Point source Q: 5" @2kHz vs 4" @3kHz - diyAudio
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Old 22nd May 2009, 11:09 PM   #1
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Default Point source Q: 5" @2kHz vs 4" @3kHz

Which speaker configuration would act more like a point source: 5" crossed over @2kHz or a 4" crossed over @3kHz?

I'm guessing that at a certain distance there would not be much of a difference.
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Old 23rd May 2009, 01:59 AM   #2
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Driver starts to beam when the diameter is roughly half the wavelength, so:

5" -> 0.5 * 345/0.127 = 1358Hz

4" -> 0.5 * 345/0.102 = 1690Hz

Which means that at both your proposed freq. they wouldn't be point-source. Note that the baffle size needs to be considered as well.


Take for example Linkwitz' Pluto. The 5.5" driver is crossed very low (980Hz) to obtain the omni dispersion or point-source.
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Old 26th May 2009, 07:54 AM   #3
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I've been looking at the speaker threads tonight, because I just finished my latest brew, and for once I'm just thrilled with it. Here are my thoughts. Actually they might have Been Martin Colloms thoughts! LOL Anyway, the formula I use is the answer to the question: When is the dispersion -3db @ 45 degrees off axis? That is a total coverage of 90 Deg., and I am a high dispersion fanatic. - I use a well damped room, so I don't worry about reflection. The way I calculate it is, Take 13,560 as the speed of sound, in inches per second at sea level. Then divide by the width of the driver dome, or cone in inches. So my Dynaudio 3" domes would be -3db - 45 deg off axis at 4,520 HZ, a 6.5" mid-bass, (only using about 5" as usable piston width) would be 2,712. Say, if I was to cross them to a 1" dome, the tweeter would be good until, 13,560. Maybe I would go to a 3/4" dome then, which would be over 18,000. When I do off axis measurements, this bares out every time.
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Old 26th May 2009, 10:54 PM   #4
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For a 2-way": perhaps a 5" mid/woof and a 3/4" dome crossed at around 2kHz would provide the dispersion I'm looking for. 4" could cross higher, but then less bass. And a 6" or bigger is something that I have never been satisfied with.

Do you know of any low distortion 3/4" dome tweeters that can be crossed @2kHz (preferably 2nd order electrical)?
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Old 27th May 2009, 06:23 PM   #5
Face is offline Face  United States
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The Scan-Speak Illuminator D2004/6020 may work: http://www.madisound.com/catalog/pro...oducts_id=8512
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Old 29th May 2009, 08:51 PM   #6
jtalden is offline jtalden  United States
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Another option:
I finished a new design 3 mo ago. I chose the SEAS H1499 tweeter. It is 1" dia. but includes a "DXT"accoustic lens. This helps control the dispersion at the low freq end to match well the narrowing dispersion of the mid-woofer. The 2 drivers therefore blend well off-axis through the XO region.
I used 2 k for the XO, but this near the bottom end of the tweeter. A LR-24 XO to avoid overdriving it is a good idea.
There is excellent info and tech papers at:
http://www.dxt.dk/products/lens/waveguide/products.html
Of course this 1" tweeter will start to narrow the dispersion sooner than the 3/4" at the top end. I just felt the blend in the XO region was more critical. There is no best answer as everything is a tradeoff, but I am very pleased with the results.
Enjoy the process!
John
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