Loudspeaker
A loudspeaker is a combination of one or more speaker drivers, an enclosure, and electrical connections. The speaker driver is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound.
The driver is a linear motor connected to a diaphragm, which transmits the motor's movement to produce sound by moving air. An audio signal, typically originating from a microphone, recording, or radio broadcast, is electronically amplified to a power level sufficient to drive the motor, reproducing the sound corresponding to the original unamplified signal. This process functions as the inverse of a microphone. In fact, the dynamic speaker driver—the most common type—shares the same basic configuration as a dynamic microphone, which operates in reverse as a generator.
The dynamic speaker was invented in 1925 by Edward W. Kellogg and Chester W. Rice. When the electrical current from an audio signal passes through its voice coil—a coil of wire capable of moving axially in a cylindrical gap containing a concentrated magnetic field produced by a permanent magnet—the coil is forced to move rapidly back and forth due to Faraday's law of induction; this attaches to a diaphragm or speaker cone in contact with air, thus creating sound waves. In addition to dynamic speakers, several other technologies are possible for creating sound from an electrical signal, a few of which are in commercial use.
For a speaker to efficiently produce sound, especially at lower frequencies, the speaker driver must be baffled so that the sound emanating from its rear does not cancel out the sound from the front; this generally takes the form of a speaker enclosure or speaker cabinet, an often rectangular box made of wood, but sometimes metal or plastic. The enclosure's design plays an important acoustic role thus determining the resulting sound quality. Most high fidelity speaker systems include two or more sorts of speaker drivers, each specialized in one part of the audible frequency range. The smaller drivers capable of reproducing the highest audio frequencies are called tweeters, those for middle frequencies are called mid-range drivers and those for low frequencies are called woofers. In a two-way or three-way speaker system there is a small amount of passive electronics called a crossover network which helps direct components of the electronic signal to the speaker drivers best capable of reproducing those frequencies. In a powered speaker system, the power amplifier actually feeding the speaker drivers is built into the enclosure itself; these have become more and more common, especially as computer and Bluetooth speakers.
Smaller speakers are found in devices such as radios, televisions, portable audio players, personal computers, headphones, and earphones. Larger, louder speaker systems are used for home hi-fi systems, electronic musical instruments, sound reinforcement in theaters and concert halls, and in public address systems.
Terminology
The term loudspeaker may refer to individual transducers or to complete speaker systems consisting of an enclosure and one or more drivers.To adequately and accurately reproduce a wide range of frequencies with even coverage, most loudspeaker systems employ more than one driver, particularly for higher sound pressure level or maximum accuracy. Individual drivers are used to reproduce different frequency ranges. The drivers are named subwoofers ; woofers ; mid-range speakers ; tweeters ; and sometimes supertweeters, for the highest audible frequencies and beyond. The terms for different speaker drivers differ, depending on the application. In two-way systems there is no mid-range driver, so the task of reproducing the mid-range sounds is divided between the woofer and tweeter. When multiple drivers are used in a system, a filter network, called an audio 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 a woofer and a tweeter; a three-way system employs a woofer, a mid-range, and a tweeter. Loudspeaker drivers of the type pictured are termed dynamic to distinguish them from other sorts including moving iron speakers, and speakers using piezoelectric or electrostatic systems.
History
installed an electric loudspeaker in his telephone in 1861; it was capable of reproducing clear tones, but later revisions could also reproduce muffled speech. Alexander Graham Bell patented his first electric loudspeaker as part of his telephone in 1876, which was followed in 1877 by an improved version from Ernst Siemens. 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. Compressed-air designs are significantly limited by their poor sound quality and their inability to reproduce sound at low volume. Variants of the design 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.Moving-coil
The first experimental moving-coil loudspeaker was invented by Oliver Lodge in 1898. The first practical moving-coil loudspeakers were manufactured by Danish engineer Peter L. Jensen and Edwin Pridham in 1915, in Napa, California. Like previous loudspeakers these used horns to amplify the sound produced by a small diaphragm. Jensen was denied patents. Being unsuccessful in selling their product to telephone companies, in 1915 they changed their target market to radios and public address systems, and named their product Magnavox. Jensen was, for years after the invention of the loudspeaker, a part owner of The Magnavox Company.File:Edward Kellogg & Chester Rice with cone speaker 1925.jpg|thumb|Kellogg and Rice in 1925 holding the large driver of the first moving-coil cone loudspeaker
The moving-coil principle commonly used today in speakers was patented in 1925 by Edward W. Kellogg and Chester W. Rice. The key difference between previous attempts and the patent by Rice and Kellogg is the adjustment of mechanical parameters to provide a reasonably flat frequency response.
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 a 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 that the loudspeaker was connected to. 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 going to the voice coil and added to the audible hum. In 1930 Jensen introduced the first commercial fixed-magnet loudspeaker; however, the large, heavy iron magnets of the day were impractical and field-coil speakers remained predominant until the widespread availability of lightweight alnico magnets after World War II.
First loudspeaker systems
In the 1930s, loudspeaker manufacturers began to combine two and three drivers or sets of drivers each optimized for a different frequency range in order to improve frequency response and increase sound pressure level. In 1937, the first film industry-standard loudspeaker system, the Shearer Horn System for Theatres, a two-way system, was developed under the auspices of 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.Altec Lansing introduced the 604, which became their most famous coaxial Duplex driver, in 1943. It incorporated a high-frequency horn that sent sound through a hole in the pole piece of a 15-inch woofer for near-point-source performance. Altec's "Voice of the Theatre" loudspeaker system was first sold in 1945, offering better coherence and clarity at the high output 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.
In 1954, Edgar Villchur developed the acoustic suspension principle of loudspeaker design. This allowed for better bass response than previously obtainable from drivers mounted in larger cabinets. He and his partner Henry Kloss formed the Acoustic Research company to manufacture and market speaker systems using this principle. Subsequently, continuous developments in enclosure design and materials led to significant audible improvements.
The most notable improvements to date in modern dynamic drivers, and the loudspeakers that employ them, 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. At low frequencies, Thiele/Small parameters electrical network theory has been used to optimize bass driver and enclosure synergy since the early 1970s.