05 March 2009: Original
The Internet is a wonderful repository of information—both
useful and less than useful—but it has gaps. A few days ago, I looked for
information on the typical output (in millivolts) of a dynamic microphone as
used in amateur radio. Microphone manufacturers, from what I can see, use
a two-step specification process. The first step defines how the
microphone responds to a defined sound level (mechanical) input. The second step
is an estimate of the typical sound level produced by an average talker.
Combined, these two factors result in the required answer, X millivolts.
Given the difference in microphone specifications and the
squishiness in the talker output level, the best answer I came up with was the
output is "a few millivolts."
Accordingly, I decided to measure the output of the
microphone section of my old Heil "Proset" boom microphone. It's equipped with
Heil's HC-5 "full range" element.
I make no pretence of claiming my measured data is
representative of what others might measure. There's too much variation in
voice, lip to microphone distance and the like for that. And, of course,
different microphones will produce quite different outputs for the same speech
The oscilloscope image below is taken when I spoke as
loudly as would be comfortable for a short duration. The oscilloscope is, of
course, a high impedance device as are microphone input amplifiers in typical
amateur radio SSB transmitters. The peak-to-peak voltage is 140 mV. In fact,
this level overstates the expected voltage because I would not speak this loudly
for more than a minute or two.