What mechanism is causing feedback in this setup?

[dougi]

The other day, when performing near-field measurements of my study speakers (as part of designing a crossover for them), I started getting feedback when measuring close to the tweeter (Seas D2010/8513) with no test signal being generated.

Measurement setup was: PC USB->Scarlett 2i2 audio interface out ->iFi iTube ->Yamaha WXA-50 AUX in->speaker->omni mic->Scarlett 2i2 mic in->PC USB using REW.

With no REW signal being generated, with the iTube in there I get feedback when the microphone is very close to the tweeter. The iTube was in-circuit left-over from using it's 6dB gain to boost the rather deaf sensitivity of the AUX input a bit.

The iTube (see this thread) and the WXA-50 both have some ultrasonics, but if it is them, how are they getting intermodulation and folding down into the audio band? The tweeter driver itself? (probably not then you would hear it all the time). The microphone somehow? (Sonarworks calibrated Behringer ECM8000)

It's no great deal, I just would like to understand it.

 

[mtg90]

You don't perhaps have "listen to this device" selected for the microphone input in windows do you?

 

[dougi]

You don't perhaps have "listen to this device" selected for the microphone input in windows do you?

No. It is all fine when I bypass the iTube. It's own ultrasonics must be being amplified a bit and getting through the tweeter and microphone enough and back into it to create some sort of feedback loop that eventually becomes audible somehow.

 

[somebodyelse]

The other obvious one to check is the direct monitoring setting on the Scarlett's front panel