The Wessel Illusion demonstrates how timbre can determine the way in which we perceptually group notes in a melody.

Three notes, rising in pitch but alternating in timbre, are played slowly. When this sequence is played faster, it’s possible to hear the trajectory of the melody change.

What’s going on?

When the notes are played slowly, most people hear a sequence that is rising in pitch. This is because there is enough available attention to track both the sound of the voice and the vibraphone as distinct timbres, and then group them together into one ascending melody.

When the melody speeds up, it’s no longer possible to track everything at once. At a certain point we are forced to cut our losses—we begin to group the notes solely based on their timbre. This creates the illusion of a descending sequence instead.

The Wessel Illusion was discovered by David Wessel of U.C. Berkeley's Center for New Music and Audio Technologies (CNMAT).

Animation: Isaiah Saxon
Recording Engineer: Jeremy Harris
Vibraphone: Andrew Maguire
Voice: Meara O'Reilly

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