# Doppler Effect

1. May 22, 2010

### pat666

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

(a) A siren emits a frequency of 900 Hz. Wind is blowing at a steady speed of 15.0 ms-1. The speed of sound in calm air is 343 ms-1. What is the frequency heard by someone approaching at 15.0 ms¹,

(i) when the wind blows from the person to the siren,
(ii) when the wind blows from the siren to the person?

2. Relevant equations

f'=(1+v_o/v)

3. The attempt at a solution

i figured out the relative velocity of the sound (for i 343-15) and then subbed into the formula which became 1+15/328=1.04573HZ. I'm pretty sure this isnt right because the frequency should get higher but mine inst even close.... also i didnt use the given frequency at all????????

2. May 23, 2010

### rl.bhat

Check the formula.

3. May 23, 2010

### pat666

That formula is the one in my text book for the observer moving and the source stationary

4. May 23, 2010

### pat666

hey, i really need some help with this if you know a different equation i should use?

5. May 23, 2010

### rl.bhat

The correct formula is

$$f' = [1 + \frac{v_o}{v}]f$$

Velocity of sound will be depending on the direction of the wind.

6. May 23, 2010

### pat666

ok thanks i dont know how i missed the second f...thanks

7. May 27, 2010

### EzaMoo

I get (i) 941Hz and (ii) 938Hz

Does this sound right?