DK Science: Sound Reproduction

Most sounds happen only briefly and are then lost to us. Fortunately, there are two ways in which we can record sounds and later reproduce them (play them back). One way is to convert and store sound using other forms of energy, such as electricity and magnetism. The other, DIGITAL way involves converting and storing sound in the form of numbers.



Inside a recording studio, a microphone is turning the sound energy of this singer’s voice into electrical energy. She holds it close up to cut out background sounds. A wire carries the pulses of electricity to sound recording equipment elsewhere in the studio.


Lots of different singers and instruments can appear on a record, and each one has to be recorded by a separate microphone. The knobs on this mixing desk control the signals from the different microphones. Each knob can make a singer or player louder or quieter in the mix.


The final version of the recording is put on sale on CD. The music is recorded in the surface of the plastic disk as a series of tiny bumps (seen as red and yellow in this highly magnified photograph). The bumps are covered by fine metal film and a layer of plastic.


Every CD player contains a laser that reads the series of bumps on the CD surface as a long string of numbers. The CD’s shiny metal film reflects back the light of the laser. The numbers are converted back to the same pulses of electricity that originally made the bumps in the CD.


Before CDs became popular, music was recorded on flat discs made of a special plastic called vinyl. Discs containing up to an hour of sound were known as long-playing (or LP) records. The sound was stored in tiny bumps in a long spiral groove on the LP’s surface. Both sides of the disc were used.


The pulses of electricity are fed through an amplifier into a loudspeaker, which works in the opposite way to a microphone. It turns electrical energy back into sound by using electricity to make the air around it vibrate. In this way, the music is reproduced exactly.


A microphone changes sound waves into tiny bursts of electricity. Inside it, sound waves cause a flexible disc (diaphragm) to vibrate. The up-and-down movement of a wire coil fixed to the diaphragm interacts with a magnet to produce a varying electrical current that can be stored and played back.


The reel of tape inside an audio cassette stores sound as a pattern of magnetic pulses laid out along its length. When the tape is used in a cassette player, the magnetic pulses are turned back into electricity and sound.


When a piece of music is ready for transfer to CD, a blank master CD is set up to spin round at very high speed in front of a laser that switches on and off very quickly. Each time the laser switches on, it burns a tiny bump onto the CD’s plastic surface. The pattern of bumps is a coded version of the music stored on the CD. Copies of the master CD have the same surface bumps, which are read by CD players.


Sounds can be stored in digital form by using electronics to turn them into patterns of numbers. A CD stores music on its surface as a pattern of bumps. The bumps represent a coded pattern of numbers that the CD player turns back into sound waves. Digitally recorded sounds are not affected by background noises. Digital sounds are easily edited and mixed with the aid of computerized equipment.


A sound wave is stored digitally by a process called sampling. The amplitude (height) of the wave is measured every so often and stored as a number. When all these numbers are written together, they make a longer number that represents the entire wave. The more times the wave is measured, or sampled, the better it sounds during playback.


Many people now get their music from the Internet, where each piece of music is stored in a file called an MP3. These files can be downloaded using an ordinary computer, then copied onto a plug-in memory card. The card is unplugged from the computer and inserted into a portable MP3 player. The downloaded MP3 music files then load directly into the player.

Copyright © 2007 Dorling Kindersley