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Old 23rd July 2010, 03:53 AM   #11
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Originally Posted by panomaniac View Post
So you've seen THD at 55dB below the test tone? Do you remember what the SPL was?
Correct. I haven't done a gain cal of my mic preamp to work back to absolute SPL from Herb's sensitivity cal on the mic but mostly what I care about is THD at the crests of typical listening levels. So roughly 75 or 80dB SPL @ 1m---can't test into the mid 90s as I'd wake up the neighbors. It'd be nice to lift the signal farther off the ambient, but I've been able to get fairly decent results in quiet mornings with little wind, traffic, and not too much flow in the river.

I should point out that's above 1kHz or so; below that the noise floor comes up and by 20Hz the test tone's often lost in the ambient. The dropout on meaningful THD's usually around 700Hz; low enough to get a decent sense of how most woofers perform. The few hundred Hz range is still good enough you can estimate where the wall is on most mids and cross accordingly, though there's less information than I'd like. One of these days I'll get around to getting a cal'ed supercardiod and seeing how much it improves the SNR; omnis aren't really a good choice for assessing raw driver performance.
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Old 23rd July 2010, 05:47 AM   #12
Pano is offline Pano  United States
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Thanks for the info twest. Sounds good and practical. I'd love to have a nice calibrated supercardiod, too - but it may not be in the budget!

You must live very close to the river!
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Old 23rd July 2010, 04:50 PM   #13
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Hi all, Herb here.

Percy emailed me yesterday about putting together THD/IMD tests. I did a little experimenting and I think ARTA gives me the building blocks to do THD measurements. IMD is a little less certain since, to do it right, I need to good identical sources which I'm not sure I have right now.

I have a shipment of ECM8000s coming in later today that I'll play around with and I'll try to post back here in a couple of days.
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Old 23rd July 2010, 05:43 PM   #14
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Herb, ARTA will do IMD in spectrum analyzer mode with a 2-sine signal. Go to Generator - Configure to set the frequencies. Go to View - Info and check Distortion to show it in the status line. Go to Setup - Spectrum scaling (yeah I know, really illogical) and check what kinds of distortion you want to show. I just leave them all checked.

For HD sweeps, you can either do it in ARTA or STEPS.

In ARTA, use Impulse mode with a single channel swept sine signal. In the Generator setup, check the 'Center impulse response' box. Then go to Analysis - Frequency response and distortion after you record the impulse.

STEPS takes longer but it can display the curves in % distortion rather than dB.
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Old 23rd July 2010, 08:11 PM   #15
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It's a little complex in that I'll be comparing the distortion from my ref mic to the sample mic, so I need to make sure that the raw data is of a form that I can do the calculations to back out the difference. Hence the time for some experimentation.

Should be interesting.
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Old 23rd July 2010, 10:41 PM   #16
percy is offline percy  United States
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Thanks Herb!
Appreciate your willingness and your time. Do keep us posted. I am sure there are many of us eager to see the results.
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Old 23rd July 2010, 11:05 PM   #17
cuibono is offline cuibono  United States
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If not yet seen, this might interest some - audioXpress had an article on the distortion profiles of the wm61 capsule, stock and Linkwitz modded. I've posted it to this forum sometime in the past, but its relevant here, so I'll just post the pdf. Thanks to aX for making this available!

EDIT - its too big! Here is the link: http://www.audioxpress.com/magsdirx/...avaras2861.pdf
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Old 24th July 2010, 12:31 AM   #18
gedlee is offline gedlee  United States
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OK, I have a question: How is the THD of a mic measured? Surely its not a trivial task!
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Old 24th July 2010, 01:18 AM   #19
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On the original mic that it is tracked back to, I have no idea. On the others, it just seems to be compared to a reference mic. Same as for the FR.
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Old 24th July 2010, 01:25 AM   #20
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Awesome, Herb! Look forward to seeing your results.

Quote:
Originally Posted by panomaniac View Post
You must live very close to the river!
The rapids are about 0.5km away, 100kmh arterial about 1km away. Both create SnR problems if the wind's bringing noise from them; passband magnitude measurements are fine except for the low bass, but if I want good stopband, distortion, or phase data I have to wait for a different wind.
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