Page 51 - Winter2018

Thermophones operate by rapidly changing the temperature of an electrically conducting heater element, be it a wire or thin sheet, which interacts with gas in its immediate vicinity. This heated gas rarefies or expands and then cools and con- tracts again in accordance with the ideal gas law as the cur- rent through the heater is decreased. A video by Michigan Tech Acoustics demonstrates a simple thermophone play- ing music at a conference exhibition (see acousticstoday.org/ mDEcx). A driving requirement for thermophones is that the heater element have a low heat capacity and a large surface area by which it can exchange energy with the surrounding gas. It is perhaps ironic that Weisendanger’s (1878a,b) origi- nal thermophone, which was incited by the excitement sur- rounding acoustics following Alexander Graham Bell’s com- munication on the telephone two years before, was claimed to operate due to a thermally induced dimensional change in the wire itself, modernly characterized by a materials coef- ficient of thermal expansion.
Although it is possible that Weisendanger’s (1878a,b) ther- mophone was enhanced at certain frequencies due to a di- mensional change in the wire, modern thermophones don’t rely on mechanical actuation of the element itself. There was, in fact, much confusion and doubt as to what the particular transduction mechanism was at the time. Preece (1880) re- ported that it was noted by De la Rive, in 1843, that sounds were produced by passing current through iron wires, but the effect was attributed to magnetism. Preece (1880) also reported that Bell suggested straight pieces of iron, steel, and graphite could also produce sound when driven by a battery.
Bell and Tainter (1880) also presented the photophone in 1880, a device in which intermittent light, that is, light mod- ulated by a chopper or fan, impinging on a thin disk of near- ly any hard substance would produce a sound of frequency corresponding to the modulation rate. Bell considered this one of his greatest inventions. Even by 1898, Braun (1898), to whom many have presumptively attributed the invention of the thermophone, described the acoustic sound as being partly produced by a change in length of wire. It becomes understandable then, especially considering the limitations of observing thermal changes at acoustic frequencies, that it was uncertain as to which mechanism produced sound, temperature fluctuations causing mechanical strain in the material or temperature fluctuations causing mechanical strain in the air.
In actuality, both mechanisms mentioned above occur to one degree or another. Which of these a user wishes to in- terrogate is the subject of various photoacoustic techniques.
For example, in thermal wave imaging, a sample containing optical absorbers is placed in a water-filled cavity with ul- trasound detectors placed along its edge. Short laser pulses excite the sample-producing acoustic waves due to thermo- elastic expansion of the material that is recorded by the de- tectors. In photothermal beam deflection spectroscopy, the refractive index gradients in a coupling liquid produced by the “mirage effect” will deflect a laser beam that is near the sample surface. In a gas-microphone approach originating from Bell, periodic or intermittent monochromatic light impinges on a sample, is absorbed, and thus produces pe- riodic heating. Heat diffusion to an adjacent inert gas then produces thermal rarefactions and compressions in the gas as a thermally driven acoustic wave. The acoustic signature is recorded using microphones mounted flush within a reso- nant absorption cell that houses the sample and inert gas.
These various photoacoustic techniques can be utilized for imaging, spectroscopy, or material characterization. Initial photoacoustic theory established in a series of articles by Preece (1881) and Mercadier (1881a-c) was more compre- hensively formulated many years later by Rosencwaig and Gersho (1976). This has led to various applications for pho- toacoustics, particularly in regard to biomedical imaging ap- plications. Manohar and Razansky (2016) provide a much more extensive historical review of photoacoustics for the interested reader.
The Thermophone
The first quantitative theory for thermophone sound pro- duction was developed by Arnold and Crandall (1917), which paved the way for the thermophone to be used as a functional device. Since then, the thermophone has his- torically found most use as a precision source of sound for microphone calibration. Such thermophones consist of an active element, such as gold leaf or thin platinum wires, sus- pended above a metal backplate that is then coupled with a front plate housing the microphone element to be calibrated. Two narrow capillaries in the backplate serve as an inlet and outlet to supply hydrogen gas to the cavity formed when the two sides of the device are brought together. Hydrogen gas has a much higher sound speed than air and shifts the inter- nal cavity resonances higher in frequency, thereby extending the usable bandwidth of the calibration instrument. One of these thermophones on its backplate is shown in Figure 2A along with a diagram in Figure 2B.
The usefulness of a thermophone is due to its predictable and relatively smooth frequency response over a wide band-
Winter 2018 | Acoustics Today | 49