Before magnets got cheap, some loudspeakers were powered by compressed air
Around 1900 a few record players pushed sound out with blasts of compressed air. Horace Short patented the idea in 1898 and sold it to Charles Parsons, and companies like Pathé built machines around it. The sound was poor and could never play quietly, yet versions of the design now shake spacecraft parts to mimic rocket launches.
Speakers began inside telephones. Johann Philipp Reis fitted one to his telephone in 1861, and it managed clear tones and, later, muffled speech. Alexander Graham Bell patented a moving-iron speaker that could carry intelligible speech in 1876, and Ernst Siemens improved on it a year later. Thomas Edison toyed with compressed air for his cylinder phonographs before settling on a metal horn driven by a membrane.
Oliver Lodge built the first experimental moving-coil speaker in 1898. Peter Jensen, a Danish engineer, and Edwin Pridham produced the first practical ones in 1915 in Napa, California. Denied patents and unable to interest telephone companies, they sold to radio and public address users under the name Magnavox. The design found in most speakers today was patented in 1925 by Edward Kellogg and Chester Rice, whose key step was tuning the mechanics to give a fairly even response across pitches. Current in a coil sitting inside a magnetic field shoves it back and forth, and the attached cone pushes the air.
Early models relied on electromagnets, because big permanent magnets were too costly, and the field coil doubled as a filter for the amplifier's power supply, though it added hum. Jensen offered a fixed-magnet speaker in 1930, but heavy iron made it impractical until light alnico magnets spread after the Second World War.
Splitting the job came next. A single driver works well only over a limited range, so systems pair small tweeters for treble with larger woofers for bass, with a crossover sending each the right frequencies. In 1937 Metro-Goldwyn-Mayer produced the Shearer Horn, the first cinema-standard speaker system, with four 15-inch bass drivers. In 1954 Edgar Villchur worked out acoustic suspension, which got better bass from smaller cabinets, and founded Acoustic Research with Henry Kloss to sell it.
Source: Loudspeaker
