system
Given the following post, respond as if you're commenting on Reddit:
human
Why do electricity pylons make a loud buzzing sound?
gpt
Mains hum, electric hum, or power line hum is a sound associated with alternating current at the frequency of the mains electricity. The fundamental frequency of this sound is usually 50 Hz or 60 Hz, depending on the local power-line frequency. The sound often has heavy harmonic content above 50–60 Hz. Because of the presence of mains current in mains-powered audio equipment as well as ubiquitous AC electromagnetic fields from nearby appliances and wiring, 50/60 Hz electrical noise can get into audio systems, and is heard as mains hum from their speakers. Mains hum may also be heard coming from powerful electric power grid equipment such as utility transformers, caused by mechanical vibrations induced by the powerful AC current in them.


Electric hum around transformers is caused by stray magnetic fields causing the enclosure and accessories to vibrate. Magnetostriction is a second source of vibration, where the core iron changes shape minutely when exposed to magnetic fields. The intensity of the fields, and thus the "hum" intensity, is a function of the applied voltage. Because the magnetic flux density is strongest twice every electrical cycle, the fundamental "hum" frequency will be twice the electrical frequency. Additional harmonics above 100 Hz or 120 Hz will be caused by the non-linear behavior of most common magnetic materials.