Relative motion between a sound source and listener shifts perceived pitch

Evidence

Imagine a free-field condition with a single receiver and a single sound source emitting one sinusoid with frequency f. If the distance between the source and the receiver diminishes with time, meaning that the source moves towards the receiver or vice versa, as shown in Figure 2.18, each successive wave is emitted from a position closer to the receiver than the previous wave. Therefore, the time between the arrival of successive waves at the observer decreases, causing an increase in the frequency compared to the static situation. … Conversely, if the wave source moves away from the observer, each wave is emitted from a position farther from the observer than the previous wave, thus increasing the arrival time between successive waves and reducing the frequency. The distance between successive wavefronts is increased, so the frequency of the sinusoid is lower. This can be observed when a vehicle producing a harmonic tone (such as an ambulance) passes by at high speed, the perceived pitch of the tone lowers rapidly when the vehicle goes past the observer. This phenomenon is called the Doppler effect. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 263

Categorization

• DEFINE: Establishes the mechanism underlying the Doppler effect in acoustics.
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 263