Page 54 - Winter Issue 2018
P. 54

Advancements in Thar-muphanas
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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 loosebl connect the thermophone to a rigging apparatus (not shown) for calibrated acoustic testing.
can be tuned by choice of the properties and dimensions eter thermophone). W'hen the inert gas within the pouch is
of the encapsulation media. The advantage of the thermoa- replaced with a liquid, the bubble is removed and the device
coustic approach is that these mechanical resonances are in- becomes a broadband projector (Mayo et al., 2017).
dependent of the Properties of the active material‘ The main selling point of thermophones is just that, their
Aliev’s encapsulated thermophones drew interest from the cost. The simplicity of thermophone design (you could lit-
US Navy, who have worked to evaluate and adapt the tech- erally make one with a fine wire and a power source) and
nology for use in underwater sonar applications. Recent the small amount of active material required puts a very low
free-field acoustic calibration presentations have looked at cost floor on production. Thus, thermophones can be made
the usefulness of various aluminum laminate thermophones very thin and lightweight, use no rare earth metals, and can
as underwater projectors (Howarth et al., 2016). One such easily conform to most surfaces. In contrast, although piezo-
adaptation (Figure 4) is something of a cross between the electric ceramics dominate the underwater projector mar-
thermophones used for microphone calibration (Figure 2) ket due to their high electroacoustic conversion efficiencies,
and bubble transducers demonstrated by Sims (1960). These most use lead or other heavy metals and require complex
thermophones consist of a CNT-active element suspended processing steps to manufacture high-quality material such
across a substrate (Figure 4A) that is then housed inside as single crystal ceramics.

an aluminum laminate composite and pressurized in an in-

ert gas environment (Figure 4B). These devices act as low- Theory

frequency resonating-bubble projectors that allow them to Arnold and Crandall (1917) along with Wente (1922) were
achieve relatively large source levels for their small package the first to significantly develop a theory for thermophone
size (143 dB re 1 uPa at 1.4 kHz from a single 6.35-cm-diam- transduction as a precision source of sound. Their calcula-
52 | Acnuaeics Thday | Winter 2018






























































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