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Something really weird happened while working out soundcard issues on my rockmite tonight. While holding two unconnected ends of a 3.5mm cable in each of my hands, the audio signal passed through my body. Is this a thing?

schematic

simulate this circuit – Schematic created using CircuitLab

More context:

Something in my mic input was fading all the morse code antenna signals away over ~1s each time I plugged the cable into the soundcard. So I was thinking about how I should attenuate the headphone jack before sending it to my mic port. Then discovered I could use... my body? Each time the rockmite powers on, it sends a welcome message in morse code over the headphone jack. Imagine my surprise when I heard the welcome message coming through my speakers while holding two ends of unconnected wire.

I wanted to eliminate my unshielded wires acting as antennas. I placed the ends close, parallel, and not touching. I could hear the welcome message very faintly. Next, I wrapped my fingers around each of the TRS connectors and powered on the radio. The sound was much louder. Plugging the two wires in together allows louder sound. Also note that the computer is hooked up to mains, but the rockmite is running on batteries an unconnected by wire to any part of the PC.

I can hold two "unconnected" wires in each hand and become part of the circuit just that easily? Why isn't my body insulating the audio signal like it does when working with DC? Or is DC actually passing through me too? Is my whole understanding that the human body insulates <24v wrong?

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Very nice example of interference. The signal could pass though, especially to high impedance and sensitive microphone input. With your body you are adding series resistance (range of some tens of kohms), but also resistance to ground that attenuate the signal. (it is actually more complex, as the signal is AC and and body is not pure real resistance, but a combination of inductance and capacitance too resulting in compex impedance).

I imagine that RF signal from the transmitter couples to your body and then excite the mic input.

To your last question, yes, your body can conduct also from a low voltage source, but you hardly power something through your body because you add too much resistance.

Your body is actually generating a small electrical signal (couple mV) that you can pick up by ECG.

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