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Old 19th January 2013, 11:15 AM   #21
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Quote:
Originally Posted by thalis View Post
Agreed. Very nice study.
In some old papers by l-acoustics, it was claimed that for an array to produce a true cylindrical wavefront, there are not just frequency vs driver spacing requirements, but also driver diaphragm percentage coverage of the line requirements (>=70-80%), which can be (practically) met only with isophasic waveguides (or true ribbons). Truth is that in some comparable (with speaker dave’s) measurements of mine but with ribbon arrays, polars looked somewhat better.
Continuous lines and multi element lines give the same performance at lower frequencies and deviate at higher frequencies. Its a sampling theory or aliasing "thing". When the point density is sufficient then there is no difference in polar curve, when the point spacing is too great then alliasing occurs (polar lobes).

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Old 19th January 2013, 02:01 PM   #22
thalis is offline thalis  Greece
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Agreed, and I’m aware of this elementary stuff dear Dave. Regarding High frequencies, there are practically no differences in the primary lobe, but structure of off-axis lobes and nulls is changed. An almost gap-less uniform phase and amplitude array (with ribbon elements or isophasic waveguides) will be always somewhat better (regarding side-lobes in high frequencies -ie when wavelengths are comparable with elements distances) from practical multi ommidirectional element arrays. Somewhat off-topic, sure, but I felt it was worth mentioning. Power tapering perhaps shouldn’t be a requirement if someone constructs an array with ribbon elements sufficiently close-spaced
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Old 19th January 2013, 07:25 PM   #23
ScottG is offline ScottG  United States
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Originally Posted by mayhem13 View Post
Interesting design....not something i think i've ever seen before. I understand your point in regards to getting the forward lobes right, but not sure why you recommend a 2" fullranger instead of a dedicated tweeter?

....and i thought 14ft of listening distance would still be before the far field transition for a 4ft tall line. Maybe i should just build a good ol WMTMW instead! Much easier that's for sure. From researching it has all the acoustic advantages without the design headaches.
The 2" driver as a tweeter?

Lower crossover. 1st Order crossover. The ability to create a longer line (even if not terribly long) to better match it's character with the mid-bass array at freq.s near the crossover region. And finally, this particular driver's fantastic performance. Note: a good cone tweeter (accepting press loss off-axis) can often sound better than a good dome tweeter (..largely because the dome tweeter is over-damped by comparison). Tip: Best way to "load" the cone tweets is a long tapered lines a'la B&W Nautilus.

Of course combing will still be there for the small tweeter line, but the "peaks" and "nulls" won't start to become exaggerated until the top octave (..and even under a modestly averaged condition I don't think they would be particularly visible). Moreover, having heard larger drivers operate similarly - it's not something I've found particularly objectionable. (..with 4 inch driver's in an array the sound is not unlike a full-range electrostat.)


One of the problems with a short line source at a distance is vertical "miniaturization" of images (or vertical "shrinkage"). You can go to any Magnepan dealer and hear this effect with their smaller speakers (vs. their larger speakers). The partial "solution" is to place the treble array higher on the baffle (..as shown in figure 3 on page 6 of the Line Array paper).


A WMTMW is of course another option, but at the 14 foot distance images will tend to be much smaller but still in proportion to the soundstage (which will also be much smaller). (..it's not a vertical shrinking effect, but rather an overall shrinking effect.) Pretty much the same thing as if moving 14 feet away from a mini-monitor and listening. On top of that you could get some strange combing effects IF you don't have steep crossovers at lower freq.s.. So the typical 3 kHz crossover doesn't work very well with the two mids operating up that high. BUT move it down to 1.6 kHz or so with an LR 4th while keeping the mid.s as close together as possible and it shouldn't be a problem. (..with a similar design structure for the woofer to mid crossover region.)
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Last edited by ScottG; 19th January 2013 at 07:28 PM.
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Old 19th January 2013, 09:53 PM   #24
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I'm liking this line of thinking more and more, even as unconventional as it is.

I tried to make an arguement against the 2" 'tweeters' with poor off axis response but at 14ft listening distance AND a room whose width is only 13 ft, there really wouldn't be an off axis seat in the house with a wee bit of toe in.

So why even 1.6khz?......why not lower with a steeper slope?....or do you prefer the 1st order cross in this configuration. My thoughts are also now shifting towards a larger diameter midwoofer since the cross is lower......say 4.5"?
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Old 19th January 2013, 09:59 PM   #25
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Hi all... great posts. <mini hijack warning> I'm trying to do a small-ish (8' or so) ground-stacked PA line array. I do mostly acoustic and vocal shows, often in 150-200 person overly reverberant halls, usually with flat seating.

My all-time favorite PA driver (any driver, really) is the Stage Accompany SA8535 planar tweeter, and I've managed to accumulate 6 of them, which, if I do funny things to their waveguides to butt 3 of them together, will make about a 36" tweeter line on each side.
(drawings at http://www.stageaccompany.com/suppor...35_techdoc.pdf )

I'd like to cross this at about 1 or 1.5k to a series of 6.5" midbasses. I've been spinning my mind in circles as to how best deal with this; a double-row (flat, or in the tweeter waveguide throat), or, much more ambitiously, to try a horn-in-front-of-horn coaxial mounting scheme, of the "let the tweeters act like a phase plug for the midrange" format used in some commercial designs of the past.

I'll start a different thread to discuss this if there is interest, but with so many knowledgable people in one place, I'd like to ask if there is already some good reading on the coaxial approach.

If the idea makes any sense, I'll re-start my efforts to understand hornresp and see if the dimensions have any chance at working out.
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Old 19th January 2013, 11:04 PM   #26
ScottG is offline ScottG  United States
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Quote:
Originally Posted by mayhem13 View Post
I'm liking this line of thinking more and more, even as unconventional as it is.

I tried to make an arguement against the 2" 'tweeters' with poor off axis response but at 14ft listening distance AND a room whose width is only 13 ft, there really wouldn't be an off axis seat in the house with a wee bit of toe in.

So why even 1.6khz?......why not lower with a steeper slope?....or do you prefer the 1st order cross in this configuration. My thoughts are also now shifting towards a larger diameter midwoofer since the cross is lower......say 4.5"?
Yup. When you are sitting from that distance the off-axis pressure loss isn't as big a deal. You could even equalize it for an increase in pressure ON-axis in the treble region for a flat response OFF-axis and "aim" them inward - i.e. "toe-in" to "cross" in front of your listening position for greater image stability for multiple listeners (..similar to what Ted Jordan advocates). Under such reverberation it could be useful without a much width-contraction in the soundstage. (..unless of course you have a VERY large room and have the speakers substantially away from walls.)

1.6 kHz was for the MTM version with a standard tweeter and steep crossovers.

For the 4 driver 2" cone tweeter array - a bit less than 1.5 kHz (probably 1.2 -1.3 kHz) - it's a guesstimate based on several factors. Actually, looking back at the linesource/pointsource chart again - in relation to the midbass line length of around 30 inches, you should move the crossover a little lower to perhaps 800 or 900 Hz. (..midbass line combing above the crossover freq. is also a factor for lowering the crossover freq..) You would need to do some simulations though for your intended spl (factoring distance and room reflections) vs. excursion with a 1st order. BUT with 4 drivers I don't think it will be a problem.


You could go with a larger mid-bass, but you have a few things to factor-in like: combing at higher freq.s with a 1st order filter and variables relating to tonal similarity and "matching" between the drivers. Of course price is a factor as well. (..and those 3.5" drivers are rather excellent performers all on their own.) Like the tweeters, you would need to run the sim.s to see if it will work with those drivers (..the 3.5" drivers might not be enough for your application). Of course if you can find something similar with a more extended bass response then . Can't hurt to do some searching.
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Old 20th January 2013, 12:54 PM   #27
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Honestly Scott, I don't need extension below 100hz, but nor do I want to sacrifice midbass 'slam' in the 100-150hz range either. I'll have to sim the TC9s a bit more and see what comes out.

Now I assume that combing from the tweeter array will exist above 8khz, but I'm guessing its effects won't be audible from my listening distance?
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Old 20th January 2013, 01:59 PM   #28
opc is offline opc  Canada
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Hi mayhem13,

If you're looking at the TC9, you should give this a read:

Stupid Cheap Line Array

I built an array with them a few years back, and I'm still using them today. It's an excellent driver, and you'd be had pressed to find anything better for use in an array.

There's lots of info about them in the above thread if you keep reading through.

Cheers,
Owen
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Old 20th January 2013, 02:21 PM   #29
Pnotus is offline Pnotus  Sweden
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Hey!

After living with a couple of the different 3,5" and 3" out there iŽd say that the cost-cut for the TC9 isnŽt worth it. The TD9 is better.

Also - on your original question about tweeter line length, it comes out in whether you want the nearfiled to cover your listening space, which for me personally is what linearrays are all about. You will find the formula to use for this in Griffin's paper. Very useful that one!

I've seen Sreten's answer in many places but always without nearfield theory involved. Perhaps he could be bothered to reveal the long answer instead?

Good luck!

/pontus
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Old 20th January 2013, 02:56 PM   #30
opc is offline opc  Canada
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Hi Pnotus,

Do you mean the TG9FD? I don't think there is a TD9.

It's curious you should say that, because I listened to and measured both drivers extensively, and the TC9FD is definitely the better driver.

They both share the same motor and frame, and the only real difference is the glass cone for the TG9 vs. the paper cone of the TC9.

In the case of the TG9, the cone is heavier making MMS higher. It also goes into breakup sooner, and more prominently, causing more midband ripple and a drastically reduced upper octave compared to the TC9. In fact, the TC9's response at 30 degrees off axis still has more extension than the TG9 does on axis.

More expensive isn't always better... Are you sure you aren't letting that pretty cone material sway you?

Cheers,
Owen
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