Although at first glance, microphones and speakers appear to be very different kinds of devices, they are in fact closely related. Speakers and microphones are both transducers -- components which transform energy from one type to another. A speaker turns electrical currents into sound waves; a microphone converts sound into electrical energy. The main differences between them lie in the way audio designers have optimized each to perform its particular task efficiently.
A speaker produces sound when you drive it with an amplifier connected to an audio source. Most speakers use a electromagnet design in which a permanent magnet is situated in a metal frame that holds a cone made of paper or plastic. A wire coil attached to the end of the cone produces attractive and repulsive forces when electrical current flows through it; the pushing and pulling against the cone generates sound waves. The cone's size dictates the general frequency range it reproduces most efficiently: large cones produce low frequencies, and small ones generate high frequencies.
When you speak or sing into a microphone, the sound waves of your voice produce vibrations in a diaphragm inside the mike. Although they have a variety of basic designs, a common type called the dynamic microphone uses a magnetic principle similar to that used in speakers. The diaphragm carries a lightweight wire coil made of fine wire; the coil moves through a magnetic field and produces electrical currents which mirror the incoming sound waves. Another popular design, called the condenser microphone, places the diaphragm on one of two metal plates separated by an insulator. The vibrations in the diaphragm produce changes in the electrical capacitance between the two plates. Condenser mics require a battery, as the capacitance effect doesn't produce electrical currents by itself.
The dynamic microphone and standard speaker both employ a moving coil in a magnetic field, producing electrical currents from sound vibrations or vice-versa. It is possible, although risky, to connect a dynamic microphone to a speaker output and hear sound from the mic. As the microphone is not designed to handle electrical inputs, a loud amp setting can destroy the mic if used in this manner. In the same way, you can connect a speaker to a microphone input, but because a speaker doesn't make an ideal mike, you must yell into it to produce a detectable signal. Walkie-talkies and room intercom systems use a single speaker-microphone device that performs both functions moderately well.
Microphones produce a relatively weak output that requires preamplification to bring the signal to a standard line level. Because the signals are weak, microphone cables have shielding that reduces electrical noise picked up from fluorescent lights and appliances. A microphone picks up a wide range of frequencies with great sensitivity. The loudspeaker's purpose is to fill a room with high-fidelity sound. This means handling large amounts of power from an amplifier -- up to several hundred watts for some types of speakers. To manage the power, the speaker has a robust, heavy design. For good fidelity, a single speaker cabinet may have two or more separate speaker drivers, each suited to a particular frequency range; a single speaker does not have the wide range that a microphone has.
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