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Old 12th June 2007, 10:05 AM   #1
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Default Tweeter resonant frequency missing

Trying to use Speaker Workshop to measure impedance.

I have been through all the calibration steps and have succesfully managed to measure passive components within close tolerances.

I have no problem measuring free air impedance of woofers. AS expected, I see the Fs of the driver and rising impedance with frequency.

However, when I measure tweeters - there is no resonant frequency at all. The impedance is flat until the natural rise toward the top end of the frequency range.

I am using a cable jig with an 8.2ohm resistor.

Attached is the tweeter free air impedance measurement.

Any help much appreciated

Thanks,
David.
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File Type: jpg wharfedale_tweeter.jpg (86.5 KB, 193 views)
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Old 12th June 2007, 10:06 AM   #2
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PS: I have measured 2 different tweeters and woofers. Attached below is the Wharfedale woofer

No setup changes were made between measurements (other than switching drivers over).
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Old 12th June 2007, 10:12 AM   #3
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What type of tweeter are you using?
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Old 12th June 2007, 10:39 AM   #4
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Sorry - should have said.

The image posted is a 3/4" metal dome tweeter (Wharfedale proprietary). The other tweeter I tested is a Vifa D19-TD-05 fabric (plastic?) 3/4" dome. The Vifa plot is below (similar symptoms).

Both tweeters were removed from working speakers so nothing wrong with the tweeters.

I have tried different sample sizes, rates, repeat counts to no avail.

Thanks,
David.
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Old 12th June 2007, 10:42 AM   #5
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Do the tweeters have ferrofluid?
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Old 12th June 2007, 11:00 AM   #6
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I'm not sure. It looks like I can remove the faceplate on the Wharfedale tweeter but am reluctant to do that.

Good thinking... I thought "maybe this is a case of ferrofluid flattening out the resonant peak", but surely even the most damped / fluid filled tweeters have some sort of peak? (even if only a few dB above nominal?)

They both seem too flat - even for ferrofluid tweeters.

From memory, I don't think the Wharfedale's do. I couldn't find the D19 spec in the Tymphany archive pages. will google it now.

Cheers,
David.
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Old 12th June 2007, 11:49 AM   #7
soongsc is offline soongsc  Taiwan
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Did you do the interchannel latency calibration?
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Old 12th June 2007, 12:03 PM   #8
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Hi soongsc,

Yes I did. Found -14usec flattened the phase rise toward the upper end as shown in the attached image (zoomed in)

I also found a 500msec latency got the MLS pulse starting consistently around 400msec.

Cheers,
David.
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Old 12th June 2007, 12:31 PM   #9
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by Dave Bullet
Hi soongsc,

Yes I did. Found -14usec flattened the phase rise toward the upper end as shown in the attached image (zoomed in)

I also found a 500msec latency got the MLS pulse starting consistently around 400msec.

Cheers,
David.
Is the greem line prior to interchannel latency?
If it is, then it seems the tweeter impedance must be right. I also have a tweeter that has a flat impedance and flat phase as well.
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Old 12th June 2007, 01:02 PM   #10
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Possible reasons for "flat" impedance curve:
1. Ferrofluid
2. copper short circuit rings.
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