# thiele-small impedance measurement problem



## boze902 (Aug 22, 2011)

REW is wonderful, except I can't get it to measure TS of a speaker on either my mac or linux computers. The problem is that it says "The DC resistance value is greater than the minimum impedance, please enter a DC resistance less than 1.2 ohm". The curve is very spiky, if I do smoothing, it looks great, but the T-S Param calculation seems to use the spikey curve where there are values that drop down close to 1 ohm.

I'm using the Griffin iMic. Should I run the output through an amp? If so, the manual says to protect the inputs with back to back zener diodes. I could use more specifics, like a drawing. What resistive divider provides 20dB of attenuation? Another drawing would help, as I don't know what it is.


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## JohnM (Apr 11, 2006)

The iMic is unlikely to work well, it has a line out which would need a high value sense resistor to avoid overloading the output and the results would be noisy. It would be much better to use a headphone output if one of your computers has one.


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## boze902 (Aug 22, 2011)

Thanks for the help. I still wonder if third octave filtering BEFORE the calculations wouldn't make the iMic interface noise less of a problem.


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## JohnM (Apr 11, 2006)

Filtering changes the shape of the impedance peak, which in turn alters the results - not necessarily a big shift, but better to get a cleaner signal if possible. The curve matching does inherently reject much of the noise, so entering a lower DC R would probably still give a good result (REW would just calculate a higher dR value) but difficult to be confident of that without seeing the measurements.


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## boze902 (Aug 22, 2011)

Thank you!


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