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Old 25th August 2009, 09:56 PM   #1
tresch is offline tresch  United States
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Default Measurement trouble, noisy pulse, rising freq response

Heyo.

I'm frustrated. I've been trying for days to get some decent frequency response measurements using speaker workshop and a Panasonic capsule microphone.

I have the microphone wired up to the mic-in on the sound car (an E-MU Tracker Pre), so the sound card is providing the bias for the microphone.

I can record nice clean sin-waves all day, but I'm getting weirdness when taking measurements with MLS signals. I've played around with levels to make sure I'm not clipping the mic, and I've tried different sample sizes and such, but no matter what I do I get consistently odd results.

Here's a screenshot of a far-field measurement and a pulse response. Both taken at 1-Meter from the speaker. I'm aware the far-field will be innacurate without gating, but it's not so much that I'm seeing reflections, it's just a constant smooth and very strong (30db!?) rise from speaker resonance all the way to about 10k.

Furthermore, I'm trying to take a pulse response and no matter what I do I get output like in the screenshot. A strong initial pulse and then essentially random noise that continues indefinitely, which completely covers up any reflections it might have picked up.

The mic I'm using right now is wired up in the standard configuration, NOT with the "linkwitz" mod. I have tried one with the FET configured in reverse, like linkwitz suggested, but have had no different luck. It should be noted that I may well have overheated the microphone on the modified one, so the results from that are untrusted.

Regardless, with the unmodified capsule I get the same results even at rather low levels which should be well below the WM61A's modest limits.

I'm rather stumped and don't really know where to go from here. The unfortunate thing is I don't know anyone in the area that does this type of thing and might have a microphone I can test against. I do have a few dynamic microphones around... a Shure SM57 for instance... would that get me anything reasonable, at least for the purposes of testing to see if the problem is with my capsule microphones?
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Last edited by tresch; 25th August 2009 at 10:02 PM.
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Old 26th August 2009, 03:08 AM   #2
tresch is offline tresch  United States
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I downloaded and installed HOLMImpulse and magically I seem to be getting good results!

I must have something out of whack in the speaker workshop settings or something. Oh well. Hopefully I can do what I need to do with HOLM!
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Old 29th August 2009, 12:46 AM   #3
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I've had my fair share of troubles with getting decent measurements with a panasonic 61A and speaker workshop. I've occasionally got really good ones but most of the time have had problems. I don't remember seeing something like your rising response, but have certainly had problems with the pulse measurements. basically the same thing you are seeing.

I'd put it down to my environment, but it could well be speakerworkshop! I've got T/S parameter measurements with it down pat, but might be time to try something else for FR measurements! You might be able to export your measurements as FRD files from the new prog and import them into speaker workshop to continue your work once you have done the measurements.

The speakerworkshop source code is now available (has been for some time apparently) on Sourceforge, if any coders out there are up to the challenge of "tweaking" it Something I might (almost certainly really) have done 20 years ago, but not something I feel up for the challenge of these days

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Old 29th August 2009, 01:59 AM   #4
tresch is offline tresch  United States
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Oh man.. I do a bit of programming..

I forget that it was open source.

I really don't want to get into all the math of the equations involved in the number crunching, but I would love to have a chance to completely rework the user-interface. It's a bit of a mess!

Actually now that I think about it, the ultimate would be taking the functionality of HOLMImpulse, dropping it into speaker workshop, and making a whole new interface for the whole thing.

Last edited by tresch; 29th August 2009 at 02:01 AM.
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Old 3rd September 2009, 04:36 AM   #5
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unfortunately HOLMImpulse doesn't run under wine on linux
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