Why do I still hear audio plugging only signal pins of jacks

Question

I am debugging some audio project I am currently having fun with. I came down to the following (minimal ?) example I don't understand. The setup is as follows.

I have an audio stereo jack that comes out of my phone. It splits into 2 mono jacks. A male-male mono jack is connected to a speaker. If I make contact with only the signal pin of the jack plugged in the phone and the signal pin of the jack that goes into the speaker I can hear the sound.

Phone is working on battery and speakers on AC. Speakers also have balanced inputs. The other mono jack coming from my phone goes nowhere.

Answer 1

If your source is battery powered and your receiver is AC-powered, I can see a few explanations:

You’re inadvertently touching for example both left and right, producing the difference between left and right, played in mono. Fun experiment when intended to hear what’s going on in the mix, for example dry lead vocals panned straight to middle but stereo reverb on the same. If you’re playing a stereo mix, this should be almost as loud as when fully plugged in.

You’re actually only touching one single terminal, “breaking” KVL as there is no closed path for current to flow. The closed loop is formed by an air gap and therefore a capacitor between device chassis or plug, via either air to the cable and/or via you to cable and/or ground. This will have a distinct LF roll-off sound and lower amplitude.

Rough schematic:

^{simulate this circuit – Schematic created using CircuitLab}

If you’re touching both chassis, you will form a resistive return path between the devices. Depending on how hydrated you are, this will sound as an attenuation of sound but flat frequency characteristic.

Also, the higher your receiver input impedance, the easier it is to hear the capacitive coupling and lower frequency for the -3 dB crossover point.