Sound Science: Microphones and Speakers

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 (as it is usually conically shaped for sturdiness) 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.

In order 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 (intended) 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 (picture at right) 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. Sometimes the reproduction of the very lowest frequencies (20Hz-~50Hz) is augmented by a so-called subwoofer often in its own (large) enclosure. In a two-way or three-way speaker system (one with drivers covering two or three different frequency ranges) 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 so-called 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 speakers.

Smaller speakers are found in devices such as radios, televisions, portable audio players, personal computers (computer speakers), headphones, and earphones. Larger, louder speaker systems are used for home hi-fi systems (stereos), electronic musical instruments, sound reinforcement in theatres and concert halls, and in public address systems.

Johann Philipp Reis 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 (capable of reproducing intelligible speech) 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.

The dynamic speaker was invented in 1925 by Edward W. Kellogg and Chester W. Rice issued as US Patent 1,707,570. Apr 2, 1929.


This article includes material from “Sound.” Wikipedia. https://en.wikipedia.org/wiki/Loudspeaker Licensed under Attribution-ShareAlike 3.0 United States (CC BY-SA 3.0 US) https://creativecommons.org/licenses/by-sa/3.0/us/ Authors: https://en.wikipedia.org/w/index.php?title=Loudspeaker&action=history