Complex sentence: Mains hum, electric 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.
Split into simpler sentences: Mains hum, electric hum, or power line hum is a sound associated with alternating current at the frequency of the mains electricity. It is usually 50 Hz or 60 Hz, depending on the local power-line frequency.