How Far Must a Sound Source Move to Create Silence Due to the Doppler Effect?

[beastysk8er]

Homework Statement

Two adjacent sources each emit frequency 800 hz in air (340 m/s). how far would source 1 have to be moved so an observer in front of the sources would hear no sound? .321 .213 .123 or .312.

Homework Equations

f' = f/(1+ vsource/vsnd)

The Attempt at a Solution

i tried to calculate wavelength so that the frequency is below 20 hz (human limit) and figure it out from there but I am stuck? please help, thank you

 

[Gear300]

The observer wasn't specified to be human...so we cannot assume that we would have to take it so that the frequency is below 20Hz. Rather, what you should do is find the distance of separation between the 2 sources so that the waves they emit destructively interfere (the waves are out of phase by pi).