IMPEDANCE MEASUREMENT LOW FREQ

[JOSE BEMBIBRE]

Hi, I´m new on REW so apologies in advance..
The image above is what I get measuring the impedance of a woofer: abnormal high values on low freq. The same happens with a fix resistor.. I use a BEHRINGER UCA 202 with a sharp roll off below 20 Hz nevertheless I have properly calibrated the audio interface so I assume that this would be compensated. Is correct my assumption?
Is the UCA 202 fit to be used with REW? if not which are the minimum requirements?
Thanks
Rgds.
José

 

[John Mulcahy]

I'd suspect the problem is caused by differences in the behaviour of each channel at low frequencies, such as a difference in the frequency at which the rolloff begins. You are likely to get much better results with a card which has more low frequency extension.

 

[JOSE BEMBIBRE]

Thanks. Make sense, this means that the calibration process does not take in account this asimmetry. Is correct?
BTW some recommendations for another card?
Thks again
Rgds.
José

 

[John Mulcahy]

That's correct, the impedance calibration is accounting for channel gain difference based on a spot reading from smoothed data. It is worth trying your computer's built-in headphone output and line input if it has them, otherwise Steinberg and Focusrite make good interfaces.

 

[JOSE BEMBIBRE]

That's correct, the impedance calibration is accounting for channel gain difference based on a spot reading from smoothed data. It is worth trying your computer's built-in headphone output and line input if it has them, otherwise Steinberg and Focusrite make good interfaces.

Just to confirm that you guess was correct. The two inputs of UCA 202 have a very different response: changing the input to Left and reversing the connections accordingly make the measurement almost impossible.
Thanks for your help. As a side note will be advisable to indicate some minimal requirements for the A/D converters.
Best.
José