A source (red) emits sound wavefronts at a steady rate and moves through the medium. Ahead of it the circles bunch up (shorter wavelength → observer B hears a higher pitch); behind it they spread out (observer A hears a lower pitch). The medium speed v is fixed; only the spacing the observer meets changes. Slide the source speed v_s and read the two shifted frequencies.
A source (red) sits in the medium emitting wavefronts (teal circles) at a fixed rate; observers A and B stand either side.
nu' = nu·v/(v ∓ v_s), v = 340 m/s, nu = 500 Hz
v_s = — m/s · toward B: — Hz · behind A: — Hz