Human Perception: Illusions and hallucinations: TYPES OF ILLUSORY EXPERIENCE: Stimulus-distortion illusions.: Auditory illusions. Human Perception Auditory illusions. A common phenomenon is the auditory impression that a blowing automobile horn changes its pitch as it passes an observer on a highway. This is known as the Doppler effect, for C. Doppler, an Austrian physicist, who in 1842 noted that the pitch of a bell or whistle on a passing railroad train is heard to drop when the train and the perceiver are moving away from each other and to grow higher when they are approaching each other. The sound heard is also affected by such factors as a wind blowing toward or away from the person. Copyright (c) 1996 Encyclopaedia Britannica, Inc. All Rights Reserved