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post #31 of 54 Old 11-29-2012, 01:11 PM
 
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Originally Posted by pnw View Post

...CTC from mids to woofer won't be a problem with the relatively low crossover frequency...

That's a good approach for speakers designed to be used outdoors or in a large space, where point source summing is desired. It is also important if the woofer to midrange crossover is above the Schroeder frequency, around 200Hz. That is typical for three-ways that aren't designed with any particular directivity in mind.

But in the uniform-directivity world that Pi Speakers and other similar "waveguide" speakers seek to address, we see a different trend. Our approach is to match the directivities of midwoofer and tweeter, to crossover where the midwoofer beamwidth has collapsed to approximate the horizontal directivity of the waveguide we are using. This approach necessarily favors relatively large diameter midwoofers. The efficiencies are a better match too, but that's a different subject.

This is somewhat counter-intuitive, but as important as close spacing between midwoofer and tweeter is, the distance from midwoofer to subwoofer should be somewhat greater. We aren't looking for point source summing indoors, we are actually looking for spacing on the wavelength scale. The reason is that directivity becomes ambiguous in the modal region, so point-source summing is non-sequitur. There is no way to provide point source summing, because the modes are scattered in space and time. So the next best thing is dense interference, smoothing the sound field by averaging.

The multisub configuration is becoming pretty well understood, but I find sometimes people aren't really thinking about the transition region, which is what helper woofers or "flanking subs" address. It requires blending of two sound sources in the 80-160Hz region where sound is becoming localizable, so the midwoofer and helper woofer can't be too far apart, but they do need to be far enough apart to smooth the ripple from boundary interference. It's much like the multisub approach, but frequencies are higher so distances are smaller. You put the helper woofer between the nearest boundaries and the main speaker it's flanking, usually just a couple feet away. The main is on a stand a couple feet from the wall, and the flanking sub is on the ground beside the main speaker, pushed back against the wall.


three Pi speakers with flanking subs
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post #32 of 54 Old 11-29-2012, 01:35 PM - Thread Starter
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i just saw thise JBL 2226 whit mods done to the magnets, whats this done here to thise. or is it not the real 2226?




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post #33 of 54 Old 11-29-2012, 01:43 PM
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I'm not sure what those are but they don't look like 2226s.

This is a 2226.

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post #34 of 54 Old 11-29-2012, 01:53 PM
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Originally Posted by simple100 View Post

if you read the whole tread you can see nothing is set in stone,
the main issue being the crossover, and going active seems way to expensive. and no ill prob not be using the raal i don't want to build a 2k$ speaker, ill like to keep it under 800-900 pr speaker.

So what tweeter do you have in mind for this? Are you going to try and use a compression driver matted with a horn for the high frequencys? I wonder what would be a good conventional dome tweeter that would perform similar to the Seas mid-range driver you already have? Perhaps another Seas? I am looking forward to seeing what you end up going with. Also, have you decided on what woofer you are going to use?
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post #35 of 54 Old 11-29-2012, 02:18 PM
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Originally Posted by pnw View Post

SEAS rates the W18EX at 88db, though it is probably closer to 86. Two in parallel would yield 92-94db sensitivity which, with no BSC required, isn't too shabby.

I agree.

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post #36 of 54 Old 11-29-2012, 02:24 PM
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Quote:
Originally Posted by Wayne Parham View Post

Our approach is to match the directivities of midwoofer and tweeter, to crossover where the midwoofer beamwidth has collapsed to approximate the horizontal directivity of the waveguide we are using. This approach necessarily favors relatively large diameter midwoofers.

But with two side by side 6" mids, you would likely get the same directivity match as a single 12"...no?
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post #37 of 54 Old 11-29-2012, 02:53 PM
 
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No, a speaker array doesn't act like a point source except at low frequencies where the size of the array is acoustically small. Basically, once the directivities of each individual driver starts to narrow, they start acting sort of like horns. And with two of them, you get comb filtering, which is what makes the nulls that straddle the forward lobe. In the case of side-by-side drivers, the nulls are in the horizontal plane. They move closer together as frequency rises.

I would expect the designer of a speaker with a horn above two horizontally arrayed 6" drivers would crossover to the horn below the point where the two midwoofers are becoming directional at all. So basically, it would be omnidirectional up to the crossover point, then the horn sets the pattern above that. Under that assumption, where directivity is concerned, the pair of horizontally arrayed drivers do not act like a 12" midwoofer, they act like a driver that is "arbitrarily small".
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post #38 of 54 Old 11-29-2012, 03:07 PM - Thread Starter
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from the way the tread is going, thanks for all the feed back, darn its so difficult when HF and horn is involved.

now i think that if not a great design pops up, i might have to keep researching, as i prob have no chance making a crossover from scratch on a unproven match of horn> 2 x 6.5"/7.5" > 15"

Wayne you have been a great maker of the HF speakers long time, is there a reason you never took this angle to build, and did the corner speaker whit internal woofers instead on the 7Pi.?
i feel the more i talk whit ppl about it, its a rather hard project i set my eye on smile.gif seems active is the only way i can wing it, whit no crossover exp
Maby its time to ditch the horn and go AMT or Large Ribbon instead, and build a 2 box system, tweeter mid top box and woofer lower box.
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post #39 of 54 Old 11-29-2012, 03:16 PM
 
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Originally Posted by simple100 View Post

Wayne you have been a great maker of the HF speakers long time, is there a reason you never took this angle to build, and did the corner speaker whit internal woofers instead on the 7Pi.?

Sorry, I don't understand your question. But I feel pretty confident the answer is probably somewhere in the Pi Speakers FAQ. If not, please rephrase and I'll try to answer.
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post #40 of 54 Old 11-29-2012, 07:11 PM - Thread Starter
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i read the faq, and i saw some tread about a Midrange Horn, ? whats the build called whit this mid type, sounds really interesting.


Then i found an old tread on here whit the speaker maker - Troels Gravesen, i think its the guy making the Jantzen crossover parts!? looking on the crossovers he made dang they prob cost more the speaker parts smile.gif




Well he got 2 HF models that caught my eye, uses a modified sears excel 8" mid sound right up mu ally.smile.gif
he write about the process of desigin the HF version of the sears here, still made by sears oc.
http://www.troelsgravesen.dk/JA8008.htm


DTQWT-10
Audax TW034 tweeter w/waveguide - JA8008 8" - Eminence Deltalite II 2512 10" x2
28 cm wide and 105 cm high, but depth is 50 cm
http://www.troelsgravesen.dk/JA8008_DTQWT.htm


DTQWT-12 - Audax TW034 tweeter w/waveguide - JA8008 driver - Eminence Deltalite II 2512 12" x2
152 cm height and 33 x 60 cm (WxD)
http://www.troelsgravesen.dk/DTQWT-12.htm

12"

10"


ugh, not room for a center then, if building the 12" model , he made one to match he writes this about it:

Having the small SEAS MCA12RC 4" unit covering the 800-3000 Hz range ensures wide dispersion and mated with the TW034 dome (this time without waveguide) we are certain of overcoming this problem and voiced around 92-93 dB, the MCA12RC can fill the gap between bass and tweeter despite modest sensitivity compared to the other drivers. In the end it even had to be attenuated 1 dB.
http://www.troelsgravesen.dk/8008CENTER.htm


has anyone seen this as a build tread yet? or know of these. they look great and i live in EU so i have no big problems ordering.
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post #41 of 54 Old 11-29-2012, 11:05 PM
 
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Originally Posted by simple100 View Post

i read the faq, and i saw some thread about a Midrange Horn, ? whats the build called whit this mid type, sounds really interesting.

That's the midrange horn used in Pi constant directivity cornerhorns. That's the only hifi loudspeaker configuration I know of that maintains constant directivity through the entire audio band, all the way down to the Schroeder frequency. There is a lot more information about it in the Pi Speakers FAQ.
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post #42 of 54 Old 11-30-2012, 07:16 AM
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Originally Posted by Wayne Parham View Post

No, a speaker array doesn't act like a point source except at low frequencies where the size of the array is acoustically small. Basically, once the directivities of each individual driver starts to narrow, they start acting sort of like horns. And with two of them, you get comb filtering, which is what makes the nulls that straddle the forward lobe. In the case of side-by-side drivers, the nulls are in the horizontal plane. They move closer together as frequency rises.
I would expect the designer of a speaker with a horn above two horizontally arrayed 6" drivers would crossover to the horn below the point where the two midwoofers are becoming directional at all. So basically, it would be omnidirectional up to the crossover point, then the horn sets the pattern above that. Under that assumption, where directivity is concerned, the pair of horizontally arrayed drivers do not act like a 12" midwoofer, they act like a driver that is "arbitrarily small".

They don't act exactly like a 12" woofer, but from a horizontal pattern perspective, the combined -6db point narrows to ~90deg around the same frequency range for mating to a horn, around ~1300hz. A single 6" driver (assuming a ~5.25" piston) will narrow to ~90deg around ~2500hz so their individual narrowing should have no significant impact. Comb filtering won't play a significant effect either with a crossover in the 1200-1300hz range. That has been my experience.
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post #43 of 54 Old 11-30-2012, 11:01 AM
 
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Once the beamwidth of the individual drivers start to narrow, they begin to stop acting like a single source and start acting like an array. This is not actually because of the directivities of the individual drivers, but because of the distances between drivers and the wavelengths of sound generated by them. At low frequencies where they are acoustically close, they combine without interference. But as frequency rises, interference patterns develop.

If the two drivers are placed edge-to-edge so the center-to-center distance is equal to the diameter of each driver, they start transitioning from point source around 1/2λ to array at 1λ spacing. Frequencies below the 1/2λ point can be considered point source, and frequencies above 1λ, the drivers act as an array.

For a 6" driver, these two frequencies are 1130 and 2260. So below 1130, the pair of drivers is acting as a point source, pretty much summing coherently everywhere in front of the baffle. But above this frequency, it is starting to act like an array, meaning it is gaining directivity from interference with the adjacent driver(s).

When the system starts to act as an array, it begins to become directional. The reason it becomes directional is interference with adjacent driver(s). It is comb filtering that provides directivity in an array.

So that's what made me suggest that probably the designer of a speaker with a waveguide above a horizontal array of mids was using the mids down in the range where they were acting as a point source, and avoiding the range where they started acting like an array. One would not want nulls out to the sides, in the horizontal plane. Out beyond the nulls would be secondary lobes, which are not what one would want either. So my suspicion is these speakers are designed to crossover below the point where the two drivers acted as a horizontal array, down in the frequency range where they are not becoming directional, in order to avoid nulls and secondary lobes.
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post #44 of 54 Old 11-30-2012, 12:20 PM
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Overall shape of the source(s) seems to play a role as well. A round 12" source has a high percentage of radiating area in the first inch +/- center, while two 6" have very little.

Morphing between the two, a single rectangular source 6" high by 12" wide produces a polar pattern between a single 12" and two 6".

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post #45 of 54 Old 11-30-2012, 12:50 PM
 
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That's true. A pair of round drivers form an "8" shape, which is different than a single round driver or a single elliptical driver. So naturally the directivity characteristics are different in each case.

The radiation pattern of a single round (or elliptical) piston is well understood. So is the radiation pattern of a pair, or of an array of N units. These all create predictable radiation patterns that are well-understood. Of course, cone breakup changes things but when in the pistonic region, they're predictable.

The directivity pattern of an array is more complex than the directivity pattern of a single driver.

Directivity of a single driver is created by simple superposition of the waves eminating from all points across its surface. On-axis, they all sum coherently at all frequencies. Off-axis, summing is affected by the angle and the frequency, the proportion of energy that is in-phase to the energy that is out-of-phase. Directivity of an array of drivers includes this, but it also includes interations across all drivers in the array as well. The interactions and energy proportions are different, because the radiating shapes are different.
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post #46 of 54 Old 11-30-2012, 02:39 PM
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Yup.

...and back to Simple's speakers...

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post #47 of 54 Old 11-30-2012, 02:51 PM - Thread Starter
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thanks guys. i dont mind the talk, i can learn a thing or 2 in the process

pnw did you see the new one i found have you see those before?
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post #48 of 54 Old 12-01-2012, 04:13 AM
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Quote:
Originally Posted by Wayne Parham View Post

No, a speaker array doesn't act like a point source except at low frequencies where the size of the array is acoustically small. Basically, once the directivities of each individual driver starts to narrow, they start acting sort of like horns. And with two of them, you get comb filtering, which is what makes the nulls that straddle the forward lobe. In the case of side-by-side drivers, the nulls are in the horizontal plane. They move closer together as frequency rises.
I would expect the designer of a speaker with a horn above two horizontally arrayed 6" drivers would crossover to the horn below the point where the two midwoofers are becoming directional at all. So basically, it would be omnidirectional up to the crossover point, then the horn sets the pattern above that. Under that assumption, where directivity is concerned, the pair of horizontally arrayed drivers do not act like a 12" midwoofer, they act like a driver that is "arbitrarily small".

(I have a few newbe questions for you, Wayne, or anyone else who wants to answer)

1. What do you mean by point source at low frequencies? Does this mean that the array you are referring to is acting like it has a horn and is throwing sound in a controlled line straight to the listener?
2. Also in regards to that first sentence you posted, what do you mean by saying that the size of the array is "acoustically small"?
3. Can you give me a quick definition of what is comb filtering?
4. What is the forward lobe and how do the nulls straddle it? It is my understanding that nulls are points in space where the frequency or sound dips down, correct?
5. When you say "where directivity is concerned, the pair of horizontally arrayed drivers do not act like a 12" mid-woofer, they act like a driver that is arbitrarily small", what do you mean by saying that they are acting arbitrarily small? I am confused as to what that means.


I realize that these are stupid questions, but, I am trying to learn as much as I can about this because I have been hooked onto this hobby/passion and I plan on continuing this journey for many many many years to come!
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post #49 of 54 Old 12-01-2012, 07:36 AM
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Wayne and Paul-

Yes, two 6" woofers side by side doesn't exhibit the same directivity over its entire bandwidth, but they generally narrow their horizontal directivity down to ~90deg in a similar range to most 12" drivers (which also vary based on cone shape). Directivity matching of the dual 6" woofers works in a similar manner as most 12" woofers. The behavior of the directivity of the two configurations diverge as wavelengths get shorter. In my experience this is less important as the woofers are further out of their passband in these applications.

Ultimately, where the rubber hits the road we have to measure the behavior.

Marty-

I would suggest googling #3 and reading Wayne's posts on #4 here, http://audioroundtable.com/PiSpeakers/messages/22628.html, and here http://audioroundtable.com/PiSpeakers/messages/11054.html
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post #50 of 54 Old 12-01-2012, 08:15 AM
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Simple,
I have no experience with Troels designs. He has a good reputation but that is all I know.


Max,
Agreed. IMO, with the exception of stereo speakers reproducing a mono signal, combing usually doesn't measure or sound as bad as the theoretical predictions look.

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post #51 of 54 Old 12-01-2012, 11:59 AM
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Originally Posted by pnw View Post

Max,
Agreed. IMO, with the exception of stereo speakers reproducing a mono signal, combing usually doesn't measure or sound as bad as the theoretical predictions look.

There is also a tradeoff to be had. Well above a crossover in the 1100-1300hz range there will be comb filtering with 2 6" drivers side by side. The difference is that these likely 6" drivers already have a cleaner frequency response in this range than most likely 12" drivers. Which is better? I haven't done enough testing to say definitively. There are other tradeoffs too where the larger woofer is sensitive, the smaller woofers will have a much wider forward lobe, etc etc.

I'm not suggesting that this configuration is inherently superior. I'm simply saying it is a possible solution that still adheres to what I think are vital requirements in directivity matching and an adequately wide forward lobe. It might have advantages in it can be spec'd with some very low distortion wideband hifi drivers without sacrificing directivity and fairly high sensitivity. I'd really like to see it done with 18Sound 6ND430's or B&C 6NDL38's. I do think it would be best run with significant bass reinforcement (many ways to do this).
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post #52 of 54 Old 12-01-2012, 12:39 PM
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In addition to directivity, I like horizontal (and vertical) pairs of smaller drivers for slightly better clarity over the larger paper drivers. I just haven't tried it higher in frequency (under 400Hz for for my center and below 800 for the L&R). At some point I'd like to experiment up where handoff to a 1" CD would make sense.

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post #53 of 54 Old 12-02-2012, 09:30 AM
 
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Yes, two 6" woofers side by side doesn't exhibit the same directivity over its entire bandwidth, but they generally narrow their horizontal directivity down to ~90deg in a similar range to most 12" drivers (which also vary based on cone shape).

I think the biggest difference - besides the basic shape difference - is the fact that the single driver is a single source and the pair of drivers creates a two-source pattern. There are some pretty significant differences, beyond the matter of acoustical size.

Single source interference creates a well defined forward lobe and small secondary lobes. But dual source interference creates side lobes that are nearly as strong as the forward lobe.




Single source energy distribution




Dual source energy distribution
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post #54 of 54 Old 12-02-2012, 09:46 AM
 
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Originally Posted by Martycool007 View Post

(I have a few newbe questions for you, Wayne, or anyone else who wants to answer)
1. What do you mean by point source at low frequencies? Does this mean that the array you are referring to is acting like it has a horn and is throwing sound in a controlled line straight to the listener?
2. Also in regards to that first sentence you posted, what do you mean by saying that the size of the array is "acoustically small"?
3. Can you give me a quick definition of what is comb filtering?
4. What is the forward lobe and how do the nulls straddle it? It is my understanding that nulls are points in space where the frequency or sound dips down, correct?
5. When you say "where directivity is concerned, the pair of horizontally arrayed drivers do not act like a 12" mid-woofer, they act like a driver that is arbitrarily small", what do you mean by saying that they are acting arbitrarily small? I am confused as to what that means.
I realize that these are stupid questions, but, I am trying to learn as much as I can about this because I have been hooked onto this hobby/passion and I plan on continuing this journey for many many many years to come!

Answers by numbers:

1. A point source is something that radiates as a single tiny pinpoint source rather than as a large object (that may develop path length differences across its area) or as multiple sources. At low frequencies, even though the radiator may not really be a pinpoint, it is acoustically small and so acts like one.

2. Something that is acoustically small is smaller than a wavelength. A 12" woofer generating 40Hz is acoustically small because the wavelength of 40Hz is 28 feet. But by 1.2kHz, that same 12" driver is no longer acostically small, because the wavelength of 1200Hz is just under a foot.

3. Look at the dual source energy distribution image in my last post. That pattern is comb filtering, named because the peaks and valleys look sort of like a comb.

4. The center peak of the energy distribution images above is the primary lobe, which points forward in a well-designed loudspeaker. The first dips outside it are the nulls, followed by secondary lobes just outside, followed by another pair of nulls, another lobe, etc.

5. By saying "arbitrarily small" what I'm saying is acoustically small. Using the 40Hz woofer example, since the wavelength is 28 feet, anything smaller than that is a point source and can be considered "arbitrarily small". I don't care if the woofer is a 12" woofer, a 15" woofer or an array of four of them that's several feet across - at 40Hz, it's small enough to be a point source and so I'd see them all as being arbitrarily small. But once you get a few big hornsubs stacked, they might be large enough to be acoustically large, and then they'd act differently. And at higher frequencies they'll become acoustically large too.
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