Hub motor under vacuum (in theory)

So I was thinking on this, if we can drill a hole to the side cover and then suck the air out. Since the motor is almost completely sealed, it wont take much time to improve the sealing.

Ok, the first thing that sound can't travel in vacuum so, the motor would be quiet, if not completely silent. The air resistance would be zero, so I think that would increase the motor efficiency too. What do you think?

Nope, since the noise travels directly in the material from where it starts through the metal it doesn’t need any air to reach your ears.

The coil generates the sound and the side plate amplifies it.

And the coil is physically mounted to the axle, which is physically connected to the side plates by bearings.

You also won't be able to vacuum-seal a motor; you can't even waterseal one completely and air is much easier to fit thru gaps in bearings and cables/wires/conductor strands/insulation).

But if you have a vacuum pump, you should still try the experiment just to see what happens.

The coil acts on the magnet ring, creating resonance that produces sound from the magnets as well. In things like Drones, where the brushless motor is used as a beeper for locating a downed drone, it's the coil acting on the magnet to vibrate the outer ring, not the coil it's self that produces the sound.

A Vacuum would have lower internal air resistance, however at the RPM of a hub motor, the air resistance is so incredibly small that it is effectively zero. It might matter more in a different application, but not for an ebike.

The air in a hub motor is the main means by which the coils shed heat, so a vacuum would only serve to thermally insulate the part you want to keep cool.