Question

High frequency whistles are often used for training dogs and have a frequency of 21 kHz.
Stanley’s dog is not responding to the whistle he bought and suspects the whistle may not
be working. However, it is difficult for Stan to tell if the whistle is working because
humans cannot hear sounds above 20 kHz. In order to test the whistle, he asks a friend to
blow the whistle and hop on his four-wheeler. In which direction should his friend ride
(toward or away from Stanley) to know if the whistle is working? Explain using words or
a diagram.

Answer 1

Solution:

According to Doppler's effect, the frequency heard by the observer is equal to

=> f = (v -vL ) / v - vs) * F ..............(i)

where, v is the speed of sound, vL is the speed of listener , vs is the speed of source , f is the apparent frequency heard by the observer and F is the actual frequency with which source emits sound.

Now, as stanely would be at rest then vL =0 ,

sign of (vs) velocity of source depends on its direction when vs is positive means source is moving towards observer and according to formula in this case apparent frequency will be more than actual frequency now as the source is emitting sound with 21 kHz frequency we do not want to increase it further.

So, source should move away from the observer,

means His friend should move away from stanely which will reduce the apparent frequency heard by the stanely.

please rate thanks.