Why do my headphones hum under power lines?

Question

Whilst walking under overhead power lines, I notice a bass buzz/ rumble in my Bose Bluetooth headphones, which seems to fluctuate in amplitude. I understand there is naturally a hum/ buzz around power lines, but why would this affect my headphones?

Answer 1

Your earbuds contain high-gain amplifiers that drive the audio signals into the miniature speakers hidden inside the earbuds. Because the earbuds are not perfectly shielded against radiated electromagnetic interference as commonly produced by high-voltage power lines, it is not unusual for you to hear buzzing noises in all sorts of electronic audio equipment when you are near power lines.

Those power lines are carrying 50 or 60Hz AC power; why do you then pick up a crackly buzz that is manifestly not 50 or 60Hz? That's because the voltage on the wires is on the order of 225,000 volts or more on the big towers, and this causes intermittent currents to flow off the wires and into the air or over the outsides of the porcelain insulators that hold the wires in the air. That current "leakage" starts and stops with every cycle of the AC current and looks like a square or sawtooth wave, both of which contain lots of higher-frequency harmonics of the fundamental 50-60Hz signal. This means your earbuds will be amplifying the crackly buzz to the point where you hear it interfering with your music.