The acoustic era (1877 to 1925)[edit]
The earliest practical recording technologies were entirely mechanical devices. These recorders typically used a large conical horn to collect and focus the physical air pressure of the sound waves produced by the human voice or musical instruments. A sensitive membrane or diaphragm, located at the apex of the cone, was connected to an articulated scriber or stylus, and as the changing air pressure moved the diaphragm back and forth, the stylus scratched or incised an analogue of the sound waves onto a moving recording medium, such as a roll of coated paper, or a cylinder or disc coated with a soft material such as wax or a soft metal. These early recordings were necessarily of low fidelity and volume, and captured only a narrow segment of the audible sound spectrum — typically only from around 250 Hz up to about 2,500 Hz — so musicians and engineers were forced to adapt to these sonic limitations. Bands of the period often favored louder instruments such as trumpet, cornet and trombone, lower-register brass instruments (such as the tuba and the euphonium) replaced the string bass, and blocks of wood stood in for bass drums; performers also had to arrange themselves strategically around the horn to balance the sound, and to play as loudly as possible. The reproduction of domestic phonographs was similarly limited in both frequency-range and volume — this period gave rise to the expression "put a sock in it", which commemorates the common practice of placing a sock in the horn of the phonograph to muffle the sound for quieter listening. By the end of the acoustic era, the disc had become the standard medium for sound recording, and its dominance in the domestic audio market lasted until the end of the 20th century.
The electrical era (1925 to 1945) (including sound on film)[edit]
The 'second wave' of sound recording history was ushered in by the introduction of Western Electric's integrated system of electrical microphones, electronic signal amplifiers and electrical disc-cutting machines, which was adopted by major US record labels in 1925. Sound recording now became a hybrid process — sound could now be captured, amplified, filtered and balanced electronically, and the disc-cutting head was now electrically-driven, but the actual recording process remained essentially mechanical – the signal was still physically inscribed into a metal or wax 'master' disc, and consumer discs were mass-produced mechanically by stamping an impression of the master disc onto a suitable medium, originally shellac and later polyvinyl plastic. The Westrex system greatly improved the fidelity of sound recording, increasing the reproducible frequency range to a much wider band (between 60 Hz and 6000 Hz) and allowing a new class of professional – the audio engineer – to capture a fuller, richer and more detailed and balanced sound on record, using multiple microphones, connected to multi-channel electronic amplifiers, compressors, filters and sound mixers. Electrical microphones led to a dramatic change in the performance style of singers, ushering in the age of the "Crooner", while electrical amplification had a wide-ranging impact in many areas, enabling the development of broadcast radio, public address systems, and electrically-amplified home gramophones. In addition, the development of electronic amplifiers for musical instruments now enabled quieter instruments such as the guitar and the string bass to compete on equal terms with the naturally louder wind and horn instruments, and musicians and composers also began to experiment with entirely new electronic musical instruments such the Theremin, the Ondes Martenot, the electronic organ, and the Hammond Novachord, the world's first analogue polyphonic synthesiser.
Contemporary with these developments, the movie industry was engaged in a frantic race to develop practical methods of recording synchronised sound for films. Early attempts — such as the landmark 1927 film The Jazz Singer – used pre-recorded discs which were played in synchronisation with the action on the screen. By the early 1930s the movie industry had almost universally adopted the "sound-on-film" technology (developed by Western Electric and others) in which the audio signals picked up by the microphones were modulated via a photoelectric element to generate a narrow band of light, of variable width or height, which was then captured on a dedicated 'audio' strip on the edge of the film negative, as the images were being filmed. The development of sound of film also enabled movie-industry audio engineers to make rapid advances in the process we now know as "multi-tracking", by which multiple separately-recorded audio sources (such as voices, sound effects and background music) could be replayed simultaneously, mixed together, and synchronised with the action on film to create new 'blended' audio tracks of great sophistication and complexity. One of the best known ex