Line Array Tweeter line length?

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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).

David S.
 
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 :)
 
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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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"?
 
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/support/downloadnew/8535_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.
 
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 :cool: . Can't hurt to do some searching. :)
 
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?
 

opc

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Hey!

After living with a couple of the different 3,5" and 3" out there i´d say that the cost-cut for the TC9 isn´t 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
 

opc

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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
 
Hello Owen!

Oops! My mistake - of course it´s the TG9 I'm refering to.
It's very possible that the earlier breakups give the impression of more and better extended treble.
I haven't measured anything yet. As of now I'm 1500 kms away from home and will be for a while.

My impressions are very subjective so they may not be for everyone. Deal breaker for me was that everything through the TG9 sounded more alive and really, in lack of better words for a 3,5", more dynamic. But i have had problems with polyprops before and understans that this may be personal taste more than quality preference.

Regarding your use of the TC9. I'm sure you are right on spot with these and their measurements, I mean absolutely no disregard for your knowledge or choice.
Knowing that english is not native to me I realis(z?)e that my posts can come off as a bit plump.

/Pontus
 
Thanx Owen, I've seen your thread and it was partly the inspiration for this project. The only drawback for me is that I can't/won't be using a digital crossover or eQ, everything has to be done passively through the crossover. My in-wall rack is completely full so there's no option to add anything. Speaker wiring is in wall as well with two pairs per side on either side of the screen. One set reserved for the top end, the other for passive bass bins that already are in place. If you have any insights on doing this all passively, I'm all ears.

After back and forth with Scott, and reviewing the performance of the TC6, my mindset is now a central line of 16 TC6s flanked on either side by 12 TC9s for a driver total 40 drive units per speaker. With this close spacing, I think I can cross as low as 800hz or 1.6khz second order. To combat HF losses above 10khz from comb filtering in the tweeter line, I'm wondering if a 1st order on the midwoofers and 2nd order on the tweeters isn't the better way to go, slightly tilting the response upwards.
 
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?

You won't really get "slam", or even "punch" for that matter, from any low mass domestic line array. (..well, not without serious "boost" anyway.) BUT, with a crossover around 150 Hz you get that from the "sub".

At a 14 foot distance with a multiplicity of reflections from walls and furnishings - I think it's *highly unlikely* that combing will be audible. Even if it is, I think it's even less likely that it would be objectionable.
 
..my mindset is now a central line of 16 TC6s flanked on either side by 12 TC9s for a driver total 40 drive units per speaker. With this close spacing, I think I can cross as low as 800hz or 1.6khz second order.

To combat HF losses above 10khz from comb filtering in the tweeter line..


-a "flanking" arrangement is a bad idea UNLESS it's a ".5" baffle-step loss addition at a much lower freq.. (300 Hz crossover or lower for the low-pass on the .5 array.) (..I've tried this myself - the higher in freq. the weirder the combing and dispersion pattern, and the further away you have to get from the speakers to get any sort of integration window. Further away than 14 feet in my experience.)


Actually combing with small drivers doesn't work out as a pressure loss per se, rather very narrow band losses followed by narrow band gains - sort of a spiky/hashy behavior. As it get's higher in freq. the hash becomes less "narrow" which could leave you with a minor "dip", but with the Vifa 2" drivers I don't think it will be significant enough to need any correction.

What you will get however, is a lot of acoustic and diffraction gain at the lower end of the driver's response with the more drivers you use. Basically it "tilts" the freq. response downward as freq.s rise. Here is an example with somewhat different 2" drivers (..and note that the raw top-end response of these drivers is naturally "hashy" to begin with):

Parts Express: Project Showcase

What this means is that if the driver is naturally 86 db at the top of its range, AFTER it's been corrected passively it's still going to be 86 db unless there is voltage gain from a reduction in impedance (..and an amp that responds accordingly). In the design above the author just used a graphic equalizer to actively boost the top end.
 
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opc

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Joined 2004
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Hello Owen!

Oops! My mistake - of course it´s the TG9 I'm refering to.
It's very possible that the earlier breakups give the impression of more and better extended treble.
I haven't measured anything yet. As of now I'm 1500 kms away from home and will be for a while.

My impressions are very subjective so they may not be for everyone. Deal breaker for me was that everything through the TG9 sounded more alive and really, in lack of better words for a 3,5", more dynamic. But i have had problems with polyprops before and understans that this may be personal taste more than quality preference.

Regarding your use of the TC9. I'm sure you are right on spot with these and their measurements, I mean absolutely no disregard for your knowledge or choice.
Knowing that english is not native to me I realis(z?)e that my posts can come off as a bit plump.

/Pontus

Hi Pontus,

Not at all... I actually re-read my post and it was me who came off a little edgy :)

Certainly, the cone breakup could give an impression of more HF output, and possibly even more detail, but I personally would opt for the softer and more controlled breakup of the paper cone.

Again, the sonic preference is always going to come down to taste, and personal preference. We're certainly allowed to differ in our opinions when it comes to personal preference :)

After back and forth with Scott, and reviewing the performance of the TC6, my mindset is now a central line of 16 TC6s flanked on either side by 12 TC9s for a driver total 40 drive units per speaker. With this close spacing, I think I can cross as low as 800hz or 1.6khz second order. To combat HF losses above 10khz from comb filtering in the tweeter line, I'm wondering if a 1st order on the midwoofers and 2nd order on the tweeters isn't the better way to go, slightly tilting the response upwards.

mayhem13:

Just out of curiosity, why not do away with the TC9 drivers all together and just stick with a single line of the TC6?

Since you're using a sub to fill in the low end, there isn't really a need for anything larger than the TC6, and you'll get a cleaner HF response with none of the headaches of trying to implement a passive filter between two arrays.

If you make the line sufficiently long, you'll get a naturally boosted bottom end that will easily make it down to 125-150Hz even with just the TC6.

I can promise you it will sound better using a single full-range driver than trying to cross over between two arrays.

If you needed the LF extension, and were planning to run without a sub then the little 2" Vifa drivers would not be sufficient, but I think you'll find that with a sub, they will be more than enough all on their own.

How high is your ceiling?

Cheers,
Owen
 
Ceiling is 8ft, bass bin enclosures are 24" tall......figuring a 48" line would be ideal with equal distance from the floor and ceiling to the tops and bottoms of the lines.

Funny....been wondering why just not use the TC 6's alone either. I can get my bass bins up to 240hz cleanly and a third order low pass.
 

opc

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Ceiling is 8ft, bass bin enclosures are 24" tall......figuring a 48" line would be ideal with equal distance from the floor and ceiling to the tops and bottoms of the lines.

Funny....been wondering why just not use the TC 6's alone either. I can get my bass bins up to 240hz cleanly and a third order low pass.

If that's the case, I'd certainly go with just the TC6 drivers in a single line. Having that much flexibility with your subs means you'll definitely be able to integrate the two cleanly.

As for line length, I would make them taller if anything, but I'd be curious to hear Scott's input on this. I'm guessing a taller line with roughly 80" of actual drivers would be pretty much perfect for 8' ceilings. With a line that length you'll be loading the room in an ideal manner, and you'll have enough natural LF boost to help compensate for baffle step. More drivers also means less work from each driver which will improve distortion and increase efficiency. Based on my experience with the TC9 drivers, I'd be willing to bet a line like that would require little or no EQ as long as you get the baffle width right, and cross over to a sub somewhere between 100 and 200Hz.

The only downside I can see is that the driver already has a bit of an inherent tilt downwards from LF to HF. This might mean the end result will need a little bit of boost in the top end, or if you're using passive crossovers, you'll need to attenuate the lower midband.

Cheers,
Owen
 
I actually need to mount these on top of the sub/bins......inboard is prevented by the projection screen and outboard too close to the side walls. I could certainly increase the line length, but 60" would be max. My thinking would be the line biased towards one boundary over the other created problems in the vertical dispersion.....and some unintended bounce.

As for baffle width...it can be as wide as 12" if needed to combat baffle step losses.
 
ScottG and others,

The symmetrical (or flanking) style line array (a tweeter line with a line of mids on each side of the tweets) has been used to advantage for several line array speakers manufactured by McIntosh Labs. Robert E. Green (see his regonaudio site) did a review of their XRT28 a few years ago in 'The Absolute Sound' and later wrote a supplemental paper on the in-room advantages of the flanking style array as you can see at:

How to Tell How a Speaker Sounds in a Room: (Supplement to REG's review of the McIntosh XRT28s)

You can think of the XRT28 array as elemental MTMs placed horizontal and stacked to create the flanking (or symmetrical) line array. If you can achieve close spacing, you can essentially direct the nulls that you get with a horizontal MTM so that side wall reflections are minimized as pointed out by Green. Thus you get the reduction in floor and ceiling reflections that you normally observe with a line array plus another degree of freedom to minimize sidewall bounce.

McIntosh has taken this concept to a new level in their current XRT2K model which you can see on the McIntosh web site. Follow the support tab to select 'speakers' and enter XRT2K to download the brochure for this model. That is a mind blowing line array configuration!

All that being said about the possible advantages of a symmetrical array, I expect that a single line array of wide range drivers supported with woofers to cover the low bass and into the lower midrange may suffice for the original poster.

Jim
 
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