Sound is a kind of energy which travels in the form of waves. It can be recorded on a tape of disk by converting it into electrical signals. It can also be reproduced and heard from the tape or disk. The whole process is called sound recording ad reproduction. Sound waves are recorded on tapes in the form of magnetic patterns. On ordinary record disks, waves are recorded in the form of grooves. In a compact disk, sound is recorded as spiral patterns of microscopic pit. All these systems of recording sound are different form each other but all require a microphone for recording and a loudspeaker for reproducing the sound.
The microphone converts the sound waves into varying electric current. A simple microphone consists of thin disk or diaphragm connected to a piezo-electric crystal. When this crystal is pressed by sound waves, a feeble electric current is generated. Sound waves cause vibrations in the diaphragm exerts a little pressure on crystal and, as a result, a feeble electric current is generated. The current produced varies as the amplitude of the vibrations and produces a sound wave pattern which is recorded on the tape or disk.
To listen to this sound again electric current is produced from the disk or tape and this electric current is sent to the loudspeaker. The loudspeaker converts these electric signals into sound waves. An ordinary loudspeaker has a big cone of paper connected to a coil of wire. The cone is fitted in between the pole pieces of a permanent magnet. When electric current is passed through this coil a magnetic field is produced which causes vibrations in this cone and sound is produced. In this way the recorded sound is reproduces and heard.
The credit of recording sound on an ordinary disc goes to the American scientist, Thomas Alva Edison. He invented the phonograph in 1877.
In 1887, the production of modern type of disc gramophone started. Commercial disc recording started in 1895.
If you see the disc with a magnifying glass you will observe wavy grooves on it. These wavy grooves are formed according to the intensity of vibrations of sound. Sound waves are recorded as an image in these grooves. After recording, the disc is rotated with a needle placed on the pin vibrations re produced in it ad thus the original sound gets produced. Today many developments have been made in the field of recording and reproduction of sound. Nowadays amplifiers are also used. Mechanical reproduction has been replaced by electrical pickup system. Long play and stereo records have also developed.
Tape recorders were started by Vladimir Paulsen in 1899. Sound is recorded on a plastic tape which remains wound like a reel in a cassette. Most of the cassette tapes are coated with iron oxide which is a magnetic material.
In a tape recorder first of all electrical signals coming from a microphone are converts into magnetic signal. This is done by a small electromagnet which is called the recording head. Tape is passed through the had with the help of a motor. The iron oxide coated on it changes to a magnet by the electrical current generated due to sound waves. In this way sound is recorded on the tape in the from of magnetic field.
To listen to the sound recorded on the tape it is again passed through the head. The changing electric current due to the head resembles the electric current of the microphone at the time of recording. This electric current is amplified by an amplifier and then the original sound is prelude by the loudspeaker.
The electric signals of a microphone are recorded on a compact disc by laser beam. Laser is also used for playback. The signals are recorded in the form of digits. We have the best reproduction of sound by this method. A compact disc measures only about 12 cm but in comparison to 30 cm long playing record its playing time is much more.