# Sound: freq moving sources

#### Dx

A train is traveling toward you at 120km/h. The train blows its 400Hz whistle. take the speed of sound to be 340m/s. whats the frequency do you hear?

f'=f/(1-vs/vo)
400/(1-(4.32x10^8 / 340)) i got 4.32x10^8 = 120000m * 3600sec

what am i doing wrong?
The possible answers are: which is it and what have i done wrong to miss this test question?

a) 444hz
b) 364hz
c) 361 hz

thanks!
Dx

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#### Tom Mattson

Staff Emeritus
Gold Member
Originally posted by Dx
f'=f/(1-vs/vo)
400/(1-(4.32x10^8 / 340)) i got 4.32x10^8 = 120000m * 3600sec
You made a mistake converting the speed of the source from km/h to m/s. If you look at it, you have the source moving faster than light, which is a big no-no.

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