Momentum - The Magazine for Virginia Tech Mechanical Engineering Vol. 4 No. 2 Summer 2019 | Page 6

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demonstrate that the ears of
bats come with a "built-in
ambulance" that creates the
same physical effect.
Above: Professor Rolf
Mueller holds a robotic
'bat' used to collect data
on how bat navigation
can be understood and
applied. Opposite page:
doctoral student Xiaoy-
an Yin, in Borneo doing
field research.
Bats have an
ambulance in
their ears
Ear movement creates a Doppler shift that
helps some bats locate prey and navigate
Anybody who has been
passed by an ambulance at
high speed has experienced
a physical effect called the
Doppler shift: As the am-
bulance moves towards the
listener, its motion compress-
es the siren's sound waves and
raises the sound pitch. As the
ambulance moves away from
the listener, the sound waves
get dilated and the pitch is
lowered. A listener wearing
a blindfold could use this
Doppler shift pattern to track
the motion of the ambulance.
In a paper published by the
Proceedings of the National
Academy of Science June 3,
the authors, Rolf Mueller,
professor of mechanical
engineering in the College of
Engineering, and his doc-
toral student, Xiaoyan Yin,
“The animals move their
ears fast enough so that sound
waves that impinge on the
ears are transformed by the
motion of the ear surfaces
and shifted to higher or lower
frequencies,” said Mueller. “In
fact, the bat species studied
(horseshoe bats and Old
World Roundleaf bats) can
move their ears so fast that
Doppler shifts of around 350
Hz can be created. This is
about seven times larger than
the smallest Doppler shift the
animals haven been shown to
be able to detect.”
Doppler shifts have long
been known to play an
important role in the bioso-
nar system of bats such as the
species studied by Mueller
and Yin. The bats have the
enviable ability to hunt in
very dense vegetation, but to
accomplish this, they have to
solve the problem of how to
distinguish a moth (their pre-
ferred prey) from hundreds of
leaves that surround it.
“The solution these two
types of bats have come up
with is to tune in on the Dop-
pler shifts that are produced
by the wing beat motion
of their prey,” Mueller ex-
plained. “These "good Dop-
pler shifts" serve as a unique
identifying feature that sets
prey apart from static distrac-
tors such as leaves in foliage.”