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Old 27th July 2005, 11:18 PM   #1
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Default A question on directivity

I am working on a non-horn 3 way system. I want the directivity to be controlled (i.e. somewhat narrow) and that the driver integration is apropriate "directivitywise". How many dBs is the maximum desirable differece between drivers at 15, and at 30 degrees respectively at the crossover frequencies (mine are about 350 and 3000Hz) ?

Thanks in advance.
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Old 28th July 2005, 02:03 AM   #2
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They should be as close to the same as possible (at all angles).
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Old 29th July 2005, 03:05 PM   #3
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Perhaps I wasnt very vlear with what I meant with controlled radiation. Im not aiming at a certain fixed dispersion all over the spectrum, but only 1) trying to mimimize directivity jumps between drivers at crossover points 2) that the directivity is somewhat narrow, at least from a point up.

I`ll be using dipole woofers which have a cardioid-like dispersion, an 8 or 10" midrange, and a tweeter on top, perhaps a ribbon(-like)one.

Any ideas or suggestions are appreciated.
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Old 29th July 2005, 04:27 PM   #4
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by swak
Perhaps I wasnt very vlear with what I meant with controlled radiation. Im not aiming at a certain fixed dispersion all over the spectrum, but only 1) trying to mimimize directivity jumps between drivers at crossover points 2) that the directivity is somewhat narrow, at least from a point up.

I`ll be using dipole woofers which have a cardioid-like dispersion, an 8 or 10" midrange, and a tweeter on top, perhaps a ribbon(-like)one.

Any ideas or suggestions are appreciated.

Take a look at this site and see if it makes sense.

http://www.kettering.edu/~drussell/D...ledPiston.html
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Old 29th July 2005, 08:00 PM   #5
poptart is offline poptart  Canada
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I don't suppose anybody has the animations from that page saved do they? They don't seem to work anymore.
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Old 29th July 2005, 08:35 PM   #6
Rudolf is online now Rudolf  Germany
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Quote:
Originally posted by swak
I`ll be using dipole woofers which have a cardioid-like dispersion ...
... so it would be best to follow that dispersion pattern with a dipole midrange. A 8 or 10" will start to beam at less than 3000 Hz. Therefore the tweeter dispersion should start with less than 180 too.


BTW.: Dipoles dont radiate cardioid-like, but in a figure 8 pattern.
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Old 30th July 2005, 12:57 AM   #7
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by poptart
I don't suppose anybody has the animations from that page saved do they? They don't seem to work anymore.
I don't recall them animated on line, but they just seem to be animation results from either MATLAB or MATHEMATICA or some analysis program like those.
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Old 30th July 2005, 01:08 AM   #8
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Quote:
Originally posted by swak
Perhaps I wasnt very vlear with what I meant with controlled radiation. Im not aiming at a certain fixed dispersion all over the spectrum, but only 1) trying to mimimize directivity jumps between drivers at crossover points 2) that the directivity is somewhat narrow, at least from a point up.
No, I got that. I guess I was not clear. That is what I meant - that you should have smooth transitions between drivers. Ie, the woofer should have some polar response at the crossover frequency and the mid should have exactly the same polar response at that frequency. That's what I meant by ' They should be as close to the same as possible (at all angles)'. I didn't mean they should be omnidirectional - just that the output level at 60 degrees, for example, of the woofer should match the output level at 60 degrees of the mid, etc.

Basically you will probably have a rough time without resorting to some sort of horn or waveguide to restrict the pattern. Especially with a ribbon - you will have essentially omni radiation in one plane which will likely not match up to any decent sized driver covering the frequency range below it. Unless of course you did something like a super wide range ribbon and crossed that to a sub, but there are still issues there.

For some examples (with horns), see the polar maps on my webpage.
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Old 30th July 2005, 01:12 AM   #9
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Rudolf:

Perhaps you could share a word or two about the dipole configuration. As I said I plan to use pair of dipole subs, each with two 15" drivers, probably cheap Pyle or the one of the equally priced from steelsound.com.

If I cross to an 8" driver at about 300 Hz (I know I might have to equalize the peak moving to the crossover point), how would the dispersion of the W-frame dipole at 300Hz be?

Would the directivity of an 8" driver like a B&C 8PE in a closed box aproximately match the dipoles directivity at the crossover point (300Hz)? I`m afraid the mid will be wider radiating than the sub, but I don`t know the functions for directivity to calculate how much.
Thanks in advance.
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Old 30th July 2005, 01:20 AM   #10
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John:

I was completely aware you had got the point. I should have specified this when writing, sorry for that.


What you point out about the tweeter is true, I think there is no other way. The horn resonances should be a smaller penalty at higher frequencies, therefore triying to get the midrange to go higher (moving the crossover freq. conveniently out of the most troublesome range) would be a nice thing. Perhaps stepping down and using the widerange 6" B&C, but only when the resonance and lower crossover point is more anoying that the slight lack of dinamics from a bigger mid.

Thanks for your reply.
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