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Ken:
Dad, I learned Doppler effect in a science class today.
I know from my own experience that the pitch of an
ambulance siren differs between when it’s approaching
and leaving but the teacher did not explain how it
differs because it is too difficult for us to
understand, he said.
Dad: You call Doppler effect a situation when a
frequency of a sound changes while a source of the sound is moving. The
easiest example is an ambulance siren as you mentioned. For instance, an
ambulance moves about 17 meters per second in driving at a speed of 60
km per hour, which is 60,000m/3,600 sec. Sound travels about 340 meter
per second. When it approaches, sound reaches 5% faster than when it
leaves and sound leaves 5% slower than when it approaches. There is a
difference by 10% in frequency.
Ken: That is how the
tone of siren changes.
Dad:
There are two sounds, 770Hz and 960Hz in
ambulance siren. They are close to “So” (783Hz) and “Ti”
(987Hz) in musical scale. A man with an absolute pitch
hears the sound “So, Ti, So, Ti” when an ambulance stops
wailing siren. If it approaches at 60 kilometers per
hour, the sound becomes higher by 5% as I mentioned. The
sound changes by a semitone, sounding “So#, Do”.
 Ken:
Then a man with an absolute pitch finds the speed of an
object giving out a sound in approaching and leaving
him, doesn’t he?
Dad: What you
understand is right in principle. A man tells a speed if he finds the
change of frequency. This is correct not only for sound but light. A
wave of light looks bluish as it is compressed in approaching. On the
contrary, as it leaves faster, it looks more reddish. Light spectrum
shifting to be red by Doppler effect of light is called “redshift”, and
to be blue is called “blueshift”. The fact that the universe is bloating
was found since the redshift was seen far out in the universe.
Ken:
I first thought that Doppler effect was only for
ambulance siren phenomenon but it is actually utilized
for universe observation as well.
Dad:
There is another interesting story. Did you know there
is an animal which hunts with Doppler effect of
supersonic ray?
Ken: An animal
judging by sound, well, it has no sight, doesn’t it? It is…a bat?
Dad: Yes, it is. It
has marvelous hearing. It observes a distance from or a speed of a game
such as an insect, by hitting pulse wave of supersonic ray which is
approx. 60kHz.
Ken: What does pulse
mean?
Dad: It means a wave
of short time width. A bat uses a sound which sounds for dozens of
millimeter second, or approx. 0.03 second. When it approaches an insect,
it shortens the time width of a pulse and gives out the pulse
continuously.
Ken: How does it find
a distance or a speed by hitting pulse?
Dad: By catching a
pulse wave of supersonic ray reflecting from a game, it can tell a
distance from a target, its speed and direction, the size or the
characteristic of its surface.
Ken: I see. Finding a
distance from a game by calculating the time of reflection wave coming
back, it can calculate the speed with Doppler effect of reflecting wave.
Doppler effect is so interesting, isn’t it?
Dad: Yes, it
continues interest from the view point of natural science; it is also
very useful in the field of industry utilizing the phenomena I mentioned
now.
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