Motors as speakers?

[Al Skierkiewicz]

Originally Posted by daltore

Anyone who's ever worked on VEX, or turned on a camera flash, has dealt with the microphony effect. This is when a high-frequency changing magnetic field is induced in a wire due to an electrical wave, and the field opposes itself, causing the whole coil to vibrate. It's what makes that high-pitch whine when you turn a VEX motor on half-power, or charge a camera flash.

Not exactly, what you are hearing are two different things. The high frequency you hear in a flash charging is the actual oscillator used in the DC-DC converter. It changes frequency because the feedback that causes oscillation is dependent on load current. As the capacitor charges the load current becomes less so the frequency rises.
The Vex motor whine could be a variety of things but is most likely the windings in the motor moving or the laminations of the armature reacting to the PWM input. Manufacturers of small, cheap components rarely dip the coils to prevent individual wires from moving in the magnetic field. Moving laminations is something we all just learn to live with. There is greater acoustic output from a motor when using the IFI 844 Victors because the switching frequency (150Hz) is so low compared to Jaguars (15kHz). Old 883 Victors switched at about 2kHz and made some nice whine when used in the past.
Motors have been used to drive speaker cones directly. But since there is so much mass to move, they have been relegated to very low frequencies such as large sub-woofers.
As to microphony effect, I think what you may be referring to is "microphonics". This is an effect wherein acoustic energy adds noise to a stable signal. Vacuum tubes were particularly sensitive to microphonic if tapped or if present in a large signal acoustic environment. Modulation of video was also a microphonic effect in cameras prior to the 1990's when used on music shows with loud PA systems. CCD cameras have nothing to move in the pickup so they are not sensitive to microphonics.