Question

When a fan makes a humming noise as it turns on, is the energy transfer from electrical to sound or mechanical to sound

Answer 1

Mechanical. The surging currents in the motor windings produce mechanical forces, which, on the one hand, rotate the fan, and, on the other, produce vibrations at about 60 Hz and its harmonics (the one most easy to hear is at 120 Hz). These vibrations produce similar vibrations in the air (i.e. sound) by direct contact.

It is very difficult to go directly from electrical forces to sound, because air is not very polarizable. It's for this reason we still need speakers, which audio currents shake in order to make sound. No one has yet found a way to directly couple air movements to changing electrical forces.

Answer 2

Thank you