement.

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post #109 of 109 Old 05-06-2014, 07:18 PM
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You are not asking about that. You are asking about the ceiling sound treatments, and if needed or not with whatever speakers. Seems like if you paid for something you should be able to ask the simple question.

What speakers were assumed or used from the design perspective?
Is ceiling treatments needed?
Would it matter with different speakers?

I'd try to ask the questions, and if they can't answer you might want to inform others of such so they know what they get when they decide to pay Erksine group for plans. Nothing wrong with them answering them to you, and you not sharing the specifics with the community (if you did not want) but for them not to answer some good questions seems like BS. I'd bet if you made the offer to keep the answer a secret as options 1, or complain loudly they did not answer as option 2 - they would take option 1. I know they are really big into the entire "intellectual property" and I get giving out advice for free might go against their business interests, but since you paid I see no reason you can't ask.

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