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. 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.
Using a similar optical technology, DVD offers greater storage capacity and even more accurate sound reproduction. 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.