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Multi-Way Conventional loudspeakers with crossovers

A dip at around 10KHz. Reason and correction.
A dip at around 10KHz. Reason and correction.
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Old 21st February 2019, 09:14 PM   #1
Aucosticraft is offline Aucosticraft  India
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Default A dip at around 10KHz. Reason and correction.

I got CX120 -8 CX120-8 4" Coaxial Driver with 3/4" Silk Dome Tweeter 8 Ohm Specification Sheet for project.

In the tweeter Frequency response published, No where I see a graph having "T" drop at around 10Khz . But in my measurements it shows this drop, when I measure at a distance around 15mm from tweeter dome. I thought its my measurement equipment problem. So I checked it with the other dome tweeter I had in spare. But measurements were all Ok. I took another frequency response measurement placing mic only 3 mm away from the tweeter dome of CX120-8. and the "T" drop did not appear. Unable to understand I tried to measure frequency response of the woofer part of CX120 -8. and there too i got to see such "T" drop , although the frequency of "T" was shifted.

So
1) The Question is this a geometry of the driver creating the problem? Or its something else.
2) How to correct it?
3) And why is this "T" Drop not seen in the Manufacturer's measurement.

Note: The graph attached is of CX120-8 coaxial driver. tweeter and woofer part.
Orange Colored FR graph is of tweeter measurements at a distance of 3 mm.
Green Colored FR graph is of tweeter measurements at a distance of 15 mm.
Blue Colored FR graph is of woofer measurements at a distance of 15 mm.
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File Type: png graph 1.png (29.5 KB, 375 views)
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Old 21st February 2019, 09:29 PM   #2
David Morison is offline David Morison  Scotland
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10kHz has a wavelength of about 34mm - your measurement distance is fairly close to half that, making me think it may be some kind of cancellation?
If so, measuring a bit further out may get rid of it - or at least shift it lower in frequency, confirming it as a measurement artifact and not a driver anomaly.
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Old 22nd February 2019, 02:26 AM   #3
Aucosticraft is offline Aucosticraft  India
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Thanks, Its giving me the dip even when I carry measurements from a feet. I will update the snapshot in a day or so. I am currently away from my home location.
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Old 22nd February 2019, 06:49 AM   #4
Sonce is offline Sonce  Macedonia
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You should measure at 1 m distance.
Almost all coax tweeters exhibit strong dip on axis in the range 8 - 10 kHz, except KEF and Tannoy coax tweeters. That dip disappears off axis at 30 degrees or so.
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Old 22nd February 2019, 07:55 AM   #5
Aucosticraft is offline Aucosticraft  India
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Quote:
Originally Posted by Sonce View Post
You should measure at 1 m distance.
Almost all coax tweeters exhibit strong dip on axis in the range 8 - 10 kHz, except KEF and Tannoy coax tweeters. That dip disappears off axis at 30 degrees or so.
Sure, I will try that. And update here in a day or two, once I am back to my home location.
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Old 22nd February 2019, 11:27 AM   #6
Quip is offline Quip
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Out-of-phase radial reflection at the tweeter-midwoofer cone transition interferes with the tweeter's output at 10kHz. Geddes briefly talks about it here.
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Old 22nd February 2019, 12:44 PM   #7
Juhazi is offline Juhazi  Finland
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Yep, it is almost unavoidable, like throat resonance in compression drivers. KEF's tangerine "phase plug" seems to help, but we diyers are helpless with other drivers.

Because of being an interference cancellation, it can be equalized only partially, and it has minimal effect in farfield and power response
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Old 22nd February 2019, 07:32 PM   #8
plasnu is offline plasnu  United States
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Even if you can correct this dip somehow, I don't think you will hear any difference. Just pretend that you didn't see it.
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Old 22nd February 2019, 07:39 PM   #9
bwaslo is offline bwaslo  United States
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A dip at around 10KHz. Reason and correction.
Maybe measure off-axis a bit (maybe 20degrees)?
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Old 22nd February 2019, 07:56 PM   #10
silverprout is offline silverprout  France
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Quote:
Originally Posted by bwaslo View Post
Maybe measure off-axis a bit (maybe 20degrees)?
It will be worse.
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