A loudspeaker (or "speaker") is an electroacoustic transducer that converts an electrical signal into sound. The speaker pulses in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water.
Loudspeakers (and other electroacoustic transducers) are the most variable elements in a modern audio system and are usually responsible for most distortion and audible differences when comparing sound systems.
Terminology
The term "loudspeaker" can refer to individual transducers (known as "drivers") or to complete systems consisting of an enclosure incorporating one or more drivers. To adequately reproduce a wide range of frequencies, most loudspeaker systems require more than one driver, particularly for high sound pressure level or maximum accuracy. Individual drivers are used to reproduce different frequency ranges. The drivers are named subwoofers (for very low frequencies); woofers (low frequencies); mid-range speakers (middle frequencies); tweeters (high frequencies); and sometimes supertweeters , optimized for the highest audible frequencies. The terms for different speaker drivers differ, depending on the application. In two-way loudspeakers, there is no mid-range driver, so the task of reproducing the mid-range sounds falls upon the woofer and tweeter. Home stereos use the designation "tweeter" for high frequencies, whereas professional audio systems for concerts may designate high frequency drivers as "HF", or "highs", or "horns". When multiple drivers are used in a system, a "filter network", called a crossover, separates the incoming signal into different frequency ranges and routes them to the appropriate driver. A loudspeaker system with n separate frequency bands is described as " n -way speakers": a two-way system will have woofer and tweeter speakers; a three-way system is either a combination of woofer, mid-range, and tweeter, or subwoofer, woofer, and tweeter.
History
Johann Philipp Reis installed an electric loudspeaker in his telephone in 1861; it was capable of reproducing pure tones, but also could reproduce speech. Alexander Graham Bell patented his first electric loudspeaker (capable of reproducing intelligible speech) as part of his telephone in 1876, which was followed in 1877 by an improved version from Ernst Siemens. Nikola Tesla reportedly made a similar device in 1881, but he was not issued a patent. During this time, Thomas Edison was issued a British patent for a system using compressed air as an amplifying mechanism for his early cylinder phonographs, but he ultimately settled for the familiar metal horn driven by a membrane attached to the stylus. In 1898, Horace Short patented a design for a loudspeaker driven by compressed air; he then sold the rights to Charles Parsons, who was issued several additional British patents before 1910. A few companies, including the Victor Talking Machine Company and Pathé, produced record players using compressed-air loudspeakers. However, these designs were significantly limited by their poor sound quality and their inability to reproduce sound at low volume. Variants of the system were used for public address applications, and more recently, other variations have been used to test space-equipment resistance to the very loud sound and vibration levels that the launching of rockets produces.
The modern design of moving-coil drivers was established by Oliver Lodge in 1898. The first practical application of moving-coil loudspeakers was established by Peter L. Jensen and Edwin Pridham, at Napa, California. Jensen was denied patents. Being unsuccessful in selling their product to the phone companies, in 1915 they changed strategy to public address, and named their product Magnavox. Jensen was, for years after the invention of the loudspeaker, a part owner of The Magnavox Company.
The moving-coil principle as commonly used today in direct radiators was patented in 1924 by Chester W. Rice and Edward W. Kellogg. The key difference between previous attempts and the patent by Rice and Kellogg was the adjustment of mechanical parameters so that the fundamental resonance of the moving system took place at a lower frequency than that at which the cone's radiation impedance had become uniform. See the original patent for details.
About this same period, Walter H. Schottky invented the first ribbon loudspeaker.
These first loudspeakers used electromagnets, because large, powerful permanent magnets were generally not available at a reasonable price. The coil of an electromagnet, called a field coil, was energized by current through a second pair of connections to the driver. This winding usually served a dual role, acting also as a choke coil, filtering the power supply of the amplifier to which the loudspeaker was connected. AC ripple in the current was attenuated by the action of passing through the choke coil; however, AC line frequencies tended to modulate the audio signal being sent to the voice coil and added to the audible hum of a powered-up sound reproduction device.
In the 1930s, loudspeaker manufacturers began to combine two and three bandpasses' worth of drivers in order to increase frequency response and sound pressure level. In 1937, the first film industry-standard loudspeaker system, "The Shearer Horn System for Theatres" (a two-way system), was introduced by Metro-Goldwyn-Mayer. It used four 15″ low-frequency drivers, a crossover network set for 375 Hz, and a single multi-cellular horn with two compression drivers providing the high frequencies. John Kenneth Hilliard, James Bullough Lansing, and Douglas Shearer all played roles in creating the system. At the 1939 New York World's Fair, a very large two-way public address system was mounted on a tower at Flushing Meadows. The eight 27″ low-frequency drivers were designed by Rudy Bozak in his role as chief engineer for Cinaudagraph. High-frequency drivers were likely made by Western Electric.
Altec introduced their coaxial Duplex driver in 1943, incorporating a high-frequency horn sending sound through the middle of a 15-inch woofer for near-point-source performance. Altec's "Voice of the Theatre" loudspeaker system arrived in the marketplace in 1945, offering better coherence and clarity at the high power levels necessary in movie theaters. The Academy of Motion Picture Arts and Sciences immediately began testing its sonic characteristics; they made it the film house industry standard in 1955. Subsequently, continuous developments in enclosure design and materials led to significant audible improvements. The most notable improvements in modern speakers are improvements in cone materials, the introduction of higher-temperature adhesives, improved permanent magnet materials, improved measurement techniques, computer-aided design, and finite element analysis.
Driver design
The most common type of driver uses a lightweight diaphragm, or cone , connected to a rigid basket , or frame , via a flexible suspension that constrains a coil of fine wire to move axially through a cylindrical magnetic gap. When an electrical signal is applied to the voice coil, a magnetic field is created by the electric current in the voice coil, making it an electromagnet. The coil and the driver's magnetic system interact, generating a mechanical force that causes the coil (and thus, the attached cone) to move back and forth, thereby reproducing sound under the control of the applied electrical signal coming from the amplifier. The following is a description of the individual components of this type of loudspeaker.
The diaphragm is usually manufactured with a cone- or dome-shaped profile. A variety of different materials may be used, but the most common are paper, plastic, and metal. The ideal material would be stiff, to prevent uncontrolled cone motions; light, to minimize starting force requirements; and well-damped, to reduce vibrations from continuing after the signal has stopped. In practice, all three of these criteria cannot be met simultaneously using existing materials; thus, driver design involves trade-offs. For example, paper is light and typically well-damped, but not stiff; metal can be made stiff and light, but it is not usually well-damped; plastic can be light, but typically, the stiffer it is made, the less well-damped it is. As a result, many cones are made of some sort of composite material. This can be a matrix of fibers, including Kevlar or fiberglass; a layered or bonded sandwich construction; or simply a coating applied to stiffen or damp a cone.
The basket, or frame, must be designed for rigidity to avoid deformation, which would change the magnetic conditions in the magnet gap and could even cause the voice coil to rub against the walls of the gap. Baskets are typically cast or stamped metal, although molded plastic baskets are becoming common, especially for inexpensive drivers. The frame also plays a considerable role in conducting heat away from the coil.
The suspension system keeps the coil centered in the gap and provides a restoring force that cau
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