Re: camera flash sound

["John S. Denker" <jsd@MONMOUTH.COM>]

Carl E. Mungan wrote:

> As a camera flash charges up, one hears a whirr of increasing pitch.

Not on my camera, but I know what you mean.

> What causes this? Is there enough stray inductance to cause some
> oscillation and hence mechanical stress via a magnetic interaction

The inductance isn't what's stray.  But yes,
it's a magnetic interaction with the inductor.

You've got a switching power supply.
Central to its operation is an inductor,
which is used to stuff charge onto a capacitor.

Ideally they would use a toroidal inductor, which
has no stray fields.  But to cut costs they might
use an inductor with lots of stray fields.  These
put a force proportional to B^2 on any magnetic
objects in the vicinity.  And of course at those
frequencies any piece of metal will be diamagnetic
just due to its conductivity.  Then assuming some
less-than-infinite rigidity, you've got a noisemaker.

As the output voltage ramps up, the "inertia" of
the current in the inductor is more quickly overcome,
so the thing has to cycle faster.

> (vaguely light-bulb filament style)?

I don't know what that's alluding to.