Page 55 - Winter Issue 2018
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tions predict an acoustic pressure that is proportional to the appears to be limited by the properties of air rather than the
square root of the driving frequency (of a biased system). active element.
More recent models of CNT thermo hones in free s ace _ _ _ _ _ _
were published by Xiao et al. (2008), Vgterinen et al. (20) 10), To rompltcite flung: furthe: lt ls dlfficulttto prfovlde a cfmb
and Aliev et al. (2013). Each of these models differ slightly, panson 0 t ermop one per Ormance to t rt o_ ConveI_mOn-
but under the Same basic assumption of a negligible active_ al transducers. Most transducers operate within a region of
element heat capacity, the far-field acoustic pressure is lin- essentlally constfmt efficlency no nlatter What Power ls Pm-
early proportional to its frequency and reduces to vided to the device. Thermoacoustic devices are completely
different, and the conversion process is similar to that of
f pet a car engine or power plant that has a behavior limited by
P = 2‘/TI, (1) Carnotls cycle in which the efficiency is dependent on the
‘’ ‘ml’ temperature difference between the “hot” fluid and “cold”
where p is the acoustic pressure at a distance r from the mono- leselvffln lltlelefole’ _Sll lllllg as tlle llackgllllllld temrlelatllle
pole source that is driven with an electrical input power PE, llf_tlle colll leselvllll elllllllllldlllg ll thelmopllone ls ll_lalll'
producing an acoustic signal at frequency f in an ambient gas- tallled’  lllcleflse lll lllpllt Pllw_el_ Plovllles a ploporllollal
eons environment at temperature Tflmb with gas_SPecific heat increase in efficiency. For acousticians, this translates to a 6
capacity at constant pressure C1,. Therefore, in the “absence” of lllltlllclelllse lll Solllld Plessllle level (sptll) fol each dl_lllllllllg
the thermophone active element, the thermoacoustic trans- of lllpllt Powell as Opposed to the 3 dB lllclease Seen lll coll-
duction process is only determined by the drive power, fre- velltlllllal del/lceS'
quency, aI1dPf0P€I‘ti€S Ofthe surrounding gas- As discussed, thermophone efficiency may be greatly in-
These formulations are limited in scope to sources small creased by opefatlng at a hlghel: frequency and higher port,"
with respect to the acoustic wavelength and the ability of the er  by creatmg resonant dances‘ However’ efliclencr Stlll
thermophone to maintain its background ambient tempera- remflms Orders of magmtllde lsfwer than_ m_ convetltlotlal
ture. At high power, the acoustic pressure will thermally sat- devlces’ and_ each °_f these requlrements ltmlts flppllcatlon
urate as the background temperature approaches the surface P°tentlal' It ls f°_r thls reason that comrflerclallzauon of the”
temperature on the heater, eventually causing the active ele- mophones at thls Pomt has been StYmled'
ment to degrade, either burning or melting in extreme cases
(Aliev et al., 2014a). Both Xiao et al. (2008) and Aliev et al. canal‘-'5i°n5
(2015) have proposed that the difference in the frequency Tnerrn°Pn°ne transducers generate aeonstre signals 17)’
dependence between Various thermophones is due to a more modulating the temperature of an active element via Ioule
substantial heat accumulation of the active elements used in neatrn8- Heat tr ansrer t° gas adjacent to the element Causes
the early 20th century etnnnared with the CNTS Often uti_ thermal rarefactions and compressions producing an acous-
hzed in thermophones today tic wave. The historical origins oftthese transducers are en-
Models for the far-field acoustic pressure of encapsulated ffglsilgitlgz ttl111:,[1IZ:1:1I:)t‘:‘](:tI(11’01fel:§:§gt‘,:£::l:t1::I:;:::l)
tllellllllpllolles ale Stllllled less allll’ altllllllgll few elllst’ llfllle photoacoustic spectroscopy, thermoacoustic engines, and
:l1r1:ie::r:l:;l:;§:l:I:%:lit:l(§:::=l:;E::l1eOr::::::l:;l::I:£:3;l:::: thermoacoustic refrigeration. Arnold and Crandall (1917)
Even fewer models of underwater thermophone projectors i:l,:;1‘:,l:::l)pt;:nt:e,§:;t::1a:b1t::I:fi:::0t:;:2::(;l::£S§::l::::::
elllst’ all area_lllllcll lll lleed of development fol fllly Plactlcal of sound for microphone calibration. Decades went by with
llllplelllelltatloll of tllellllllpllolles lll naval appllcatlolm relatively few developments in thermophone technology un-
The largest criticism of thermophones by far is their low til highly porous nanoscopic materials such as porous doped
efficiency. People often think, “can you just make a better silicon and carbon nanotubes were utilized in the late 1990s
nanomaterial that converts heat to sound more efficiently,” and 2000s, respectively. The discovery that such materials
but often it isn’t the element itself that is the problem. At significantly improve thermophone efficiency has led to a
the current stage of development, the efficiency of a ther- resurgence in interest as well as new theoretical models and
mophone open to its environment (i.e., not encapsulated) potential use Cases.
Winter 2018 | Acnustics Thday | E5

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