An observer standing beside a road hears the horn of an approaching vehicle at a frequency higher than the actual frequency of the horn. When the vehicle passes and moves away, the frequency heard drops below the actual frequency. This observation is best explained by

The Doppler effect describes the apparent change in frequency of a wave due to relative motion between the source and the observer. An approaching source compresses the wave fronts, increasing the perceived frequency, while a receding source stretches them, decreasing the perceived frequency. This is a classic illustration of the Doppler effect and not related to superposition, resonance, or diffraction.