# Understanding Doppler Effect

## Homework Statement

Two trumpet players are both playing with a pitch of 440 Hz, corresponding to the musical pitch A above middle C. However, one of the trumpet players is marching away from you so that your hear a beat frequency of 4 Hz from the two trumpets. With what speed is the departing trumpet player moving away from you? (The speed of sound in air is 340 m/s)

## Homework Equations

f' = f(V+-Vo/V-+Vs)

f2 - f1 = beat frequency

## The Attempt at a Solution

How I approached the question was the following:

f' = f(V/V+Vs)
f(V/V+Vs) - f1 = beat

so
440(340/340+Vs) - 440 = 4
which solves to Vs = 3.06 m/s

However, looking at the answer (3.12 m/s) It seems I should have set the equation up as:

440 - 440(340/340+Vs) = 4
which goes to 3.119 m/s

So conceptually speaking, how am I supposed to know how to set up the equation? Is it just what yields a positive answer? or is there something I'm not thinking of. I couldn't find anything specific online. Thanks for the help.