DC motor hum

In playing with several small dc motors (3v up to 12v surplus), and controlling the speed with potentiometer-scaled pwm, I was surprised at the significant hum in the motors...seems to stay pretty noticeable until you get to about 75%-80% duty cycle. I used the set up from the ITP Phys. Computing web site....

Is their a simple explanation of what is going on? Is there a workaround? Better motors? I need the speed because I'm vibrating very thin rods, (think a guitar string with a standing wave), but I'd really like to have a lower speed without the noise......

Thanks for you help in advance!

Is there a simple explanation of what is going on?

They've forgotten the words. ;D

PWM frequency is just short of 500Hz, maybe if you increased the frequency to something like 20kHz, it would be less noticeable.

Question Why do motors hum Answer because they don't know the words. ;D

However this might be because the windings move slightly as the sudden change in current cause a change in field and the windings repel. It is a function of how well the motor is wound. You can try filtering the PWM to more of a variable DC voltage or you could try changing the frequency of the PWM.

EDIT OK then. In Switzerland the cows all have bells, why?

'cos their horns don't work?

Ok, I deserved that! (True story: walked into a lumber yard and the salesperson actually asked, "How long you want them boards?" I couldn't resist....)

Thanks for the response - I'm a newbie, so I'll do my homework and look into changing the frequency....