# Physics Problem: Sound Waves

#### pippermay

Hi,
Can someone please help me with the following problem:

Standing at a crosswalk, you hear a frequency of 560 Hz from the siren of an approaching ambulance. After the ambulance passes, the observed frequency of the siren is 480 Hz. Determine the ambulance's speed from these observations.

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#### Nylex

Have you done anything on this? What is the problem about, besides simply sound waves? That should help you to find an equation (or equations) you can use.

Last edited:

#### pippermay

I have placed the information in the Doppler equation "I quess that it what it is called." Where the observed frequency (f') = 480 Hz, frequency (f) = 560 Hz, the velocity (v) = 343 m/s, the velocity of the observer (vo) = 0 m/s, and I am solving for the velocity of the ambulance's speed (vs).

f' = ((v - Vo)/(v + vs))f

vs = ((f(v-vo))/f') - v
vs = ((560(343-0))/(480)) - 343
vs = 57.17 m/s

I get the velocity of the ambulance to be 57.17 m/s, but according to the back of the book the correct answer is 26.4 m/s. I just dont understand what I am doing incorrectly.

#### Pengwuino

Gold Member
You must have missed somethingi from the problem. 57.17 comes out correctly with the information you have given us. Maybe the equation is wrong?

#### mukundpa

Homework Helper
560 is not the actual frequency of the source at rest it is also the apperent frequency when the amb is coming towards the observer...

#### pippermay

I just wanted to say Thanks For helping me.

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