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Microphones
5 questions By Tony R. Kuphaldt
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Question 4 of 5
The basic circuit for an unamplified capacitor microphone looks like this:
Vibrations in the air cause one metal plate to vibrate next to a stationary metal plate, rapidly changing the capacitance between the two plates. Explain how this simple circuit generates an AC voltage from this rapidly-varying capacitance, employing the mathematical relationship between charge (Q), capacitance (C) and voltage (V).
Reveal answerGiven the equation Q = CV, we can see that changes in capacitance will either result in changes of stored charge for a given voltage (Q changes with V constant), changes in voltage for a given charge (V changes with Q constant), or changes in both Q and V. Remember that changes in electric charge over time ([dQ/dt]) is the mathematical definition of electric current (I).
Follow-up question: capacitor microphones are almost never used without some form of amplification. The voltage signal generated by the circuit shown in the question is simply too weak to use on its own. Explain why the signal is so weak.
Notes:Ask your students where they located this information. it is important for students to learn where and how to research for needed information.
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Question 5 of 5
An older microphone technology is the carbon microphone. Explain how this type of microphone worked, and why it was so simple to use in a circuit.
Reveal answerA carbon microphone generates a changing resistance representing sound wave oscillations. These microphones could be used as easily as this, with no amplifier circuit necessary:
Notes:Carbon microphones may represent an older, obsolete technology, but they are still illustrative for students.
