On Fri, 18 Jun 2004 22:17:31 -0700, Bob Cain
wrote:



Don Pearce wrote:


When the diaphragm bottoms out, there is no resistor at all in the
current path.

I was discounting phantom as a source of current.

When a microphone is in use the back plate is at a high DC voltage,
whatever the source of current.

The capacitor in a microphone is of a very high grade,
with virtually no losses, and there will indeed be a very high current
flow momentarily (English meaning of this word - ie for a moment, not
sometime soon) as the diaphragm hits the back plate. While I wouldn't
expect it to actually cause attachment, it would not surprise me if it
blew a tiny pinhole. Accumulate a few of those and you could well do
audible damage.

Ok, then will you buy an energy argument? I'm too lazy to
plug in the numbers right now but I can't believe there's
enough energy on that little capacitor at 48 volts to
measurably raise the temperature of a mouse tear.


Bob

No. The energy is 1/2 C V^2. I know it is not huge, but the incredibly
high quality of the capacitor means that when the diaphragm touches,
the power released is quite large. This is because the capacitor
discharges very quickly. The diaphragm is very thin, for obvious
reasons, and it is quite possible that there would be local ablation
of the metallisation where the touch occurred. You would probably need
a microscope to find it though.

d
Pearce Consulting
http://www.pearce.uk.com