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Microphone Basics
A microphone is a type of transducer -- that is, a device that turns one kind of energy into another. In the microphone's case, it turns the energy from sound waves into electrical energy. This electrical energy varies in amplitude exactly as the sound waves do, meaning that the original sound can be reproduced from the microphone's electrical signals. This can be done directly -- as in the case of a loudspeaker -- or the signals can be processed in some way, such as being recorded. A microphone is colloquially called a mic or mike, both pronounced "mike."
Dynamic Microphones
A dynamic microphone consists of an magnet wrapped in a coil of wire with a cone (sometimes a diaphragm) that vibrates easily in the presence of sound attached to the coil. When sound waves cause the cone to vibrate, the movement is transmitted to the coil which moves back and forth over the magnet. This in turn induces a varying electric current. Dynamic microphones are cheap and durable but have a less optimal response to some frequencies than more sensitive microphones.
Speakers as Microphones
A standard speaker is basically the reverse of a dynamic microphone: electrical signals go in and sound waves come out. A speaker can be used as a microphone, although the results will be poor. In some devices, such as a walkie-talkie or an intercom, a single unit performs the functions of both speaker and microphone.
Condenser Microphones
Condenser microphones take their name from an older term for capacitors. They are also called capacitor microphones. In a condenser microphone, changes in sound pressure move a thin metallic membrane in front of a static metal back plate. This changes the capacitance between the two, forcing a varying current through a resistor connected between the plates. Condenser microphones have a good frequency response but may pop and crackle if too close to the user's mouth. They also need a separate power source.
Ribbon Microphones
A ribbon microphone uses a thin strip of metal suspended in a magnetic field. When the ribbon vibrates a varying electric current is induced. Ribbon microphones are fragile; they are generally too sensitive to use outdoors unless heavily shielded. They pick up bass frequencies very well, giving a "warmer" or "richer" sound.
Crystal Microphones
When certain crystals are compressed they produce a small electric current. this is called the piezoelectric effect. Crystal microphones, also called piezoelectric microphones, use this effect by connecting a cone to a piezoelectric crystal, which then produces a varying current when pressed by the vibrations from the cone. Crystal microphones are sometimes used as contact microphones for acoustic instruments.
Fiber-Optic Microphones
Fiber-optic microphones use a diaphragm with a reflective surface, which vibrates when acted upon by sound. Laser light reflects off the diaphragm and is picked up by a fiber-optic sensor. The light is then transmitted to a receiver, which converts it into a varying electrical signal. Fiber-optic microphones are extremely useful in situations where ordinary microphones can't be used, such as an MRI room where no metal can be present.
Laser Microphones
Laser microphones can be used to record sound in distant rooms, as long as there is an exterior window. A laser beam is reflected from the window-pane and onto a sensor. Tiny vibrations of the window glass caused by sounds from inside the room affect the laser beam; these small changes are mirrored in the electrical signal from the sensor. Laser microphones are useful for law enforcement who may need to record conversations without being observed.