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Measuring loudspeaker impedance with HolmImpulse - possible?
Measuring loudspeaker impedance with HolmImpulse - possible?
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Old 12th June 2012, 09:46 PM   #1
IG81 is offline IG81  Canada
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Default Measuring loudspeaker impedance with HolmImpulse - possible?

Hi,

I use a Behringer UCA222 USB interface, which is supposed to be full duplex and has 2 channels, input and output sections.

I'd like to have HolmImpulse read the voltage at the input of the UCA222, which would be the voltage accross the 8ohm resistor in series with a driver. I'd export the voltage vs frequency data to a spreadsheet to convert to impedance vs frequqency, much like I do manually.

It did not work when I tried and I'm not certain why. One thing I'm not sure of is, where you choose the I/O devices, there is no distinction between the input or output "section" for "USB audio codec", much like how I can set the microphone for the ouput device (I don't know why this is allowed). So I am not even certain it even tried to read the UCA222's input.

Is this even feasible?

IG
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Old 8th January 2019, 04:44 PM   #2
Lojzek is offline Lojzek  Croatia
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Lightbulb Better late than never!

I was playing around and arrived at a solution how you could achieve that. Basically you connect a circuit like it is drawn and measure voltage drops across resistor and driver. These 2 plots will have to be manipulated by introducing a division of these two responses, driver voltage drop being in the nominator part of the fraction. HolmImpulse calls it C=A/B. Once you are done with that, remains to multyply the result by 15, and that means you would have to increase the resulting response by +23.52 dB. This comes from:


spl=20*log(15)=23.52dB


Now this resulting response is your impedance in dB, and in order to appear in ohm is needed this:


Z(ohm)=10^(Z/20); Z being you end result of measured impedance in dB.

Funny though, HolmImpulse can switch the amplitude scale from dB to linear scale and that is actually your Z in ohms. Still, the exported file will be in dB, so it needs a little work on the file to appear in ohms.


There has been a thread on Behringer uca 202 having issues with measuring impedance because of the capacitor in the sound card, so its own impedance rise at low frequencies adds to the one of the device under test. Perhaps doing the impedance measurements this way, will make this sound card a little more practical.


0 dB in the plots equals 1Vrms.
Attached Images
File Type: png Z measurement.png (25.2 KB, 80 views)
File Type: png V R15.png (39.3 KB, 81 views)
File Type: png V dr.png (43.8 KB, 81 views)
File Type: png Vdr divided by Vr15.png (44.3 KB, 81 views)
File Type: png Vdr divided by Vr15 and multiplied by 15=Z in dB.png (45.4 KB, 81 views)
File Type: png Amplitude in linear scale=ohms.png (48.1 KB, 11 views)
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Old 8th January 2019, 06:00 PM   #3
Pano is offline Pano  United States
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Measuring loudspeaker impedance with HolmImpulse - possible?
Thanks for that. These days we could also use REW and get a direct reading in ohms.

FWIW, I've tried to use my old WT3 with both HOLM and REW and can't get good results. Too bad, as the hardware still works, but the software for the WT3 isn't current.
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Old 10th January 2019, 05:02 PM   #4
tthorsten is offline tthorsten  Germany
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you can measure impedanz also with SATlive SATlive Audio Measurement Software
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