Clyde Smith-Stubbs wrote:

> Hence electret mikes are also widely used in telephones and
> communication radios where carbon mikes were once used.

  Carbon mikes in communication radios eh?  You know, I REMEMBER that!
(Copious warm fuzzies from childhood ensue).

  Actually, there are a few design points to be noted here.  The
required testing criterion, extended "soak testing" at 125¡C
corresponding to storage on the dashboard or rear shelf of a car parked
in still air and full sun in an alley with the windows (glass ones that
is) closed; was not easily met by electrets.

  They appear to be used in SOME contemporary CB sets due to cheapness,
and many hand-held radios (notably including mobile phones) but
"professional" grade equipment generally uses dynamic microphone
elements as do many phones still.

  It turns out that the big (economic) advantage of electrets is their
high impedance which means high signal voltage without using expensive
transformers for coupling.  Light weight is good in a headset, counter-
productive in a telephone handset (many of which contain various forms
of ballast).

  Cheers,
        Paul B.