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Old 9th May 2007, 03:58 PM   #1
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Default Tweeters for line array?

I have read Jim Griffin's paper on near field arrays with some interest, and I would like to have a go at building a pair.
Jim stipulates that the centre-to-centre distance of the tweeters should be 34mm.
I am looking at purchasing some tweeters that seem good value and good quality, but they have an outside diameter of 46mm.
I could shave some of the baffles off, but this would be time consuming and difficult to do neatly.
Please could someone explain what the affect of the larger c-to-c distance would be? I might just live with it.
The arrays will be in a smallish room (12ftx12ft), and I would imagine that the listener would be in near field at all times.
Thanks
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Old 9th May 2007, 04:58 PM   #2
sreten is online now sreten  United Kingdom
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Hi,

The short answer is one wavelength = 7.5kHz instead of 10KHz.

/sreten.
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Old 9th May 2007, 06:16 PM   #3
Zarathu is offline Zarathu  United States
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Default Line array Fanatic

Email me. I built a large line array using dome tweeters with comb filtering not starting until a pretty much inaudible 15Khz.

I can show you photos and show you how to do it.

Calipso@epix.net
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Old 11th May 2007, 07:49 PM   #4
thadman is offline thadman  United States
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Quote:
Originally posted by Zarathu
Email me. I built a large line array using dome tweeters with comb filtering not starting until a pretty much inaudible 15Khz.

I can show you photos and show you how to do it.

Calipso@epix.net
I believe you are mistaken, the comb filtering on your tweeters should begin MUCH sooner. I'm not sure which tweeter you are using, but believe it may be a dayton. There are two models the ND16FA (5/8" dome) and the ND20FA (3/4" dome).

The closest the ND16FA can be mounted to one another is 1.25" (the flange of these is 1.25"). Since you are in the nearfield, multiply that by two and divide the speed of sound by it (13,397inches-second/2.5=5358.8hz). Comb filtering will begin at 5358.8hz for the 5/8" dome...

The closest the ND20FA can be mounted to one another is 1.75" (flange is 1.75"). 13397/3.5=3827.7hz.

In the far field, loudspeakers must be within one wavelength of one another.

In the near field (line arrays operate in the near field), loudspeakers must be within one half of the wavelength of one another.

The only way to ameliorate the situation is to lower the number of places where the diaphragms are seperated. Dome tweeters centers are at the very center of the dome, while Ribbon tweeters centers are at the middle of the entire length of the diaphragm. The area below the edge of the top ribbon and the area above the edge of the bottom ribbon is the only area you have to worry about for CTC spacing for ribbons, as opposed to half the facade for dome tweeters. For this reason, Ribbons are a much better choice for line sources. You could make the situation even better by just going with a single line source such as a BG RD75.
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Old 11th May 2007, 08:07 PM   #5
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BG RD75.

Trade in a theoretical demon for a worser demon. hehe

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Old 11th May 2007, 08:44 PM   #6
Zarathu is offline Zarathu  United States
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Default Wait a doggone minute!!

My Dear Thad,

Of course you're are 100% correct---AS FAR AS YOU GO, which isn't far enough to solve this little enigma. I use the Dayton ND20FA tweeters. I notice that you didn't bother to email me to find out how I might have done this. You just jumped off completely half-cocked assuming that I couldn't possibly be correct.

You state:

"The closest the ND20FA can be mounted to one another is 1.75" (flange is 1.75"). 13397/3.5=3827.7hz."

However you obviously don't own any of these. You have also not read carefully Jim Griffin's line array paper, nor have you actually spoken to Dr. Griffin about this. You obviously have not built one of these line arrays with the tweeters.

Get a copy of the paper here: http://www.audiodiycentral.com/awpapers.shtml

I quote from page 13:

"For the tweeter line very close center-to-center spacing is difficult to attain as very small circular drivers would be necessitated for either the one wavelength or especially the half wavelength criteria. Consider operation to 20 kHz where one wavelength is 17.2 mm (0.68”) and a half wavelength is only 8.6 mm (0.34”). Without regard to their surrounding flanges, dome tweeters are available in 25 mm (1”), 19 mm (0.75”) and 13 mm (0.5”) diameters. Hence, with any mounting flange allowance at all, the one or half wavelength c-t-c criteria are very difficult—if not impossible--to satisfy at 20 kHz. But, if we relax the c-t-c criterion, more secondary lobes would appear in the 10 to 20 kHz frequency range. Fortunately, in this octave the ear is less sensitive (per FletcherMunson curves) so any secondary lobes likely would be less audible to the listener. Thus, if one wavelength spacing at 10 kHz is adopted as a compromise, then tweeter spacing would need to be 34.4 mm (1.35”) c-t-c apart. While more off axis secondary lobes would be generated in the far field, small flange tweeters are available to meet this dimension. The tradeoff is possible sound degradation from comb lines near 20 kHz. "

My line array cuts the flanges so that the C-to-C distance is closer to .9 inch. Using the 1 wave length criteria: 13560/.9= 15066hz. Additionally my lines are designed to be listened to very close to the edge of the nearfield-farfield boundary.

Since I have actually built such an array, and you have not, I assure you that sitting at about 9-10 feet from the tweeter portion of the array, using cut flanges with the tweeter array at about 34 inches in length,

THERE IS NO AUDITORY EXPERIENCED COMB FILTER DISTORTION(even standing and moving my head up and down).

You can rely on your theoretical discussion if you want. I will continue to rely on my own and all who have listened, auditory observation.

Until you have actually built a line array of this design, quite honestly, you don't know whereof you speaketh.

Zarathu
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Old 11th May 2007, 09:49 PM   #7
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Quote:
Originally posted by thylantyr
BG RD75.

Trade in a theoretical demon for a worser demon. hehe

Not sure you've ever heard a "properly" utilized RD-75... or you wouldn't be so negative... I've used them to advantage, and, compared to Griffin style multidriver arrays, there's no comparison. Of course, personal preference always plays a strong hand, so if your opinion of them is different, so be it. Mine sound great, better than any purely cone driver setup I've heard in 40+ years of audio pursuits (and I've heard alot).


Combing is an obnoxious artifact, very annoying even in the 10 -20 Khz range, especially vertically oriented...an obvious flaw in multidriver arrays
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Old 11th May 2007, 09:55 PM   #8
thadman is offline thadman  United States
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Default Re: Wait a doggone minute!!

Quote:
Originally posted by Zarathu
My Dear Thad,

Of course you're are 100% correct---AS FAR AS YOU GO, which isn't far enough to solve this little enigma. I use the Dayton ND20FA tweeters. I notice that you didn't bother to email me to find out how I might have done this. You just jumped off completely half-cocked assuming that I couldn't possibly be correct.

You state:

"The closest the ND20FA can be mounted to one another is 1.75" (flange is 1.75"). 13397/3.5=3827.7hz."

However you obviously don't own any of these. You have also not read carefully Jim Griffin's line array paper, nor have you actually spoken to Dr. Griffin about this. You obviously have not built one of these line arrays with the tweeters.

Get a copy of the paper here: http://www.audiodiycentral.com/awpapers.shtml

I quote from page 13:

"For the tweeter line very close center-to-center spacing is difficult to attain as very small circular drivers would be necessitated for either the one wavelength or especially the half wavelength criteria. Consider operation to 20 kHz where one wavelength is 17.2 mm (0.68”) and a half wavelength is only 8.6 mm (0.34”). Without regard to their surrounding flanges, dome tweeters are available in 25 mm (1”), 19 mm (0.75”) and 13 mm (0.5”) diameters. Hence, with any mounting flange allowance at all, the one or half wavelength c-t-c criteria are very difficult—if not impossible--to satisfy at 20 kHz. But, if we relax the c-t-c criterion, more secondary lobes would appear in the 10 to 20 kHz frequency range. Fortunately, in this octave the ear is less sensitive (per FletcherMunson curves) so any secondary lobes likely would be less audible to the listener. Thus, if one wavelength spacing at 10 kHz is adopted as a compromise, then tweeter spacing would need to be 34.4 mm (1.35”) c-t-c apart. While more off axis secondary lobes would be generated in the far field, small flange tweeters are available to meet this dimension. The tradeoff is possible sound degradation from comb lines near 20 kHz. "

My line array cuts the flanges so that the C-to-C distance is closer to .9 inch. Using the 1 wave length criteria: 13560/.9= 15066hz. Additionally my lines are designed to be listened to very close to the edge of the nearfield-farfield boundary.

Since I have actually built such an array, and you have not, I assure you that sitting at about 9-10 feet from the tweeter portion of the array, using cut flanges with the tweeter array at about 34 inches in length,

THERE IS NO AUDITORY EXPERIENCED COMB FILTER DISTORTION(even standing and moving my head up and down).

You can rely on your theoretical discussion if you want. I will continue to rely on my own and all who have listened, auditory observation.

Until you have actually built a line array of this design, quite honestly, you don't know whereof you speaketh.

Zarathu
You have taken my post out of context, It was not an attack against you. I was merely informing the thread starter how to properly design a line array and avoid comb filtering, you obviously have not created an *optimal* line array but I have no doubts that you are happy with it and that is all that matters.

I also NEVER debated whether comb filtering was audible or not, I simply stated when it should occur. ...and yes I have read Griffin's white paper on line arrays. It calls for CTC spacing of <1 wavelength in the far field and CTC spacing of <1/2 wavelength in the nearfield (of which line arrays propagate). I contemplated building one myself using those VERY same tweeters (Yes, I read the white paper thoroughly and did ALL THE MATH) and decided an ESL with a dynamic bass array would've been a more economic approach to a line source with greater sound quality potential.

I have a hard time believing you cut the tweeters down to .9" wide...as the cutout is 1-5/16". Could you provide some pictures please?

ND20FA-6
Specifications: *Power handling: 15 watts RMS/30 watts max *VCdia: 3/4" *Impedance: 6 ohms *Re: 5.2 ohms *Frequency range: 3,500 - 25,000 Hz *Fs: 1,800 Hz *SPL: 92 dB 2.83V/1m *Dimensions: A: 1-3/4", B: 1-5/16", C: 3/4".

Either way, with a flange measuring .9", comb filtering would begin at 7442.8hz in the nearfield...which I assume the threadstarter would envelop himself in as nearfield propagation is one of the line arrays greatest benefits (very little floor-ceiling reflections and -3dB for every doubling of distance)...I dont understand why you'd construct a line array and purposely listen to it outside of the nearfield? hmm...

.9(2)=1.8, 13397/1.8=7442.8hz

^What of these facts are you disputing? Please don't act arrogant and assume things (specifically that my posts were a threat against you, we're hear to inform people not force our beliefs on others)
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Old 11th May 2007, 10:01 PM   #9
thadman is offline thadman  United States
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Default Re: Wait a doggone minute!!

Quote:
Originally posted by Zarathu
My Dear Thad,

Of course you're are 100% correct---AS FAR AS YOU GO, which isn't far enough to solve this little enigma. I use the Dayton ND20FA tweeters. I notice that you didn't bother to email me to find out how I might have done this. You just jumped off completely half-cocked assuming that I couldn't possibly be correct.
Zarathu
As far as I go? hmm...please elaborate. I'm really not keen to your elitist attitude and I'm sure others aren't either. Please don't make personal attacks and assume things, this is a place of learning not a wrestling ring. Hurling insults and being an "e-thug" isn't going to intimidate or impress anyone.

What have I claimed or debated that is false? Please inform me...I AM CORRECT, YOU'RE WRONG, YOUR DESIGN IS FLAWED, PLEASE ACCEPT THAT AND MOVE ON.

I have used the information currently available, to come to perfectly plausible conclusions. You obviously have not taken the time to construct your line array "optimally" or engineer it "perfectly" as you seem to claim you have...why should we allow you to spread your ignorance and arrogance without a response?

How many line arrays have you constructed? Have you built one with an optimal arrangement and compared them subjectively and objectively?

Quote:
Originally posted by Zarathu
My Dear Thad,

You state:

"The closest the ND20FA can be mounted to one another is 1.75" (flange is 1.75"). 13397/3.5=3827.7hz."

However you obviously don't own any of these. You have also not read carefully Jim Griffin's line array paper, nor have you actually spoken to Dr. Griffin about this. You obviously have not built one of these line arrays with the tweeters.
Zarathu
...quoted from partsexpress


* Compact neodymium motors have low stray field
* Ferrofluid enhances power handling and helps damp resonance
* Front-mount pressfit design
* Center-to-center spacing as low as 1-3/4"
* Extremely flat and extended response

*Dimensions: A: 1-3/4", B: 1-5/16", C: 3/4".

The cutout is larger than .9"...
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Old 11th May 2007, 10:08 PM   #10
thadman is offline thadman  United States
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Default Re: Wait a doggone minute!!

Quote:
Originally posted by Zarathu
My Dear Thad,

Since I have actually built such an array, and you have not, I assure you that sitting at about 9-10 feet from the tweeter portion of the array, using cut flanges with the tweeter array at about 34 inches in length,

THERE IS NO AUDITORY EXPERIENCED COMB FILTER DISTORTION(even standing and moving my head up and down).

You can rely on your theoretical discussion if you want. I will continue to rely on my own and all who have listened, auditory observation.

Until you have actually built a line array of this design, quite honestly, you don't know whereof you speaketh.

Zarathu
How many line arrays have you constructed? Have you built one with an optimal arrangement and compared your flawed design with it subjectively and objectively?

If you havent, I wouldn't consider your biased opinion a reliable source.
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