Momentum - The Magazine for Virginia Tech Mechanical Engineering Vol. 4 No. 2 Summer 2019 | Page 7

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Researchers became aware
early on that the bats’ own
flight motion also produces
Doppler shifts that would
interfere with the perception
of the prey-induced Doppler
shifts. In the late 1960s a solu-
tion to this conundrum was
discovered when it was found
that horseshoe bats decrease
their emission frequency by
an amount that is carefully
controlled to exactly elimi-
nate any of the "bad Doppler
shifts" caused by the bats'
flight velocity.
“Since these groundbreaking
discoveries, the general belief
in the scientific community
has been that the role of
Doppler shifts in the biosonar
systems of these animals has
been completely understood,”
said Mueller. “Doppler shifts
due to prey motions are
‘good Doppler shifts’ that
the animals' entire hearing
system is optimized to detect
whereas Doppler shifts due
to the bats' own flight motion
are ‘bad Doppler shifts’ that
the animals eliminate through
feedback-control of their
emission.”
While Mueller and Yin
found speculation in the
literature of the early 1960s
that bats may be producing
Doppler shifts with their
own ear motions, the idea
was never followed up with
experimental work.
The work conducted by
Mueller and Yin has mea-
sured the motion of the
ear surfaces carefully using
stereo-vision based on high-
speed video cameras, and the
authors were able to predict
how fast surfaces move in
different portions of the ear.
They also estimated the angle
between the directions of the
ear motions and the direc-
tion the bat has its biosonar
pointed in and found that
motion speeds and directions
were aligned to maximize the
Doppler shifts produced.
To show that Doppler
shifted signals entered the
ear canal of the biomimetic
pinna and would be accessible
to bats, the researchers built
a flexible silicone replicate of
a bat ear that could be made
to execute fast motions by
pulling on an attached string.
“The final piece in the
research has been to find
possible uses for the ear-gen-
erated Doppler shifts,” said
Mueller. “We were able to
show that the Doppler shifts
produce distinct patterns
over time and frequency that
can be used to indicate the
direction of a target. This is
interesting in the context of
the biosonar systems of the
bat species that were studied,
because these animals typi-
cally concentrate the lion's
share of the ultrasonic energy
they emit with each biosonar
pulse in a narrow frequency
band. However, for telling
the direction of a target, it is
usually convenient to look at
how different frequencies are
transmitted by the ear and the
‘spectral color’ that results.
This is what humans due to
tell the direction of a sound
in the vertical where compar-
isons between two ears are
of no use. The Doppler shift
patterns produced by the ear
motions could give these bat
species the option to concen-
trate their energy in a narrow
frequency band yet be also
able to tell target direction.”
The authors of the study
hope the study of ear-gen-
erated Doppler shifts in bat
biosonar could give rise
to new sensory principles
that could enable small, yet
powerful sensors; e.g., for
drones that can operate in
dense foliage or autonomous
underwater vehicles navigat-
ing near complex underwater
structures.