# Doppler effect

1. May 28, 2010

### matt_crouch

1. The problem statement, all variables and given/known data

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

2. Relevant equations

FD=CD/Cs *Fs

3. The attempt at a solution

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

Cheers

2. May 28, 2010

### jamesmo

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. May 28, 2010

### jamesmo

Also, write out how to get your C's.

4. May 28, 2010

### matt_crouch

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. May 28, 2010

### matt_crouch

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. May 28, 2010

### jamesmo

Awesome! Have a good one.