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Old 31st August 2010, 06:20 PM   #1
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Default Question about driver impedance data

Hi,

There's up to 3dB difference in FR between two SEAS P18RNX/P drivers at about 800Hz region when measured in same cabinet.

I measured impedance for both drivers in free air. See the attachment. Is this kind of difference in impedance normal between drivers?

Thanks,
Timo
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File Type: jpg P18RNX_P imp data.jpg (125.8 KB, 63 views)
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Old 31st August 2010, 07:04 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

For both drivers you have an impedance kink at around 800Hz, this
is not typical and indicates IMO sort sort of cone / surround resonance.
I would not be happy if my bass / mids measures like that in free air.

rgds, sreten.
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Old 31st August 2010, 08:23 PM   #3
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Hi Timo,

Some SEAS drivers of this Sd show this little bump on their impedance plot around 800hz, usually those which are relatively flat above this fr, like i.e. my U18RNX/P pair; I didn't yet measure their fr response because no box built at this time but here are two free air impedance plots:
Click the image to open in full size.

Click the image to open in full size.

This type of anomaly doesn't obligatory show a problem not manageable in the fr response (filtering)...but what is problematic to me is your 3dbs difference! I would return one or both of these drivers because if we do pay for European drivers we do pay for their supposed consistency...at least it's my opinion.
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Old 1st September 2010, 06:31 AM   #4
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Quote:
Originally Posted by tipo1000 View Post
Hi,

There's up to 3dB difference in FR between two SEAS P18RNX/P drivers at about 800Hz region when measured in same cabinet.

I measured impedance for both drivers in free air. See the attachment. Is this kind of difference in impedance normal between drivers?

Thanks,
Timo
You say is not the xover?
The difference of 3dB (fr. curve) you mention is not shown. You show a curve for impedance of both drivers that is typical from the manufacturer of a not matched pair. More details are needed to understand. Probably I would buy a third driver to match with one of the existents, if problematic (3dB difference).
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Old 1st September 2010, 11:48 AM   #5
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I recently refoamed two vintage Thiel 5 inch midranges and they had an impedance 'kink' in the 800-900 hz range. These mids were originally heavily coated - both cones and foam; perhaps to address resonance issues. I coated the foam with 3 coats of a PVA solution, but it didn't alter the impedance test.

The mid was hooked up to my function generator and I listened for any anoying resonance based sounds but couldn't detect any. So, I then did a quick respose test using HolmImpulse simply holding the driver in air. It looked fine.

Sometimes when I see a kink in the impedance test I look for a loose spider which can cause them also.
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File Type: jpg Shanahans Thiel 5 inch midrange refoam A.jpg (250.8 KB, 38 views)
File Type: jpg Shanahan thiel midrange resp.jpg (902.6 KB, 36 views)
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Old 3rd September 2010, 06:31 AM   #6
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Back to original question about impedance difference between these two drivers: Based on batch numbers there´s almost three years between their production and according to distributor there is differencies from batch to batch. So maybe that explains it....

But where this impedance bump comes from in first place? SEAS data sheet shows small bump in FR in same region.

Cheers,
Timo
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Old 3rd September 2010, 06:34 AM   #7
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And Seas data...
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File Type: jpg P18RNX_P.jpg (108.8 KB, 14 views)
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