7.3.2—The expression fο = f sv / (v ± vs) for the observed frequency when a source

For a source moving relative to a stationary observer in a medium, f_o=f_s v/(v±v_s), using the sign that gives higher frequency for approach and lower for recession.

Define source speed and wave speed in the medium, then choose the sign from the physical motion rather than memorising it blindly.

An approaching siren produces compressed wavefronts, so f_o exceeds f_s; after passing, the denominator changes and the observed frequency falls.

The source speed is measured relative to the medium in this model, not automatically relative to the observer; the sign must match approach or recession.