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Advancements in Thermophones
Figure 4. A: 6.35-cm-diameter G10 circuit board substrate populated with CNT sheets. The device resistance is specified by the separation gap and height along with the number of overlapped CNT sheets. B: fully assembled aluminum laminate thermophone pouch (labeled P033) inflated with argon gas and supported along its perimeter by a black carbon fiber ring. The black and green leads have been soldered to the positive and negative tabs, respectively. The needle port along with the brass valve below the pouch allows the pouch to be pressurized with inert gas. The two metal wires with clips loosely connect the thermophone to a rigging apparatus (not shown) for calibrated acoustic testing.
 can be tuned by choice of the properties and dimensions of the encapsulation media. The advantage of the thermoa- coustic approach is that these mechanical resonances are in- dependent of the properties of the active material.
Aliev’s encapsulated thermophones drew interest from the US Navy, who have worked to evaluate and adapt the tech- nology for use in underwater sonar applications. Recent free-field acoustic calibration presentations have looked at the usefulness of various aluminum laminate thermophones as underwater projectors (Howarth et al., 2016). One such adaptation (Figure 4) is something of a cross between the thermophones used for microphone calibration (Figure 2) and bubble transducers demonstrated by Sims (1960). These thermophones consist of a CNT-active element suspended across a substrate (Figure 4A) that is then housed inside an aluminum laminate composite and pressurized in an in- ert gas environment (Figure 4B). These devices act as low- frequency resonating-bubble projectors that allow them to achieve relatively large source levels for their small package size (143 dB re 1 μPa at 1.4 kHz from a single 6.35-cm-diam-
eter thermophone). When the inert gas within the pouch is replaced with a liquid, the bubble is removed and the device becomes a broadband projector (Mayo et al., 2017).
The main selling point of thermophones is just that, their cost. The simplicity of thermophone design (you could lit- erally make one with a fine wire and a power source) and the small amount of active material required puts a very low cost floor on production. Thus, thermophones can be made very thin and lightweight, use no rare earth metals, and can easily conform to most surfaces. In contrast, although piezo- electric ceramics dominate the underwater projector mar- ket due to their high electroacoustic conversion efficiencies, most use lead or other heavy metals and require complex processing steps to manufacture high-quality material such as single crystal ceramics.
Arnold and Crandall (1917) along with Wente (1922) were the first to significantly develop a theory for thermophone transduction as a precision source of sound. Their calcula-
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