The Doppler Illusion: Rising Pitch with Falling Frequency

John G. Neuhoff, Michael K. McBeath

ABSTRACT

The Doppler effect refers to the change in frequency that occurs at a point of observation as a moving sound source passes that point. A familiar example may be the pitch change heard in a train's horn as it passes a crossing or an ambulance as it passes on the street. Three studies were performed to illustrate and explain a new auditory illusion associated with the Doppler effect. Study one was a survey that confirmed the existence of a popular belief that the pitch of an approaching sound source rises, even though its physical frequency at the point of observation falls. The fact that this belief has worked itself into the literature with seemingly no physical explanation for the phenomena is further evidence that the belief is widespread. Study two confirmed that subjects presented with simulated Doppler shifted stimuli actually perceive a pitch rise, so the belief was not due to an error in memory. These results demonstrate that a rise in pitch can be experienced under various conditions of falling frequency. Specifically, a Doppler shifted tone in which there is no rise in frequency can produce an illusory rise in pitch. We therefore refer to this perceived rise in pitch as the "Doppler Illusion." Wave form complexity and frequency range were found to significantly affect the magnitude of the illusion.

As the sound source approaches, perceived pitch rises while physical frequency remains constant, then falls. The rate of change in intensity may affect judgments about changes in pitch.

Study three was a control condition that confirmed the pitch rise found in experiment two does not occur under matched static conditions. Subjects were presented with static tones that corresponded in frequency and intensity to the portion of the dynamic Doppler tone where the pitch rise was perceived.

Our results support dynamic change as a fundamental element in the perception of the Doppler illusion. The findings provide evidence for marked differences in the perception of pitch and loudness under static and dynamic conditions. It appears that the illusion is at least in part due to the dynamic nature of Doppler stimuli. Observers may be inaccurate in judgments of pitch change because of the complexity in the rate of change in intensity and frequency. A doppler shifted tone exhibits an accelerating rate of change in intensity, even when the velocity of the sound source remains constant. As the source draws closer to the observer, this increasing rate of change in intensity may influence real time judgments about changes in pitch (which is also changing at an increasing rate in the opposite direction).

Degree of pitch rise increases with frequency and wave form complexity.

In any event, it is clear that in the case of Doppler shifted stimuli, the relationship between pitch and loudness takes on new characteristics that have not previously been addressed. While this is important, it is likely that these types of characteristics are not limited to the dimensions of pitch and loudness, but are revealed by an appropriately dynamic means of studying dynamic dimensions. Further research that considers holistic dynamic stimuli will likely yield a more thorough and ecologically valid understanding of other perceptual dimensions.