Page 34 - Fall_DTF

Acoustic Leaky Vllave Antenna
The ability to localize objects in the environment using Although highly useful, beamforming also requires complex
omnidirectiona.l sources and sensors requires the use electromechanical systems, postprocessing algorithms, and
of either additional source-receiver pairs distributed in significantpower. Theexplosioninrecentyears ofautonomous
space, known as arrays, or structures that have an inherent systems for exploration requires navigation systems that are
directionality such as an acoustic lens like the melon of a physically smaller, with reduced energy needs. LWAs have
dolphin. In man-made structures, the latter case is limited potential to fill all of these needs because they utilize very
by the fact that its directionality cannot be changed once few acoustic sensors coupled to an analog aperture. Recent
it is fabricated. For this reason, acoustic localization is demonstrations by Esfahlani et al. (2016) and Naify et al.
most often performed using arrays of transducers such (2013) and subsequent development of acoustic LWAs have
as hydrophones and microphones. As a simple example, shown promise as a game-changing technique for acoustic
humans partially localize objects by using time-of-arrival sensing compared with array processing.
differences between our ears. Estimates of source location in
arrays is achieved by array processing that uses knowledge VVBVQB The‘: "Leak" and
of the location of each source or receiver and applies phase SP3”?-"3|"5°'SP5"-ial N'3PPin9
delays to electronlcally alter the dlrectlvlty of the array Fundamental to the operation of an acoustic LWA, as one
through anrocess known ashearnforrnlngr plgure l shows a might guess, is that sound must “leak” out of the device
cartoon of how beamforming works. In Figure 1A, an array hm’ ‘he 5'h"°““‘hhS medie in 3 whlmhed mehheh 1“
of acoustic point sources are equally spaced along a line. If this Cohlextv leekihg Iefe“ t0 the 3hihtY °f 3 W3Ve in ‘me
all of the sources are electronically coded to emit sound of medhh“ ‘h3‘ is "3Veh“S P3-'3-"el t0 ‘he h°““d3"Y Whh 3
equal magnitude at the same tone the resultlng wave front second medium to lose some energy to an acoustic wave
propagates at an angle norrnal to the array_ -l-he red ls-nes in all radiating into the second medium. According to Snell’s law,
cases indicate acoustic phase fronts. If, instead of activating this °“1Y °CC“’5 lflhe W3Ve "3VehhS 3t ‘he hherfece between
all of the sources slrnultaneously_ we lrnnart a Phase delay the media is faster than the wave in the second medium. An
of ¢ across each successive element in the array, the emitted hhereshhg eX3mPle of ‘his 055"“ when ‘’he thmws 3 ‘Mk
wave front is steered at an angle gr -l-hls phenomenon is onto an ice-covered lake. Because the elastic wave at the
shown ln plgure "3, with steerlng occurring at dlfferent point of impact travels faster than sound in air, some of the
angles elastic wave energy leaks into the air, producing an audible
sound reminiscent of the Star Wars blaster. The initial wave
A Mama! t\[-‘Ah . 5 W-st-f-‘t-n-T-:93 «:5»-s»_~-.w'»~_-s:n generated by the impact is a multifrequency, short-duration
“Mm r ' 3’ e l e impulse. Due to dispersion, each frequency component
.. ‘/K 4., .. propagates at slightly different speeds and thus the wave
E _ L E _, -;,"’ produced by the impact spreads out in time as it propagates
‘g: M if '3 — — — -v » _ —_—a — — — in the ice sheet. An excellent demonstration and discussion
’' 5: L '—-it‘, of this phenomenon can be seen in this short National
" “ '0 Public Radio video “Why Does a Frozen Lake Sound Like a
Star Wars Blaster” (awilable at n.pr/2xHBiOW).
Figure 1. Schematics afphase—deIay steered active arrays. Active The dispersion that turns an impulse into 3 Chirp is the
transducers (i.e., laadspeakers ar hydraphanes) are in in a unxfnrm same phenomenon that is exploited by an acoustic LWA to
ane—t1imensianal linear array. The transducers behave as paint saurc— generate f[equgncy.dgPgndgnt djrgctjonajjtyr Snell’; 13w tg]_]5
95 0" ’9C5i"e’5- R34 ‘"_"V‘- Ph"59 f"”"‘5 "'_'1i“tif‘.§’f"7'” “Ch I’‘“:"‘ us that the angle of refraction [9(f)] depends on the ratio of
saurce. A: when all active elements are excited simultaneously, with the wave 5 eed al an interface to and the sound 5 aed
zera phase delay, the mdmtwn dlrectwn IS hemmz ta the antenna (9 _ P _ "“ _ _ P
= 0). B, left: by impasinga phase shift increment ef¢ ta each paint ‘“ ‘he "°°“d "‘°d‘“‘“ (‘D ‘h'°“8h ‘he ’°h“°“5h‘P 907 =
saurce alang the line array, it is passilzle ta steer the phase fiunt in a sin"[e2/£s...(f)]- In the icy ieke example» es..t(f) is the speed of
specific direetian; right: by reversing the delay alang the length af the elastic waves in the ice sheet and :2 is in air. For the case of an
“"“5""“- the bmm is 5t”’5'1 1'" '1 '1‘f5’9"t di’€Cii”"- acoustic LWA, cu,t(f) is the dispersive wave speed in the LWA
and £2 is the speed in the medium surrounding the LWA. If
as 1 AI:uulI:lr:l Tbday 1 much;