Recording - May 2017 - 39
Voltage at the point where the resistor meets the capsule goes down.
What happens when the momentarily decreased air pressure from the
rarefaction segment of the sound wave pulls the capsule's diaphragm
outward? Now the capsule has less capacitance, and will hold less charge,
so the sequence of events shown above is reversed. As the charge rushes out,
the voltage difference now works in the other direction, and the voltage at the
bottom of the resistor now rises, to +40.1V (Figure 7). The sequence of events
now becomes:
Air pressure pulls diaphragm out.
Capsule capacitance goes down.
Voltage at the point where the resistor meets the capsule goes up.
As the diaphragm moves in and out in response to sound, the voltage at
the point where the capsule meets the resistor goes down and up (Figure 8);
if it's a good microphone the electrical voltage produced by the capsule
will correspond fairly closely to the pressure changes on the diaphragm. (The
polarity will be wrong, but that can be fixed later in the circuit.)
So that's how a condenser mic capsule works to translate variations in air
pressure (sound) into corresponding variations in electrical voltage (audio signal).
We say that the microphone is behaving as a transducer-that is, it's converting
one type of varying signal (sound) to another type (electrical).
Unfortunately, a naked condenser mic capsule wouldn't be much good in the
real world; to be usable, the capsule needs to be connected to a head amplifier.
Why? We'll discuss that next time.
Paul J. Stamler (stamler@recordingmag.com) is an audio engineer, educator,
musician, and collector of vintage and historical audio recordings, living and
working in St. Louis.
7
8
http://www.RoswellProAudio.com
Table of Contents for the Digital Edition of Recording - May 2017
Contents
Recording - May 2017 - Intro
Recording - May 2017 - Cover1
Recording - May 2017 - Cover2
Recording - May 2017 - 1
Recording - May 2017 - 2
Recording - May 2017 - 3
Recording - May 2017 - 4
Recording - May 2017 - 5
Recording - May 2017 - Contents
Recording - May 2017 - 7
Recording - May 2017 - 8
Recording - May 2017 - 9
Recording - May 2017 - 10
Recording - May 2017 - 11
Recording - May 2017 - 12
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Recording - May 2017 - 15
Recording - May 2017 - 16
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Recording - May 2017 - 18
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Recording - May 2017 - 20
Recording - May 2017 - 21
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Recording - May 2017 - Cover3
Recording - May 2017 - Cover4
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