On Fri, Jan 02, 2015 at 09:20:00PM +1300, IVP wrote:
> I've not experienced it myself but my belief is that noise from
> switching in the audible range comes from the load, eg motor or coil
> windings vibrating, and some loads are better made than
> others. Occassionally I've heard the semi ringing if it's not bolted
> down hard enough to the heatsink, or like a loose transformer that
> hums until you give it a clout

A motor and mechanical load can be considered an audio transducer with
a very limited frequency and dynamic range.  But unlike transducers
designed for the job, the motor is full of resonant cavities and
components.

So if you drive it with any frequency, you will get an emission, and
the emission will be on frequency, and on harmonics of that frequency,
and on the frequency of the resonances, and on the harmonics of those
resonances, and so on.  That's excitement.

Placing that set of frequencies outside human hearing is common.
First iteration of design may neglect the high frequency capability of
young humans.

Then for certain applications there's low frequency.  In a recent
bench test driving a Delta AUB0812HH fan, with an ATmega328, a 7.8 Hz
PWM was sufficient to operate the fan and not cause audible noise.

--=20
James Cameron
http://quozl.linux.org.au/
--=20
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