Aside from whatever gear reduction and drive train you need to use, what makes these motors so noisy on their own? Would they run quieter with sinusoidal commutation? If it is the commutation that is causing the sound, then this is a predictable waveform right? It would be easy to fit a speaker and produce destructive interference, because you know the waveform and can predict it... ?
Why are outrunners so noisy?
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Do you mean RC outrunners? Because hubmotors are generally outrunners, too. 
I think that a part of the sound comes from the coils vibrating with the PWM, as they're not usually lacquered down solid and immovable. If they were, then either the small amplitude high frequency vibrations would be damped or eliminated, or the low-frequency vibrations/resonances would increase because they'd now be moving the laminations easier.
I also think that gluing the lams together might help, basically coating them in varnish too, as they're being assembled (maybe would work after assembly, too, if the gaps are big enough for varnish's surface tension to pull it down into them).
The magnet ring itself could also be being flexed by the pull of the fields, and acting as a resonator, creating sound.
Then there is the resonance of the entire housing, especially for large enclosed outrunners like the 9C hubmotors.
Another issue is that at different speeds, air turbulence within the motor will cause vibrations, flutter, etc., which will resonate inside the motor cavity and be amplified.
I'm not sure there is any single solution that will mitigate all the possible sources of the noise, but I suppose it's possible the sinusoidal waveform might help, if vibration from switching is part of the issue, as the switching will be "slower", and the field will not pull as hard during on and off transitions that way. But it will probably still *have* the noise, just be lower amplitude and/or have different resonances.
All of this is just intuitive guesswork, as I don't know enough about any of these things to be sure.
I think that a part of the sound comes from the coils vibrating with the PWM, as they're not usually lacquered down solid and immovable. If they were, then either the small amplitude high frequency vibrations would be damped or eliminated, or the low-frequency vibrations/resonances would increase because they'd now be moving the laminations easier.
I also think that gluing the lams together might help, basically coating them in varnish too, as they're being assembled (maybe would work after assembly, too, if the gaps are big enough for varnish's surface tension to pull it down into them).
The magnet ring itself could also be being flexed by the pull of the fields, and acting as a resonator, creating sound.
Then there is the resonance of the entire housing, especially for large enclosed outrunners like the 9C hubmotors.
Another issue is that at different speeds, air turbulence within the motor will cause vibrations, flutter, etc., which will resonate inside the motor cavity and be amplified.
I'm not sure there is any single solution that will mitigate all the possible sources of the noise, but I suppose it's possible the sinusoidal waveform might help, if vibration from switching is part of the issue, as the switching will be "slower", and the field will not pull as hard during on and off transitions that way. But it will probably still *have* the noise, just be lower amplitude and/or have different resonances.
All of this is just intuitive guesswork, as I don't know enough about any of these things to be sure.
REdiculous
10 kW
High speed motors are just louder. Mine isn't an outrunner but it's pretty loud all by itself when I push it to 9k rpm.
Jeremy Harris
100 MW
I've tried to tackle this on a rebuilt motor, by immersing a rewound stator in silicone conformal coating for a while, then letting the excess run off and curing the stator in an oven. To be honest, it didn't make that much difference to the noise, so I'm pretty sure most of it comes from the can. It was also a pain to clean the coating out from the inner bore to bond the stator back on to the bearing carrier, so I don't think I'd bother to do it again.
As well as pulling the motor around, the pulses in the stator are also pulling the can in and letting it spring out at certain points. This makes it act pretty much like a loudspeaker - the principle is just the same. I'm sure that this is where the majority of the noise comes from.
Hub motors are probably quieter because they have far more rigid outer cans. It would probably be possible to make an RC outrunner a fair bit quieter by just stiffening the can up, maybe by wrapping it with unidirectional carbon fibre and epoxy or even just adding a secondary steel can press fitted over the original.
Jeremy
As well as pulling the motor around, the pulses in the stator are also pulling the can in and letting it spring out at certain points. This makes it act pretty much like a loudspeaker - the principle is just the same. I'm sure that this is where the majority of the noise comes from.
Hub motors are probably quieter because they have far more rigid outer cans. It would probably be possible to make an RC outrunner a fair bit quieter by just stiffening the can up, maybe by wrapping it with unidirectional carbon fibre and epoxy or even just adding a secondary steel can press fitted over the original.
Jeremy
John in CR
100 TW
Jeremy Harris said:
I like the idea of more steel, because the magnet shouldn't be leaking through the retaining ring anyway. I understand they go for minimum weight in RC, but wouldn't locking the magnetic field on the inside like my hubmotor seems like it would give the motor have more torque.
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