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Principles of sound recording and reproduction

Information on the history of sound recording and reproduction.

Sound recording and reproduction is a 100 year old science, the principles of which remain largely unchanged today. Essentially, to record sound a microphone captures sound waves and converts them into electrical energies to be encoded and then either stored or transmitted by a current. To reproduce the sounds playback devices read the encoded data and convert them back into electrical energy that when fed to a loudspeaker is recreated as the original sound waves.

Microphones

Common to all microphones is a thin, metal diaphragm that vibrates in response to the frequency and amplitude of sound waves. As the diaphragm of an early microphone moved, carbon dust was compressed and decompressed and its resistance altered. The flow of an electric current passed through the carbon was affected by its changing resistance thereby creating a specific electrical signal. The carbon microphone was invented in 1878 by Welsh scientist and music professor David Hughes.

In a modern microphone the diaphragm is suspended within a magnetic field which, when disturbed, creates electric signals that represent the pattern of the sound wave. Electromagnets, capacitors and crystals are all used for this purpose.

An example of an image of an early microphone is held at the Archives in the Midgley Collection.

Playback Devices

A playback device, such as a turntable or CD player, accesses the stored electrical signal, reads the data and converts it back into electrical energy to be fed into a loudspeaker.

Speakers

Loudspeakers take electrical signals and covert them back into audible sound waves. A diaphragm suspended in a magnetic field is vibrated by the electric signal causing air to be pulled toward and pushed away from the diaphragm and sound waves identical to the originals to be produced