process of converting the acoustic energy of sound into some form in which it can be permanently stored and reproduced at any time. In 1855 the inventor Leon Scott constructed a device called a phon autograph that recorded tracings of the vibrations of sound. Thomas Edison, starting about 1877, made great improvements in mechanical sound recording and was the first inventor to achieve the actually audible reproduction of recorded sound. The greatest advances, however, were made after the adoption in 1925 of electromechanical systems using electronic amplifier's (see record player). Generally, in recording, the sound waves impinge on a microphone and are converted into an electrical signal that is recorded by a tape recorder.
The tape can be edited if desired. When a commercial phonograph record is to be made, a disk of soft acetate composition coated on an aluminum base, called an original, is placed on a rotating turntable. The tape is played back and controls a stylus that cuts a spiral groove starting from the outer edge and moving to the inner edge of the original. For monophonic sound the stylus vibrates from side to side as it cuts the groove.
For stereophonic sound the stylus vibrates vertically, as well as from side to side, recording one sound channel in the left wall of the groove and one in the right. In a series of steps the original is used to make a metal stamper that presses the groove into commercial records. In order to play a commercial record, a stylus, or needle, is placed in the disk's groove while it is in motion on a turntable. The vibrations of the stylus cause the transducer to which it is attached to produce a varying voltage. This voltage is amplified and fed into a loudspeaker. In magnetic tape systems the varying electrical voltage is converted in a small electromagnet, called a head, into a varying magnetic field that causes magnetic particles embedded in the tape to become aligned in varying degrees as the tape passes through the magnetic field.
On playback, the magnetic tape moves past the head, generating a varying voltage in the coil of the head, which is boosted in an amplifier and converted to sound by a loudspeaker. Compact discs, first introduced commercially in the early 1980 s, employ laser technology to inscribe and "read" digital information in a way that avoids actual physical contact between the disc and any type of stylus. The optical properties of the disc's tracks are measured by a sensor and converted to digital signals and then to sound. Compact discs have the advantage of minimal wear and a greater possible dynamic range.
Compact disc technology was super ceded by digital versatile disc (DVD) technology with the introduction of the DVD-Audio format in 1999. Using a similar optical technology, DVD offers greater storage capacity and even more accurate sound reproduction. Recorded music and other sound may also be stored on and played from computer disks using several different computer-program file formats. Motion picture soundtracks are called optical recordings. The sound to be recorded is converted into an electrical signal that is used to modulate the intensity of a beam of light. This modulated beam exposes a moving film to make a recording of the sound.
Reproduction is effected by shining a steady beam of light through the developed film that is the sound track. As the film moves across the light beam, some of the light passes through it into a photocell, the amplified output of which activates a loudspeaker. See tape recorder.