, is a common term for a loudspeaker
. A loudspeaker is a device that converts an electronic signal
into sound vibrations
. They are differentiated from buzzers
, and gongs
in that speakers reproduce signal energy. In other words, their output preserves the character of the electronic signal. Speakers vary greatly in size
and basic design
depending on their purpose.
Dynamic speakers are far and away the most common type of speakers. They are found in almost all applications. A cone which is usually made of stiff paper is suspended within a round metal frame and attached at all sides. At the center is the voice coil The coil moves with the cone, and is suspended in a fixed magnetic field Electrical energy is sent through the coil, and the interaction produces movements in the coil, which in tern moves the cone, producing acoustic signal energy.
Piezoelectric speakers are used primarily for very high frequency sounds. Sound waves are produced when electrical energy produces a corresponding shape change in a piezoelectric material. That material is usually a crystal.
Electrostatic speakers use a very light conducting diaphragm is used as one pole of a condenser or capacitor. The diaphragm is suspended, and vibrates when electrical energy is applied. Electrostatic speakers have a relatively low impedance, and require significant input power, making them difficult loads to drive. Their primary advantage is freedom from distortion produced by the differences between suspension and coil shape in the cones of dynamic drivers. They also require a very large area to produce long wavelength lower frequencies, which is one reason they are used only in very high quality applications. The Quad ESL-63 "US Monitor" is a well known electrostatic design, and often used with a subwoofer.
In musical or high quality applications, speakers are also differentiated in terms of their frequency output. Tweeters produce high frequency sounds, generally greater than 8,000 hz, though there is no hard rule on the frequency boundaries of any individual speaker driver. They tend to be small in size, sometimes less than one inch in diameter, in order to improve high frequency dispersion. The slit effect dictates that at high frequencies the angle of hoizontal dispersion is inversely related to speaker diameter. Almost all piezoelectric drivers are used as tweeters, and tweeters have been subject to more experimentation than any other driver.
Midrange speakers tend to be larger, between two and five inches in diameter. Depending on design and application they reproduce acoustic energy between 100 and 10,000hz. When designing musical speakers, the midrange is probably the most important driver, because it reproduces 90% of the musical notes, including overtones. Like tweeters, midranges vary greatly in basic design.
Woofers concentrate on the lower frequencies, generally below 200 hz. They are physically larger, generally with a diameter of six inches or greater. The larger size helps them move more air, and is helpful reproducing long wavelengths. Dispersion does not matter at low frequencies, because the lower the frequency the more it tend to omnidirectionality.
Subwoofers reproduce very low frequencies, generally under 100, and often under 50 hz. They are very large, generally with a diameter of over 12 inches. They are popular mostly in Home theater and very high end audio systems where the last few hz are important. They have very high power requirements, ass low bass notes require more energy than higher frequencies. The term subwoofer has been greatly abused in the media. Most "subwoofers" in the audio and computer jargons are actually lower midranges, or woofers.
A crossover is used to harmonize the actions of the two or more drivers found in a speaker system to produce a "flat" frequency response. It may be very simple, just a capacitor or induction coil, or extremely complex and adjustable.
Loudspeakers are greatly affected by cabinet design. The firm Acoustic Research pioneered the acoustic suspension speaker, where a sealed cabinet filled with damping material is used to control the back pressure on the inside of dynamic speaker systems, making very low bass possible in a small system, at the price of high power consumption.
Ported loudspeakers come in many forms, but generally a small opening in the speaker cabinet allows air to flow in and out of the speaker cabinet. As cone speakers move backwards and forwards, sound pressure is produced inside the cabinet when the speaker moves backward. The can be used to reinforce bass output at a chosen frequency. These speakers are more efficient than acoustic suspension designs, but great care must be taken in design if the port frequency is not to dominate.
Horn speakers take advantage of the tube effect in physics to reinforce sound waves. They posses unmatched efficiency, however true horn drivers are very large, requiring enclosures that may exceed forty feet in length! The folding horn designs pioneered by Paul Klipsch capture much of the efficiency of true horns at a size that works in large rooms. Horn speakers are preferred when sound output is the primary goal, such as in theaters.
The debate on what constitutes the best design is endless. Trust your ears.