1. The problem statement, all variables and given/known data A sound source has frequency 1000Hz and moves at velocity v along a straight line. A person stands a perpendicular distance d away from the line. At time t=0 the source is at its distance of closest approach to the listener. Express the frequency as heard by the listener as a function of time. 2. Relevant equations v=fλ 3. The attempt at a solution I can't work out how the wavelength is affected! When the source moves directly towards the listener then the listener hears λl = λs±Tsvs. So I have to do something involving angles, but I really don't know what! I think because of the definition of t=0, the ± won't be necessary in my expression, it will take care of itself. Is the velocity of the wave approaching the listener vscosθ, if θ is the angle between the velocity of the source and a line drawn from source to listener?