Page 51 - Winter Issue 2018
P. 51

Thermophones operate by rapidly changing the temperature For example, in thermal wave imaging, a sample containing
of an electrically conducting heater element, be it a wire or optical absorbers is placed in a water-filled cavity with ul-
thin sheet, which interacts with gas in its immediate vicinity. trasound detectors placed along its edge. Short laser pulses
This heated gas rarefies or expands and then cools and con- excite the sample-producing acoustic waves due to tl1ermo-
tracts again in accordance with the ideal gas law as the cur- elastic expansion of the material that is recorded by the de-
rent through the heater is decreased. A video by Michigan tectors. In photothermal beam deflection spectroscopy, the
Tech Acoustics demonstrates a simple thermophone play- refractive index gradients in a coupling liquid produced by
ing music at a conference exhibition (see the “mirage effect” will deflect a laser beam that is near the
mDEcx). A driving requirement for thermophones is that the sample surface. In a gas—rnicrophone approach originating
heater element have a low heat capacity and a large surface from Bell, periodic or intermittent monochromatic light
area by which it can exchange energy with the surrounding impinges on a sample, is absorbed, and thus produces pe-
gas. It is perhaps ironic that Weisendangefs (1878a,b) origi- riodic heating. Heat diffusion to an adjacent inert gas then
nal thermophone, which was incited by the excitement sur- produces thermal rarefactions and compressions in the gas
rounding acoustics following Alexander Graham Bell’s com- as a thermally driven acoustic wave. The acoustic signature
munication on the telephone two years before, was claimed is recorded using microphones mounted flush within a reso-
to operate due to a thermally induced dimensional change in nant absorption cell that houses the sample and inert gas.
;l::e::r:fltt;:1r§I1I:l0ei:anfi:i§l::racteflzed by a mammals coef. These various photoacoustic techniques can be utilized for
imaging, spectroscopy, or material characterization. Initial
Although it is possible that Weisendange1"S (l3783>l3) t-he!‘ photoacoustic theory established in a series of articles by
mophone was enhanced at certain frequencies due to a di- Preece (1881) and Mercadier (1881a—c) was more compre-
mensinnal change in the Wire, modern thermophones <l0n’t hensively formulated many years later by Rosencwaig and
rely on mechanical actuation of the element itself. There was, Gersho (1976). This has led to various applications for pho_
in fact, much confusion and doubt as to what the particular toaeoustjcs, particularly in regard to biomedical imaging ap.
transduction mechanism was at the time. Preece (1880) re- plicatjons Manohar and Razansky (2016) Provide a much
P01”ted that it W35 Il0teCl hY De la RlVe. in 1843, that S01ln€lS more extensive historical review of photoacoustics for the
were produced by passing current through iron wires, but interested reader_
the effect was attributed to magnetism. Preece (1880) also
reported that Bell suggested straight pieces of iron, steel, and The 1-hannophana
g1'3Phlte Could also Produce 5°‘-md when driven bl’ 3 batter?‘ The first quantitative theory for thermophone sound pro-
Bell and Tainter (1880) also presented the photophone in dnetlnn W35 CleVel0Ped l)Y Arnold and Ctandall (1917):
1880, a device in which intermittent light, that is, light mod- which paved the Way for the thermophone to be used as a
ulated  a chopper of fan,  0n 3   of near- f11I1CtIOI18.l device. SIIICE then, the thermophone has his-
ly any hard substance would produce a sound of frequency t0I'tC9~llY thnntl 1'n0St “Se 35 3 Pl'eCl5l0n 5011TCe Of Sound for
corresponding to the modulation rate. Bell considered this Inle1'0Ph0ne C9~lll3f3tl0n- Such the1‘1‘n0Ph0ne5 C0nSl5t Of an
one of his greatest inventjons_ Even by 1393, Braun (1398), active element, such as gold leaf or thin platinum wires, sus-
to whom many have presumptively attributed the invention Pended 3l>0Ve 3 tnetal backplate that 35 then C011Pled Wlth 3
of the thermophone, described the acoustic sound as being fl’ Ont Plate housing the FnlCF0Ph0ne element t0 be e3~lll3fated-
partly produced by a change in length of wire, It becomes Two narrow capillaries in the backplate serve as an inlet and
understandable then, especially considering the limitations Outlet t0 S1lPPlY hydrogen 835 t0 the C‘<‘~VttY termed When the
of observing thermal changes at acoustic frequencies, that two Sides Of the device are brought together. Hydrogen gas
it was uncertain as to which mechanism produced sound, haS 3 Inneh higher S011nCl SPee<l than air and Shifts the inter-
temperature fluctuations causing mechanical strain in the nal C3VltY 1'eS0n3nCe5 higher ln frequency, the1'el3Y extending
material or temperature fluctuations causing mechanical the usable  Of the calibration instrument. 0116 Of
strain in the at; these thermophones on its backplate is shown in Figure 2A
In actuality, both mechanisms mentioned above occur to along wlth a dlagmm m Flgure 2B"
one degree or another. Vfhich of these a user wishes to in- The usefulness of a thermophone is due to its predictable
terrogate is the subject of various photoacoustic techniques. and relatively smooth frequency response over a wide band-
Winter 2018 | Acuuseics Thclay | 45

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