Page 52 - Winter Issue 2018
P. 52

Advancements in Thermophones
width. This is primarily the result of a lack of mechanical A 1.3;.‘-._,u,‘, , >  ' ‘ C - 1’
moving parts that always have accompanying passive struc-  $5." .
tural resonances. Normally, these resonances are “pushed R
out” of the band of interest through a combination of ma- ' t 4. .-_ .
terial choice and appropriate sizing of structural parts. For '“ A .t V t " ll’
thermophones used as a precision source of sound, the only .7 . - _ ' W7’ t.‘ 
dimensions of concern that would limit bandwidth are those _, . - ‘  "' l  t  . .. 
of the cavity encasing the thermophone and microphone I 0‘ _t___ .. :
element being calibrated. It remains today that one of the 1 ‘$3 _t=___ V
most attractive features of thermophones is that their acous- ‘Q Q?‘ v‘ “
tic response is largely decoupled from any mechanical parts. ' .
Thermophones saw utility as a precision source of sound but - . I ’ . _ 3’ ‘:1: E. 
were never widely used for any other purpose due to their 3“ .' ' '  . . _ V v, (gr. .__-5.) _v I It .,  :""t.’"’-ll,‘
poor efficiency compared with the electrodynamic loud- ' ‘ ‘.‘-  -~ , '  ‘ : ‘’''"’‘.7‘'’' I
speaker and other more conventional transduction sound ; 3 z .
sources. As with some other transducers, thermophones are 3    
also hindered when it comes to reproducing arbitrary wave-    
forms due to their intrinsically nonlinear tra.nsduction. The     ‘
acoustic response is quadratic with respect to the driving volt- \    \
age or current and requires a DC bias to linearize the response.    T,   
Similar to what occurs in a variable reluctance transducer, this    '\ ‘
bias current continuously generates excess heat and reduces \\)\\
efficiency. Other modern conventional signal-processing ‘
techniques such as amplitude modulation, pulse width mod-
ulafion’ and Pulse amplitude modulation can be used as well Figure 2 A' thermophone on its backplate used as a precision source
to rectllfy arbltrarl“ slgna'1s' Bouman et  (2016_) Pmvlde 3' ofsound for microphone calibration (Ballantine, 1932). B:aschematic
Companson of the“ thermophones efficlency wlth 3 blased of an assembled thermophone and microphone from Sivian (1931). I
inPut signal Versus an amP1itude'm°dulated °ne- Even with thermophone active element; D, electrostatic microphone diaphragm.
rectified signals, however, the various frequency components Images reprinted from Nokia Bell Laboratories, with permission.
of an arbitrary input signal will still generate corresponding  
h°teT°dYne5 35 Well 35 Sewnd h3Tm°niC5- Althmlgh tYPi°a1lY For example, CNTs used in thermophones have a diameter
undesired, such nonlinearities can be exploited to probe ma- on the order of 10 nm (about 1/10,000 the thickness of a
terial Pmperfies (Heath and Horsell’ 2017)" human hair) but are hundreds of microns in length. A CNT

“sheet” useful for thermophone applications is made from
Modern Thermophones a CNT “forest” (a highly oriented dense vertical array of
Relatively few articles concerning thermophones were pub- CNTs) made by a process called chemical vapor deposition
lished after the 1940s until Shinoda et al. (1999), inspired by (CVD). The CVD process for growing the CNTs now used in
photoacoustic studies on porous silicon, presented a porous thermophones was adapted by Zhang et al. (2005). Catalyst
doped silicon thermophone for ultrasonic emission. Inter- nanoparticles are deposited on a silicon wafer and individual
est grew substantially after Xiao et al. (2008) from Tsinghua CNTs grow vertically from these catalyst particles during the
University reported a flexible thermophone with an active CVD process as a heated feedstock gas is passed over the wa-
element composed of a CNT sheet. CNTs are nanoscopic or fer. With careful adjustment of various growth parameters,
nanoscale cylinders of carbon atoms arranged in a hexago- the edge of the resulting forest can be mechanically pulled
r1a.l lattice (much like a piece of wrapped chicken wire) that into a less dense porous sheet of horizontally aligned CNTs
can have exceedingly high aspect ratios. (Figure 3). The roughly 50-um-thick CNT sheet can be con-
SO I Acnuaeica Thday | Winter 2018

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