loudspeaker or speaker, device used to convert electrical energy into sound. It consists essentially of a thin flexible sheet called a diaphragm that is made to vibrate by an electric signal from an amplifier. The vibrations create sound waves in the air around the speaker. In a dynamic speaker, the most common kind, the diaphragm is cone-shaped and is attached to a coil of wire suspended in a magnetic field produced by a permanent magnet. A signal current in the suspended coil, called a voice coil, creates a magnetic field that interacts with the already-existing field, causing the coil and the diaphragm attached to it to vibrate. To provide a faithful reproduction of music or speech, a loudspeaker must be able to reproduce a wide range of audio frequencies (i.e., 20 Hz to 20 kHz). Since it is difficult for a single speaker to do that adequately, many quality sound systems employ speakers of three different sizes. The largest ones, or woofers, reproduce low frequencies; the medium-sized ones, or midrange speakers, reproduce middle frequencies; the smallest ones, or tweeters, reproduce high frequencies. The three sizes were traditionally contained in the same cabinet, with a pair of such speakers used for stereophonic sound, but increasingly in the late 1980s and early 1990s home sound systems used smaller pairs of speakers enclosures containing only the midrange speakers and tweeters and a single woofer cabinet designed to be placed in an out-of-the-way location. This design takes avantage of the fact that the human ear is not good at determining the direction of low-frequency sound, and thus stereo perception is not adversely affected by the location of the woofer. A crossover network is a filter in a speaker system that ensures that each speaker receives only signals in the frequency range it is designed to reproduce. A properly designed enclosure can reduce unwanted resonances and extend the frequency range of a speaker system. Along with dynamic speakers, other speaker types include electrostatic and piezoelectric.