Re: More on mics

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

> Tim Moody said the old timers call crystal mics " carbon mics" There is 
> such a thing as a carbon mic, and I don't think it is crystal. I 
> understand you have to apply an electric charge to a carbon mic to make 
> it work. They here used on old telephone handsets. This is my 
> understanding and not to be confused with absolute truth. ;^)

Hmmm... I didn't see the original by Tim...

This is true.  In fact, carbon mics are still in widespread use in
telephones, although they've been largely replaced by electret mics in newer
electronic phones.

Carbon microphone capsules consist of a container of carbon granules, about
the consistency of table sugar.  A current (20 mA) is applied.  When you
speak, the granules are compressed and decompressed, causing a decrease and
increase in resistance respectively, thus causing an increase and decrease
in current respectively.  The output of a carbon mic is VERY hot - the
hottest of all microphones, in fact.  This is why they were used in
telephones before the advent of solid state electronics.

Crystal mics use a solid piezoelectric crystal.  When piezo crystals have
physical stress applied, they produce electricity.  Try breaking a sugar
cube in a totally dark room sometime :-)

Many substances (such as our sugar cube, quartz, etc.) have a piezoelectric
effect.  Rochelle salt crystals are used in most crystal microphones.  I'm
not sure exactly why, but would guess that they have higher output and/or
better audio quality.

The crystal is typically attached to the diaphragm with a stiff metal wire. 
When sound is applied, the crystal is flexed, producing electricity.  No
current (or voltage in this case - crystals are hi-Z) is applied.



 -- mike