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Doppler effect wavelength from car

  • #1
161
1

Homework Statement



A car is travelling at 30m/s in still air. the frequency of the siren on the car is 600hz. The velocity of sound in still air is 330 m/s. what is the wavelength of the sound waves

a)directly in front of the car
b)directly behind the car

What is the frequency of the sound heard by a stationary observer

c)directly in front of the car
d)directly behind the car

Homework Equations



FD=CD/Cs *Fs


The Attempt at a Solution



Im not really sure where to start if someone could point me in the right direction.

Cheers
 

Answers and Replies

  • #2
34
0

Homework Statement



A car is travelling at 30m/s in still air. the frequency of the siren on the car is 600hz. The velocity of sound in still air is 330 m/s. what is the wavelength of the sound waves

a)directly in front of the car
b)directly behind the car

What is the frequency of the sound heard by a stationary observer

c)directly in front of the car
d)directly behind the car

Homework Equations



FD=CD/Cs *Fs


The Attempt at a Solution



Im not really sure where to start if someone could point me in the right direction.

Cheers
OK, let's kick this off. Can you first get the frequency of the doppler shifted sound wave? I think your equation above (though you haven't defined what each variable represents) should tell you that.
 
  • #3
34
0
OK, let's kick this off. Can you first get the frequency of the doppler shifted sound wave? I think your equation above (though you haven't defined what each variable represents) should tell you that.
Also, write out how to get your C's.
 
  • #4
161
1
well i can find the wavelength of the car but i think its only when its stationary im not sure

If Cs of the source is 30 m/s an the frequency is 600 hz
then the wavelength is

0.05 m

but im not so sure where to go from there.. if i have Cs and Fs how do i find what the wavelength in front and behind is
 
  • #5
161
1
actually i think i have the answer.. i used an equation that was stated in my text book..

Wavelength behind = (vm+us)/Fs

where Vm= velocity of sound in medium
Us= velocity of source


and it gets an answer of 0.6 m

and

wavelength infront = (vm-us)/Fs

given an answer of 0.5 m

and from here i can calculate the frequency of each wavelength from V= F*(Lambda)
 
  • #6
34
0
actually i think i have the answer.. i used an equation that was stated in my text book..

Wavelength behind = (vm+us)/Fs

where Vm= velocity of sound in medium
Us= velocity of source


and it gets an answer of 0.6 m

and

wavelength infront = (vm-us)/Fs

given an answer of 0.5 m

and from here i can calculate the frequency of each wavelength from V= F*(Lambda)
Awesome! Have a good one.
 

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