# Microphones

## AC Electric Circuits

• #### Question 1

Explain how a moving-coil, or dynamic microphone works. Identify how the principle of electromagnetic induction is put to practical use in this device.

• #### Question 2

Certain substances are known to physically deform (shorten or lengthen) when an electrical voltage is applied across their length. The effect is known as piezoelectricity. What types of substances are known to be piezoelectric? Identify some possible applications of this phenomenon.

• #### Question 3

Explain how a crystal microphone works. Identify how the principle of piezoelectricity is put to practical use in this device.

Follow-up question: characterize the typical output impedance of this microphone type. Is it generally considered a low-impedance or a high-impedance type of microphone? Why do you think this is?

• #### Question 4

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).

• #### Question 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.