Thermophone
A thermophone is a type of transducer that converts an electrical signal into heat, which then becomes sound. It can be thought of as a type of loudspeaker that uses heat fluctuations to produce sound, instead of mechanical vibration.
The basic principle of the thermophone has been known since the 19th century. Thermophones have been used to calibrate acoustical apparatus since the 20th century. In recent years, the name thermoacoustic speaker has also been used.
Beginnings
is the study of the interaction between heat and sound. It is the basis of the thermophone. Byron Higgins in 1802 reported "singing flames" which occurred when the necks of jars were put over a hydrogen gas flame. Sondhauss and Rijke performed further experiments. A theory of thermoacoustics was produced by John [William Strutt, 3rd Baron Rayleigh|Lord Rayleigh] in 1878.The theory and practice of creating sound with electric heat emerged in the late 19th century. In 1880, William Henry Preece observed that, upon connecting a microphone transmitter to a platinum wire, sounds were produced:
In 1917, Harold D. Arnold and of Bell Labs developed a quantitative theory for the thermophone. Since then, thermophones have been used as a precision device for microphone calibration. However, they did not see widespread use elsewhere due to their poor efficiency.
Description
When alternating current is passed through a thin conductor, that conductor periodically heats up and cools down following the variations in current strength. This periodic heating and cooling creates temperature waves which the conductor propagates into the surroundings. As the temperature waves propagate away from the conductor, the thermal expansion and contraction of the transmission medium produces corresponding sound waves.An ideal thermophone is made of a conductor which is very thin and has a small heat capacity.