Simplify the following text for a Grade 11 native English-speaking person.
Original: Electric hum , mains hum , or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity , which is usually 50 Hz or 60 Hz , depending on the local power line frequency .
Simplified: Mains hum is a signal at a nominal 50 or 60 Hz , depending on local alternating current frequency , and usually some harmonic content .