Doppler Effect
description
Transcript of Doppler Effect
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Honorato C. Perez Sr. Memorial Science High School
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The oppler EffectJuly 6, 2009
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Doppler Effectphenomenon happened when a source of
sound and listener are in motion relative toeach other, the frequency of sound heard
by the listener is not the same as the
source frequency.first described by Christian Dopplerin the
19thcentury.
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Resting sound source
source
at rest
observer
at rest
Frequency fs Frequency foV=340m/s
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Stationary Source
Stationary Source
c = f
=c
f
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Moving ListenerA listener moving with a velocity vL toward astationary source S, the source S emits a sound
wave with frequency fs and wavelength, = v /fs. The wave crest approaching the moving listener
have a speed of propagation relative to the listener
of (v + vL) . So the frequency f
Lwith which the crest
arrive at the listenersposition is
fL = v + v L/ = v + vL / v / fS
fL= ( v + v L/ v) fS
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A listener toward a source ( vL> 0)hears a higher frequency than does astationary listener. A listener moving
away from the source ( vL< 0) hears alower frequency.
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observer
source observer
at rest
Frequency fsFrequency fo
o sf f
Observer hears
increased pitch
(shorter wave length)
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Sound source moving awayfrom observer
o sf f
sourceobserver
at rest
Frequency fsFrequency fo
Observer hears
decreased pitch(longer wave length)
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Moving Source and Moving Listener
= v / f SvS/ fS = v vS/ fS
( wavelength in front of moving source)
= v / fS+ vS/ f = v + v S / fS( wavelength behind a moving source)
In the region in front and behind the source wavelength are compressed and stretched out respectively, by the
motion of the source.
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To find the frequency heard by the listenerbehind the source:
fL= ( v + v L/ v + vS) fS
(Doppler effect, moving source and
moving listener)
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ConventionWe choose the positive direction (+)
for both vS and vL from the listener tothe source.
L SThe speed of sound v relative to the
medium is always positive.
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Sample Problem
1. A police siren emits a sinusoidal wave with
frequency fS = 300 Hz. The speed ofsound is 340 m/s. a) Find the wavelength
of the waves if the siren is at rest in the
air. b) if the siren is moving at 30 m/s, findthe wavelengths of the waves ahead and
behind the source.
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2. If a listener L is at rest and the
siren in ex.1 is moving away from L
at 30 m/s, what frequency doesthe listener hear?
3. If the siren is at rest and thelistener is moving toward left at 30
m/s, what frequency does the
listener hear?
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seatwork: An ambulance travelling at 18.0m/s emits
sound waves with frequency 400 Hz. The speed ofsound in air is 344m/s.
1. find the wavelength of the sound waves.
2.Calculate the frequency of the waves as
perceived by a man standing while the ambulanceis moving away from him. Make a sketch of thesituation
3. A girl standing at the side of street sees the
oncoming ambulance. What is the frequency of thesound waves as perceived by the girl?
What is the frequency perceived by a passenger in aca moving behind the ambulance in the same
direction but a speed of 15 m/s?