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Old 10th April 2007, 02:45 PM   #1
Jay_WJ is offline Jay_WJ  United States
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Question How to interpret reverse null?

How do we interpret a reverse null?

For example, suppose we have two different 2-way LR4 XO designs whose XO frequency points and responses are almost identical, but one has a 15dB reverse null and the other has a 30 dB null when the tweeter polarity is flipped. Then which one is better and why? I would appreciate some help.
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Old 10th April 2007, 03:12 PM   #2
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I would ignore the reverse connection happenings as they are irrelevant. Just look what's different between your correctly connected responses.

I imagine that the one with the greater null is actually aligned best when connected correctly, but that's a sweeping generalisation that may not mean that is actually the better design.
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Old 10th April 2007, 04:28 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

As said its fairly meaningless to judge the quality of the deepest null.

It will vary massively depending on minor changes of vertical angle,
consistent nulling over a vertical range means no dips in reality.

/sreten.
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Old 10th April 2007, 04:40 PM   #4
banana is offline banana  Hong Kong
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Yes, the amount of reverse null greatly depends on mic alignment.
But once you've got patient to get the drivers physically aligned, reverse null tells you a lot about your crossover performance.

No just the amount of null, center frequency and symmetry were also important. 1 or 2dB of response irregularity in correct polarity could easily be mix up by baffle deflection or cone resonance.
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Old 10th April 2007, 04:57 PM   #5
Jay_WJ is offline Jay_WJ  United States
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Thank you for your replies!

Actually, I'm trying to build 2-way bookshelf speakers with Dayton RS180 and Seas 27TBFC/G. I'm currently in a simulation stage and came up with two different XO designs. In simulations, two XOs' performance is virtually identical with each other w.r.t frequency response. But the modeled reverse null of one is 15dB down and the other, 30dB down. That's why I asked the question.

Actually, I'll start a thread for this project. So, your comments will be more than welcome!
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Old 10th April 2007, 04:57 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by banana

But once you've got patient to get the drivers physically aligned,
reverse null tells you a lot about your crossover performance.
Hi,

It tells you a lot about meaningless "textbook" performance,
nothing else, precisely because of what you are saying.

/sreten.

note : talking about the maximum reverse null point.
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Old 10th April 2007, 05:06 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Jay_WJ

Actually, I'm trying to build 2-way bookshelf speakers with Dayton
RS180 and Seas 27TBFC/G. I'm currently in a simulation stage and
came up with two different XO designs. In simulations, two XOs'
performance is virtually identical with each other w.r.t frequency
response. But the modeled reverse null of one is 15dB down and
the other, 30dB down. That's why I asked the question.

Hi,

If you trust such figures from purely simulating,
you know something I do not.

FWIW to get a very deep null the outputs of both drivers must
match exactly AND the phase be exactly 180 degrees at the
same frequency, and be measured at an equidistant point.

You usually have far bigger fish to fry ........

/sreten.
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Old 11th April 2007, 12:36 AM   #8
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Are you planning on a flat or slanted baffle? If on a flat baffle, don't forget to enter an acoustic offset to align the drivers time wise, otherwise you will simulate phase alignment, but in reality will not be aligned due to acoustic offset. The simple math required for this can be found (I think) on the rjbaudio.com site.

Cheers,
David.
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