There is a source of sound (a tuning fork) moving towards a reflecting wall with a speed of 30 m/s. The velocity of the
sound in air is 330 m/s and the frequency of the tuning fork is 600Hz. The observer is between the source and the
wall and is moving with some velocity v, towards the wall. The beat frequency heard by the observer is 4Hz. If the
tuning fork is waxed, the frequency beats heard by man becomes 3Hz. The new frequency of tuning fork is _____
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Let n
d
and n
′′
be the frequencies heard by the observer O coming directly from the tuning fork and after reflecting from the wall respectively.
Velocity of sound in air v
sound
=330m/s
Velocity of the source v
source
=5m/s
Doppler effect when a source moves towards the stationary observer:
n
d
=n[
v
sound
−v
source
v
sound
] =256[
330−5
330
]=260Hz
Doppler effect when a source moves towards the wall:
n
′
=n[
v
sound
−v
source
v
sound
] =256[
330−5
330
]=260Hz
Now apparent frequency after the reflection from the wall n
′′
=n
′
=260Hz
Beat frequency heard by the observer b=n
d
−n
′′
=0
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