Thermophone

thermophone [′thər·mə‚fōn]

(engineering acoustics)

An electroacoustic transducer in which sound waves having an accurately known strength are produced by the expansion and contraction of the air adjacent to a strip of conducting material, whose temperature varies in response to a current input that is the sum of a steady current and a sinusoidal current; used chiefly for calibrating microphones.

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Thermophone 

an acoustic emitter whose operation is based on the thermal generation of sound.

The main element of a thermophone is a thin conductor (a strip of metal foil or a wire 2–6 micrometers thick) through which an alternating current of frequency f is passed. The cyclical changes in temperature of the conductor and the surrounding layer of air cause corresponding pressure variations, which propagate through the medium as a sound wave. The frequency f₁, of the emitted sound is equal to 2f, since the quantity of heat liberated in the conductor is proportional to the square of the current strength. To obtain f₁, = f, a direct current whose strength exceeds the amplitude of the alternating current is also passed through the foil or wire. The emitting conductor is usually placed in a chamber with rigid walls whose dimensions are less than the wavelength λ of the sound. The amplitude of the sound pressure in the cavity of the chamber may be calculated from the amplitude of the current, taking into account the heat capacity, thermal conduction, and temperature of the surrounding medium and the conductor, as well as the pressure of the surrounding medium and geometric parameters. Thus, thermophones are used as primary sound sources for the calibration of microphones.

To extend the frequency range of a thermophone, its chamber is filled with a gas such as hydrogen or helium, in which the speed of sound is higher than in air; in this case, the condition of lesser size of the chamber relative to the wavelength is satisfied for higher frequencies.

REFERENCE

Beranek, D. Akusticheskie izmereniia. Moscow, 1952. Pages 93–99. (Translated from English.)