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Old 28th August 2009, 03:25 PM   #1
Stu is offline Stu  Australia
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Default Measured Fs much higher than spec

Okay, I finally got around to rebuilding a pair of Vifa P17/D25 speakers that I've had for a while, but never really been too happy with the sound. I originally pulled the drivers out of a second-hand Jaycar JV60 kit.

Measuring the response in Speaker Workshop seems to suggest that the T/S parameters are pretty far out-of-spec, particularly the free-air resonance for the woofer. I know it's typical to see some variation, but I'm getting close to 100 Hz when it should be 37 Hz (strangely it's happening on all three of the woofers - they all seem to be defective by the same amount).

They're probably about 10 years old at this point - is it typical for the parameters to degrade by that much over time? Or even to vary that much out of the factory?

I guess it's possible I'm doing something wrong in speaker workshop, but my impedance jig works for other components and I've tried a few different mounting methods with the same result.

Any suggestions, or are these woofers rubbish?
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Old 28th August 2009, 08:33 PM   #2
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That's way more than I've ever seen. Indeed, the worst cases I'm aware of aren't nearly that bad even with drivers where the surround is just a rolled out/pleated edge of the diaphragm. These can become hard enough to split with minimal motion.

Spiders OTOH once loosened up usually stay fairly flexible once their starch is heated/flexed out of them, although I've seen some so dried out they cracked too.

Anyway, try measuring while heating them up with a hair dryer or better yet, measure them outdoors in a warm/hot noonday sun. If there's no dramatic drop, then WYSIWYG......... Ditto if there's a drop and they rise back up once cooled off.

GM
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Old 28th August 2009, 08:55 PM   #3
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Old JBL Lansaloy surrounds that have hardened over time will measure like that, but it's also quite obvious that they are stiff....
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Old 28th August 2009, 08:57 PM   #4
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Quote:
Originally Posted by Stu View Post
I guess it's possible I'm doing something wrong in speaker workshop,
Do you have any known good drivers to test? Or can you borrow some? That might tell you if your SW setup is good. As GM says, that's quite a difference!
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Old 28th August 2009, 09:09 PM   #5
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Originally Posted by ZilchLab View Post
Old JBL Lansaloy surrounds that have hardened over time will measure like that, but it's also quite obvious that they are stiff....
Oops! I'd forgotten about reading about this and its brake fluid potential solution, but like Altecs, none of the JBLs I've measured shifted much at all except for early D130s.

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Old 28th August 2009, 11:11 PM   #6
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Hmm... strange.

Don't have another driver at the moment but for my next project (once I decide what that is) I'll definitely measure the drivers first. If they're also significantly off, it's probably my method that's off. I could also try to loosen them up, but after years of use I'd assumed they'd reached their final state by now.

Otherwise it's a little puzzling why they'd all be off by the same amount if it's rare for even one to be out by that much to begin with.

Thanks, guys.
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Old 29th August 2009, 12:19 AM   #7
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Or you could be measuring on a different point in the T/S curves than the factory -- althou with that large a difference i'd suspect something is not quite right. With 3 drivers so closely matched i'd tend to suspect the measuring kit 1st.

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Old 29th August 2009, 02:09 AM   #8
Stu is offline Stu  Australia
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Or you could be measuring on a different point in the T/S curves than the factory -- althou with that large a difference i'd suspect something is not quite right. With 3 drivers so closely matched i'd tend to suspect the measuring kit 1st.

dave
You mean a different signal level? I'm just using the line-level output and input from my computer's sound card. I wouldn't have thought it would make much of a difference since the T/S parameters are small signal anyway, but I guess I could try it with an amplifier.

For what it's worth, the tweeters are right on the money - Fs is within 10% of the 850 Hz spec for each driver. So if there is a problem with the setup, it's only manifesting itself at lower frequencies.

Edit: I've also tried a frequency sweep with oscilloscope software (rather than what seems like an analysis of white noise with speaker workshop). Same response... although I wasn't expecting any difference.

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Old 29th August 2009, 02:25 AM   #9
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How are your speakers positioned? If laying on a desk or bench, that'll mess it up bigtime.

I position them on a post about 60cm above the table, on their magnet, removing as much obstructions from the rear as much as possible.

Cheers!
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Old 29th August 2009, 03:02 AM   #10
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I wouldn't have thought it would make much of a difference since the T/S parameters are small signal anyway
You wouldn't think...

Attached is a comparison of measures i made & then Mark Fenlon made on the same driver. Factory for reference.

dave
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