When real world physics and our perception of the real world differ, the result is often a perceptual illusion. Most of the time the frequency of a sound (a physical measurement) and the pitch of a sound (a perceptual characteristic) are related. 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. We found that despite the fact that frequency falls, most listeners hear a rise in pitch as the sound source approaches. Since the physical measurement of frequency and the perceptual experience of frequency (i.e. pitch) were going in opposite directions, we called this phenomenon the Doppler illusion. The pattern of pitch change that is heard (rising then falling) resembles the pattern of loudness change that occurs. We argue then that the Doppler illusion is due to the change in loudness that occurs as the train approaches. More generally, dynamic changes in loudness can influence perceived pitch in a previously undocumented way.

As a sound source approaches a listener it grows louder. Our laboratory task required that listeners attend to the change in pitch and to ignore the change in loudness. If listeners are asked to attend only to the change in pitch that occurs and ignore the loudness change, they almost always fail. Their judgments are influenced by the change in loudness. These results suggest that under dynamic conditions, the perception of pitch and loudness interact. Specifically, dynamic increases in loudness, like those that occur when a train approaches, can lead to the perception of rising pitch. The effect is noteworthy because as the train approaches, frequency, which is the primary determinant of pitch, actually falls.

To further explore the dynamic pitch-loudness interaction we presented listeners with tones that had different combinations of changing pitch and loudness. This was to see if loudness change influences perceived pitch in situations that are not specified by a Doppler shift. Loudness would rise, fall or remain constant, while pitch either rose, fell or remained constant. In the first experiment we asked listeners to attend to changes in pitch. We found that in all of the cases dynamic intensity change influenced perceived pitch.

In a second experiment we presented listeners with the same types of tones but asked them to track changes in loudness. We wanted to see if dynamic frequency change would influence loudness change in the same way that dynamic intensity change influenced perceived pitch. We found a similar pattern of results. That is, dynamic changes in frequency influenced perceived loudness. The results suggest that under conditions of dynamic change, the perception of pitch and loudness interact. This dynamic interaction is qualitatively different from any interaction that occurs under similar static conditions (where static tones change in pitch or loudness from trial to trial). While there is certainly much more work to be done on the topic, the current findings regarding dynamic interaction may have implications for improving emergency vehicle warnings, auditory virtual reality, and perhaps even contributing to our understanding of speech perception.