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Old 18th January 2013, 05:51 PM   #11
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It's all about the tweeters at this point. There's so many capable, affordable 3-4" midwoofers but when you start looking at the need of 50 tweeters per side,....well...
Cost considerations become a factor quickly!

If I use the Apex Jr Audax tweeters, cost is no longer a concern but now the crossover point becomes an issue. As a stand alone tweeter, 6khz with steep slopes would be indicated but given 40-50 per side I would image in the XO point could effectively be lowered to 4khz, more in line with the directivity of a 3.5" Fullrange unit. C to C spacing to the mids would be within array limits while the very small Apex Jr units would be ok up to 12 kHz or so relative to each other.

......or the alternative is to search for a robust tweeter that can cross lower and still stay cost effective. Not an easy quest.
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Old 18th January 2013, 06:48 PM   #12
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Quote:
Originally Posted by Overkill Audio View Post
Hi Dave,

Fab that you saw this thread, your line array knowledge is as good as it gets...

I have a question about combe filtering please!

In theory my BMR Line Array 112mm centre to centre driver spacing should result in nasty, audible peaks and troughs / lobing starting at 3KHz (ish).

Yet at anything over 2 meters listening position ( I listen at 4 meters) they sound amazing, seamless and even soundscape all over the room.
The 4.5 inch BMR does have broadest polar respose I have seen, but I also believe that room reflections play a huge part in " filling in the gaps" and or leveling out the peaks...
What do you think?

Thanks in advance and and all the best
Derek.
Hi Derek,

If you are interested in my AES paper then go to this thread and post #59. In fact the whole thread is good, as are the very interesting Keele designed CBT arrays.

Constant Beam Width Transducers line arrays

I did a lot of modeling of typical arrays to help understand the measurable phenomona. The paper also explores level tapering plus the near field/far field aspects of arrays.

Look at figure 26 and it shows the lobing issue in 2 ways: one is with regard to some fixed up and down distance, the other with regard to an up and down angle. Since we are more likely to move up and down a fixed distance (say sitting vs. standing) then figure 26 is most relevant and shows that the lobing undulations get gentler at greater distances. The top 3 curves show results to 4 meters which is about as far away as you'll get in the typical living room.

Also note that at those nearer distances you have roughly constant level for positions within the array end points: if you need a 2 foot vertical window then a 2ft long array will roughly do it. Still, figures 34 and 35 show how much of an improvement in vertical smoothness you can get with simple level tapering.

As to the audibility of the lobing, may experience was with the multi-tweeter Mac lines. Definitely with pink noise you could stand and sit and hear a "swish, swish, swish" as you moved. As you got farther back in the room it was still there but perhaps somewhat softened. I understand the argument of the reverberent field filling in the dips, but I am a big believer in the ears ability to focus in on the direct sound even if measurements don't reveal the issue at a distance.

Still, the effect was barely noticable on music (you had to focus in on it and move to hear the effect) and never seemed to hinder your enjoyment.

I don't remember any phasey effect below 10k Hz and our center to center spacing was 90 mm, so I'm guessing your array should be good to at least an Octave higher than you mention.

One last note, if anyone want to do some simple array modeling you can do it on Tolvan "the Edge". This is primarily a diffraction model but you can put multiple sources into it and effectively create a line and play with listening distance and observation height.

Regards,
David S.
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Old 18th January 2013, 07:04 PM   #13
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Default Thanks for your help

Thanks David,

I really appreciate your help and advice, I will read and re-read your post and follow on with the links you recomend.
I am hooked on line array's now and want to read and absorb (well as much as my 49 year old brain will allow!) as much info as I can find on the subject.

As I want my speakers to be on wall ( or maybe in wall in the future ) this adds another twist with the low frequency on wall and / or in corner gain.

The usual problem when just Googling around is sorting out the well meaning but ill informed opinion from the real deal qualified opinions.
Needles to say I value your input very highly as you certainly fall into the " real deal" group!

Have a great weekend and all the best
Derek.
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Old 18th January 2013, 08:16 PM   #14
ScottG is offline ScottG  United States
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Quote:
Originally Posted by mayhem13 View Post
It's all about the tweeters at this point. There's so many capable, affordable 3-4" midwoofers but when you start looking at the need of 50 tweeters per side,....well...
Cost considerations become a factor quickly!

If I use the Apex Jr Audax tweeters, cost is no longer a concern but now the crossover point becomes an issue. As a stand alone tweeter, 6khz with steep slopes would be indicated but given 40-50 per side I would image in the XO point could effectively be lowered to 4khz, more in line with the directivity of a 3.5" Fullrange unit. C to C spacing to the mids would be within array limits while the very small Apex Jr units would be ok up to 12 kHz or so relative to each other.

......or the alternative is to search for a robust tweeter that can cross lower and still stay cost effective. Not an easy quest.
From Zaph's blog page:
Zaph|Audio

"tweeter"
Vifa TC7FD00-04:
http://www.zaphaudio.com/temp/Vifa-TC7FD00-04-FR.gif
http://www.zaphaudio.com/temp/Vifa-TC7FD00-04-HD.gif
http://www.zaphaudio.com/temp/Vifa-TC7FD00-04-IMP-2.gif
http://www.zaphaudio.com/temp/Vifa-TC7FD00-04-CSD.gif

mid-"bass"
Vifa TC9FD18-08:
http://www.zaphaudio.com/temp/Vifa-TC9FD18-08-FR.gif
http://www.zaphaudio.com/temp/Vifa-TC9FD18-08-HD.gif
http://www.zaphaudio.com/temp/Vifa-TC9FD18-08-IMP-2.gif
http://www.zaphaudio.com/temp/Vifa-TC9FD18-08-CSD.gif



Jim Griffin's Line Array Paper:

http://www.diy-audio.narod.ru/litr/nflawp.pdf


Specifically page 8 figure 5 for the line length vs. distance..

For 14 feet listening distance, which is a bit over 4 meters, you go up the vertical axis 3 marks above the base line. Moving over horizontally (in freq.). To maintain line-source behavior at 1 kHz you'll need a line length somewhere between 1.5 and 2 meters in length.

If you want something more point source in nature then you'll need a line length less than that 1.5 meters (at 1 kHz).

The most important thing is "matching-up" the respective lines, (treble vs. mid-bass) over as much distance as possible.


Considering the rather far listening distance, my suggestion would be going for a more point source character beyond about 2 meters (..or perhaps even 1.5 meters). In other words matching-up both lines for line source operation up to 2 meters distance. That would put the line length at 1 kHz at just over a meter in length (around 1.3 meters) for a 2 meter transition. For a 1.5 meter transition it would be a little less than one meter in length (.9 meter) at 1 kHz.
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Last edited by ScottG; 18th January 2013 at 08:21 PM.
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Old 18th January 2013, 11:41 PM   #15
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Thanx Scott. So if i understand the line length at 1khz....that would mean at and above? which would require a line of tweeters as long as my total midbass array length of 48" or so.

You referenced the Vifa TC7 from Zaph as a tweeter. Does that imply that you suggest using them in that capacity?
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Old 18th January 2013, 11:44 PM   #16
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Quote:
Originally Posted by speaker dave View Post
Hi Derek,

If you are interested in my AES paper then go to this thread and post #59. In fact the whole thread is good, as are the very interesting Keele designed CBT arrays.

Constant Beam Width Transducers line arrays

I did a lot of modeling of typical arrays to help understand the measurable phenomona. The paper also explores level tapering plus the near field/far field aspects of arrays.

David S.
thanks from me as well Dave. The information listed in the white paper you wrote is invaulable and til today, i was unaware of it's existence. I've read it once but need another pass to thoroughly absorb it all before asking any questions.....at the risk of seeming ignorant.
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Old 19th January 2013, 12:10 AM   #17
thalis is offline thalis  Greece
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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.
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Old 19th January 2013, 02:42 AM   #18
18Hurts is offline 18Hurts  United States
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My revision 1 and 2 attempt crossed the Apex Jr. tweeters at 4.3 KHz at 18 dB per octave.

I figured they would be fine power wise since there were 48 of them running. Very true but output wise they did not work passively. Maybe some EQ but I wanted a passive solution for garage use.

The 3" full ranges crossing at 6KHz with a bessel filter worked well. The full ranges started to dip in output at over 5 KHz so the natural bump the bessel filter has at the XO point of 6 KHz cured that problem.

My initial paranoia faded away once I heard them play. Yes, the C to C distance and all that jazz--but if the frequency response was uneven, then I'd have a problem anyway. I went with even frequency response since I could control that much easier than center to center distance.

My general listening distance is 2 to 3 meters away--farther than that usually indicates I'm drinking beer and smoking something on the grill that demands greater attention.

Since I built them--don't listen to me. So I just listen to various friends that come over and the results are positive. "Those sound better than my Bose!" Ummmm, OK...don't use that one. My guitarist buddy loves the mid range and the sound of the guitar is very natural. That tells me I nailed the critical midrange (after the third try) so all is well. The only issue I have is the overall balance as the bass response is there but rolled off. Easily expected since 5" woofers in a sealed box is not the hot setup for deep bass response.

I went with sealed because in a garage exist critters--I know a snake lives in there which keeps the field mice from nearby from eating my cones. I respect the snake and it respects me but I won't have a port for the thing to set up a comfy living arrangement...it lives behind the freezer.

If you go with the Apex Jr. tweeters--cross them at 6KHz and they will need a bump in EQ at 10KHz+. Don't expect 20KHz out of them--it is not going to happen. If you want that--go with a super tweeter rolling in at around 12KHz and point it at your general listening area. I'd assume the seated position would be fine for critical listening.

As I stated in my other post, I do bring up the bottom end from 32 to 140 Hz with a pair of 15" PA woofers and an amp in mono. The arrays run full range and the mono amp rolls the woofers off with an electronic filter. A "1.5 way" if you choose calling it that. Since there are 24 five inch woofers and the old receiver provides less than 100 watts total, those 5 inch woofers never exceed Xmax even when getting hit with deep bass at max.

Arrays can handle a ton of power without distortion so use that advantage when setting them up.

Once you start building the beast--be aware it won't be correct the first time. My tweaks involved changing the tweeter line from 36 to 48 and then adding the twenty 3" full ranges and completely changing the crossover points and filter types.

If I was going to do it again--and I won't--but if I did, it would be a little different. I would of used eight 10" woofers in a sealed box and made the line arrays 7.4 feet (2.25 meters) tall. 25 of those 3" full ranges would of
done the trick (5S/5P) and 60 of the Apex Jr. tweeters to finish it off.

Such is the "knowing what I know now" part of life. If the center to center distance gets you all jumpy and hesitant--don't build them. It is impossible to get that right and hit 20 KHz. It can't happen--not with a bunch of 50 cent tweeters even if you shave them down with a sander.

I was motivated by my wife--after blowing the money to get all those speakers, crossover parts, glue, screws (over 1,000 were used) nylock nuts, terminals, handles, speaker carpet, round over bits, sand paper, solder flux, paint, brushes, wood, stain and polyurethane...then go through 3 revisions over a year. If I did not finish them I'd be living in them as she'd of thrown me out of the house!

I could build new boxes and just use six 12" woofers to replace the 12 five inch woofers--I'll give it a few years so I'll know I have a place to sleep when I'm done. The DIY mantra of "I'm saving money!" does not work anymore.
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Old 19th January 2013, 04:27 AM   #19
ScottG is offline ScottG  United States
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Quote:
Originally Posted by mayhem13 View Post
Thanx Scott. So if i understand the line length at 1khz....that would mean at and above? which would require a line of tweeters as long as my total midbass array length of 48" or so.

You referenced the Vifa TC7 from Zaph as a tweeter. Does that imply that you suggest using them in that capacity?
Yes.

Honestly though, I don't think it's a good idea. Not that line sources aren't a good idea, but rather that listening so far away from them (..under this domestic condition) - isn't a good idea.

If it were me with 14 foot listening distance I'd go with a small line of 4 of the 2" drivers on the bottom and 8 of the 3.5" drivers on top - 1st order with a crossover a little lower than 1.5 kHz. The tweeter line a little lower than your position and rotating the driver's to get polar tilt to work for you by tilting up toward the midbass line. Narrow profile with a sub underneath as a "platform" or on the side of the cabinet integrated into the loudspeaker.
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Old 19th January 2013, 12:12 PM   #20
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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.
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