A recording done with just the mic cable, without a mic

[jopatius]

I have too much noise in my signal path. While examining the causes for it, I confronted a strange phenomenon:

I plug the mic cable to the mic preamp, but do not connect a mic to the cable. Then I turn the amplification in my mic preamp up as far as it goes.
Then I tap the end of the cable (the female-end of the XLR cable) with my index finger, rather gently. It sounds like this when recording it in Reaper:

https://stash.reaper.fm/oldsb/802291/cabletest.mp3

I can hear my finger tapping the cable very clearly!

Different cables behave differently, some I need to tap quite hard in order to hear anything. It does not seem to matter if I tap the very end of the cable, or the metal surrounding the end of the cable.

Did I just invent "poor man's ultra-fast cable tester" or what is this phenomenon? Can I say that "the less there is tapping noise, the better quality the cable is"?


(OK I have some reverb in this recording too :-) but don't let it distract you)

[toyhouse]

Funny you'd bring that up.
I have one cheaper guitar cable that I'm sometimes forced to use. It's a noisey thing.

If I step on it I can clearly hear it. lol.
When I pick it up to move it I can hear many sounds, not just the one you describe. The gain is very high of course.
Could I shout at it and hear myself? I wouldn't be supprised. Lol.
So yes, I've heard the effect before many times.

You can hear your fingernail plinking against the glass of a preamplifier tube in a highgain circuit too.
In that case, the filiments vibrate and act like a mic.
As for the cable;
I guess we're generating a current by squeezing the atoms in the cable? And generating a current.
Similar to piezo element? Can't imagine what else it'd be.
A good test for bad cables? Maybe? Not sure.

Jim P.





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[Captain Damage]

Someone smarter than me may correct me, but as I understand it it's not a piezo effect, but rather an capacitance effect (not that I understand it much further than that). As it was explained to me, that's why more expensive low-capacitance cables are worth the extra money. I've had cheap guitar cables that did this (including lot of diy cables), but my Mogami cables do not.

[toyhouse]

Originally Posted by Captain Damage

Someone smarter than me may correct me, but as I understand it it's not a piezo effect, but rather an capacitance effect (not that I understand it much further than that). As it was explained to me, that's why more expensive low-capacitance cables are worth the extra money. I've had cheap guitar cables that did this (including lot of diy cables), but my Mogami cables do not.

Ahh, yes, - I've heard that too now that you've jogged my memory.
Perhaps the insulator of the center conductor is a cheap vinyl-type plastic that charges up...or stores charges rather than insulating like it should? The better cable using more exotic materials for those parts?

Funny, if it was a piezo effect like I was thinking, you probably wouldn't be able to move the cable at all without "explosive" efx. LOL.


Jim P.





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[Staccato]

Invalid test, the balanced nature of the cable requires it to be terminated at both ends. Opening one end of the cable defeats the circuits ability to achieve it's "CMRR": common mode rejection ratio, whereas signals/currents induced into the circuit are cancelled due to their relative phase.