Beyond the Doppler Illusion. When real world physics and our perception of the real world differ, the result is often a perceptual illusion.
Pitch is our subjective perceptual experience of frequency.
The change in frequency that occurs when a train passes a crossing or an ambulance passes on the street is due to the Doppler effect. If you are standing at a railroad crossing for example, the frequency of an approaching train falls slightly as the train approaches, falls rapidly as the train passes, and then continues to fall at a slower rate as the train fades into the distance. Despite this fact of physics, most people tend to hear a pitch rise as the train approaches and a pitch fall as the train departs. We tested this phenomenon in the laboratory by presenting listeners with simulated Doppler shifted tones and asking them to track with a joystick the changes in pitch that they heard. Listeners moved the joystick forward as they heard pitch rise and backward as they heard pitch fall.