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 Advancements in Thermophones
bustion explosions, none of which held up successfully to scrutiny. It was not until after experiments were performed by Sondhauss (1850) and Rijke (1859) that an adequate the- ory was developed by Lord Rayleigh (1878).
Experiments were performed by Sondhauss and Rijke be- tween 1850 and 1860 resulting in the setups that bear their names. A Sondhauss tube is a thermoacoustic device with a long cylindrical neck that is closed off at one end, some- times in a bulbous structure (Figure 1A). When the bulb or closed end is heated, sound may emit from the opening in the neck, with a frequency dependent on the resonant struc- ture created by the bulb and neck. This effect had long been known by glassblowers who, at times, noticed a sound pro- duced as blown glass bulbs began to cool. In the Sondhauss tube, a parcel of cool air enters the heated bulb and heats up as it is compressed further into the bulb. The heated parcel subsequently expands and further cools as it comes in con- tact with the colder tube. The rarefied region then collapses again toward the bulb as it heats, completing a cycle. Ray- leigh posed that the condition that enables the amplification of sound (which occasionally needs a small “kick” to jump start) is that the parcel is heated during compression and cooled during rarefaction by displacing to the hot and cold regions of the tube, respectively.
A Rijke tube is a simple vertical cylinder open on both ends, with a heat source, often a heated wire mesh or gauze, in- serted in the bottom half of the tube (Figure 1B). Here, convective flow rises through the bottom of the tube and is heated as it passes through the mesh. The gas first expands and then contracts as it interacts with the sidewalls of the tube. Eventually, a standing wave is created (which can be mathematically represented as a combination of traveling waves moving up and down the tube, being partially reflect- ed at the openings) superimposed on the convective flow. Early embodiments of both devices by Herschel (1874) can be seen in Figure 1C.
The source of heat for “singing flames” was originally through the combustion of hydrogen gas, although, the ability to pro- duce an acoustic response is largely independent of how the heat is generated. Instead, what matters most is how heat is distributed and how it propagates throughout the system. Some Rijke tubes, even early on, used battery-powered re- sistive heating (Joule heating) of coiled wires. Furthermore, if the heating element is fine enough, sound can be produced without the resonating tube by oscillating the temperature of the element at acoustic frequencies. Such a device is called a thermophone, a term coined by Weisendanger (1878a,b). 48 | Acoustics Today | Winter 2018
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 Figure 1: A: a simple bulb-end Sondhauss tube. Typically, a flame is used to heat the bulb externally. B: a Rijke tube with open ends and a heated wire mesh inserted in the bottom half of the tube. A and B: heuristic representations of the hot (red) and cold (blue) regions and are not accurate representations of the temperature profiles. See text for details. C: various embodiments of Rijke and Sondhauss tubes, al- though not referred to as such, by Herschel (1874). 1 and 4: Bunsen burners, emit a flammable gas that rises into a short (1) or long (3 and 4) vertical Rijke tube. The gas combusts at the hot wire gauze located partway up from the bottom of the tube. Even after the gas is shut off, a loud sound can be heard that fades as the gauze cools. 2 and 4: Two short bulbous Sondhauss tubes.

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