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Old 3rd October 2011, 07:55 PM   #1
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Default 2.5 way d'appolito?

Everybody knows this configuration: four (mostly identical) midwoofers in a box, with in the middle one tweeter.

Upon close listening, you can clearly hear the two midwoofers that are CLOSEST to the tweeter reproduce more mid than the other two.

My question is: are there any rules for the cross-over frequencies used in these systems? Is there a certain ratio between these, or has it anything to do with the f3 of the box as a whole, or.....??

I just don't know, but I want to understand how these things work...


Thank you all,

Timmo.
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Old 3rd October 2011, 08:34 PM   #2
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Nothing to do with f3 of the box. The best way to understand is to start the design in the middle and work out.

A conventional tweeter in the middle will likely have the usual crossover point of 3 to 5 kHz. Based on that the next two units, the inner mid/woofs will work the full woofer/mid range from f3 to (lets say) 3 kHz. If we stopped there we would have a full D'Appolito array. The horizontal performance would be wide and the vertical performance would be determined by the spacing from mid/woof to mid/woof. We want the units to be tightly clustered so that the vertical polar response isn't too messy but usually there will be lobes due to the distance from top woofer to bottom woofer.

Now if we added yet another pair of mid/woofer units, then we would make the polar response even worse since the array is now twice as tall and probably very long compared to a wavelength at 3kHz (about 3 inches). Rather than take the hit on polar response we will roll off the midrange of the outer pair. Done carefully we can have the response of the outer pair quite low in level before they reach frequencies where lobing will come into effect. Perhaps we will roll them off from 400 Hz on up.

This rolloff will have to be done experimentally because it is not only impacting the polar response, it is directly determining the midrange frequency response. For the total design to work we need woofers with a fair amount of midrange climb, say 4 to 6dB. We also need a low order rolloff so that we don't get a lot of phase shift between the inner pair and outer pair, that would tend to put a notch in the response. With that we can let the 3rd and 4th woofer add bass level without midrange level and, with careful crossover design, get the whole assembly flat.

I hope that all makes sense.

David S.
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Old 3rd October 2011, 08:41 PM   #3
18Hurts is offline 18Hurts  United States
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I built one for a center channel speaker

Four 5.25 inch woofers and a 30mm dome tweeter running "2.5 way" with a 18dB slope on the tweeter at 1,850 Hz. The outside two woofers are set around 450 Hz with a single inductor so it fades out before hitting the meat of the midrange. Works very well, the center to center distance is slightly under one octave for the mid to tweets and same with the inductor.

Quite a juggle but it "matches" my Infinity Overture 1 main speakers so gives a smooth sound stage.
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Old 4th October 2011, 06:57 AM   #4
alspe is offline alspe  Finland
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Quote:
Originally Posted by 325addict View Post
Everybody knows this configuration: four (mostly identical) midwoofers in a box, with in the middle one tweeter.

Upon close listening, you can clearly hear the two midwoofers that are CLOSEST to the tweeter reproduce more mid than the other two.
In 2,5-way configuration their is NO four midwoofers but two. Other two are woofers which are usually crossed over around where baffle step frequency is. So XO's 400 Hz (0,5 way) and 3000 Hz.
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Old 4th October 2011, 01:56 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

Most people are not familiar with a 2.5way D'Appolito. Your right, it needs four drivers.

Most would assume you meant a MTM, bottom M 0.5, but that's not D'Appolito.

And strictly speaking neither is the first case, see here :
Zaph|Audio - ZA5 Speaker Designs with ZA14W08 woofer and Vifa DQ25SC16-04 tweeter
The MTMMM is more a 2.5way D'Appolito, compared to the MMTMM.
But strictly speaking a D'Appolito is a MTM 2-way only.

Interesting discussion of the MMTMM.

rgds, sreten.

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Last edited by sreten; 4th October 2011 at 02:03 PM.
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