The Shepard sound effect
Unmasking Auditory Illusions with Spectral Analysis
Autor:
Título 4
Learning objectives :
This activity allows students to discover auditory illusions and understand how our perception can be deceived by complex acoustic phenomena. It develops the spectral analysis of sounds and the understanding of the mechanisms of perception.
Concepts covered
Auditory illusions; Spectral analysis; Pitch perception; Wave superposition; Psychoacoustics
What students will do :
The student analyzes a Shepard sound (auditory illusion of an infinite continuous rise) using FizziQ sound analysis tools. Using the frequency meter and the frequency spectrum the student observes that despite the impression of infinite rise the sound is in reality composed of several sinusoids spaced an octave apart whose amplitude gradually varies, thus creating the illusion.
What is required :
Smartphone with the FizziQ application; 'Shepard' recording from the sound library; Headphones (recommended) for better perception; FizziQ experience notebook
Scientific background :
The Shepard effect, discovered by Roger Shepard in 1964, is a fascinating auditory illusion where the listener perceives a sound that appears to rise or fall indefinitely without ever reaching a limit. This paradoxical “acoustic Penrose staircase” is based on an ingenious superposition of sinusoidal sounds. The mechanism is as follows: several sinusoids are generated at frequencies separated by exactly one octave (2:1 ratio). When all frequencies increase simultaneously, the amplitude of each component is modulated: higher-pitched sounds gradually attenuate while new lower-pitched components appear. Because our brain primarily interprets the direction of melodic movement rather than absolute frequencies, we perceive a continuous rise, even if the frequencies return cyclically to the same values. The spectrogram reveals this structure: rising parallel lines that vary in intensity, creating a perfect cycle. Analysis with the FizziQ frequency meter shows that the detected fundamental frequency does not rise indefinitely but falls periodically, while the frequency spectrum reveals the simultaneous presence of several components. This illusion exploits a fundamental characteristic of our auditory perception: the pitch of a sound is perceived in a relative and circular way (notes separated by an octave are perceived as similar). Variants like the Shepard-Risset glissando (continuous version) or the Deutsch effect (tritone paradox) exploit similar principles. These illusions have applications in electronic music, cognitive psychology and sound interface.