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Old 23rd November 2005, 07:58 PM   #1
ch83575 is offline ch83575  United States
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Default Need help measuring impedance.

I am starting my first DIY project, so I am very new to this. I bought a pair of Aurum Cantus 130MKII 5.25" drivers for a small 2-way. They are listed as having a Fs = 38 and Vas = 27L. With speaker workshop I measure them at Fs = 50 and Vas = 15.5L. I have read that actual measurements can stray from published, but this seems like a bit much to me. I measured both drivers and they match each other within 1% for Fs, Vas and Qts. Should I be coincerned about the accuracy of my measuremets, or should I procede with enclosure design with the measured results?

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Chad
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Old 24th November 2005, 01:48 AM   #2
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Hi Chad,

1st have you broken in the drivers?? the fs will probably drop at least 10% (which would be around 45Hz) after breakin, and that would be well within the expected range I'd say.... Certainly with the higher FS the VAS should be lower, so your overall ratios are probably ok, and shouldn't affect the enclosure design per se.

The other thing is what level did you do your impedance measurements at?? I always do mine at a very low level, (around 4K on the VU meter), T/S params will vary depending on the level you drive the woofer at, Vance Dickason recommends using the lowest level that your measuring equipment can use. whether you can get clean results at that low a level will depend on your sound card though.

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Old 24th November 2005, 04:31 AM   #3
ch83575 is offline ch83575  United States
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That makes perfect sense. I hadn't thought at all about break-in. Do you have a recomended break-in procedure? My VU meter is about 4500 for the left and 4000 for the right (i guess thats because of the resistor ) for the maximum, is that right or were you refering to the average? Should I experiment with lower levels? When changing level I should actualy drive the speaker with less power, not just back off the recording level right? Also, should I recalibrate my jig with each level change or will it stay constant.

PS. I am using a seperate integrated amp for these tests, not an output direct from the card.

Thanks for your help,
Chad
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Old 24th November 2005, 04:47 AM   #4
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Probably best to do a search on breakin, lots of different oppinions and methods... My method didn't appear to be very successful the drivers virtually didn't change, then again maybe they weren't going to I used 20Hz for a few hours, some pink noise for a few hours, and the measured Fs for a few hours, all at reasonably high spl levels.

4k was the peak VU value, though if you are using a separate amp, with attenuating resistors then 4k could well be too high....does the pssssshhhhhhh coming from the speakers when doing the impedance measurement sound pretty quiet?? if it is loud then the level is probably too high (this will of course depend to a degree on the speakers sensitivity) I have an Audigy II ZS which is quite capable of driving the speakers direct for the impedance measurement I don't use the separate amp for this test.

I don't think it should be necessary to recalibrate for different levels, as the resistors you use for calibrating shouldn't be affected by voltage level too much, voice coils in loudspeakers are a different animal though and are effected by drive level, but it certainly can't hurt to redo the calibration

Yes you should keep the recording level constant and reduce the power going to the speaker... there will probably be an optimum record level for your particular sound card, where you get the optimal results, experimenting with RMAA is a good way to find the optimal settings for your card. I know on mine increasing the recording level increases noise and distortion a lot!

Tony.
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Old 24th November 2005, 08:31 AM   #5
sreten is offline sreten  United Kingdom
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Default Re: Need help measuring impedance.

Quote:
Originally posted by ch83575
.....They are listed as having a Fs = 38 and Vas = 27L. With speaker workshop I measure them at Fs = 50 and Vas = 15.5L. I have read that actual measurements can stray from published, but this seems like a bit much to me. I measured both drivers and they match each other within 1% for Fs, Vas and Qts. Should I be coincerned about the accuracy of my measuremets, or should I procede with enclosure design with the measured results?

Thanks,
Chad
Hi,

your measured results indicate a stiffer than specified suspension,
and the measured Qts should also be higher than specified.

If the box you are planning is less than around 10 litres the lower
measured Vas of the drivers will barely affect the alignment, in
fact if you are planning a small bass reflex the stiffer suspension
is actually an advantage when dealing with sub-sonic bass.

/Sreten.
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