Calculate observed frequency for moving sources.
Motion changes how often wavefronts arrive, not the wave speed. A source moving toward the observer emits each successive crest from closer in, compressing the wavelength; an observer moving toward the source runs into crests sooner. The two effects enter the formula differently — source motion appears in the denominator and observer motion in the numerator — which is why they are not simply interchangeable even at the same speed.
Doppler shift for sound
f observed = f emitted x (v + v observer) / (v - v source), with speeds positive when moving toward the other party
Because the medium provides a reference frame for sound. Source motion physically compresses the wavelength in the medium, while observer motion only changes the encounter rate with unchanged wavefronts. For light there is no medium and a single relativistic formula applies.
The denominator shrinks toward zero and the predicted frequency rises without bound. Physically the wavefronts pile up into a shock front — the sonic boom — and the simple formula stops applying, which is why this calculator floors the denominator.
Approaching, the source speed is positive and the pitch is raised. After it passes, the source recedes so the speed becomes negative, the denominator grows, and the pitch falls below the emitted frequency — a single audible step at the moment of passing.