The document discusses the Doppler effect, which is when there is a difference between the perceived and transmitted frequency of a sound due to relative motion between the source and receiver. It explains that the perceived frequency is higher if the source is moving towards the receiver, and lower if moving away, due to the change in distance over time. An equation is provided to calculate the relationship between the transmitted and received frequencies based on the speed of sound and speeds of the source and receiver relative to the air. An example problem is then given about using the Doppler effect to determine if a mosquito is approaching or receding based on the perceived frequency of its buzzing.

1. LO5
-­‐
The
Doppler
Effect
What
is
the
doppler
effect?
It
is
the
phenomenon
of
perceiving
a
different
frequency
than
is
being
transmitted
when
there
is
relative
motion
between
the
source
of
the
sound
and
the
receiver
of
the
sound
;
if
two
objects
are
moving
towards
each
other,
the
received
frequency
will
be
higher,
and
if
the
two
objects
are
moving
apart,
the
received
frequency
will
be
lower.
For
example,
it
is
the
shift
in
pitch
(frequency)
you
hear
as
a
siren
or
loud
car
just
passes
you.
the
general
relationship
between
the
two
frequencies
can
be
described
by
the
following
equation:
in
which..
f’
=
the
received
frequency
f0=
frequency
of
the
source
v
=
the
speed
of
sound
in
air
(343m/s)
v0=
speed
of
the
receiver
(relative
to
the
air)
vs=
speed
of
the
source
(relative
to
the
air)

2. Practice
Question
1.
One
way
to
tell
if
mosquito
is
about
to
bite
or
has
already
bit
you
is
to
listen
for
the
Doppler
shift
as
the
mosquito
is
flying.
The
buzzing
sound
of
the
mosquito
is
emitted
at
a
frequency
of
1250
Hz.
a)
If
you
perceive
a
frequency
of
1136
Hz,
does
this
mean
the
mosquito
is
coming
towards
you
or
flying
away?
b)
At
what
velocity
is
the
mosquito
flying?

3. Solution
a)
because
the
received
frequency
is
lower
than
the
actual
frequency,
we
know
that
the
mosquito
is
flying
away.
b)
-­‐34.4m/s