Calculating Frequency of Siren in Windy Conditions

[pat666]

Homework Statement

(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?

Homework Equations

f'=(1+v_o/v)

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 isn't right because the frequency should get higher but mine inst even close... also i didnt use the given frequency at all?

 

[rl.bhat]

Check the formula.

 

[pat666]

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

 

[pat666]

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

 

[rl.bhat]

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

The correct formula is

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

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

 

[pat666]

ok thanks i don't know how i missed the second f...thanks

 

[EzaMoo]

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

Does this sound right?