What is a flutter echo, and how can parallel walls cause it?

A flutter echo is a rapid, repetitive pulsing of sound that occurs when sound waves reflect back and forth between two surfaces.1

This phenomenon is caused by parallel walls in an enclosed space, such as an auditorium, which trap sound waves and reflect them repeatedly between the opposing surfaces. Because the sound is reflected back and forth in quick succession, it creates a distinct, pulsing auditory effect. In addition to the repetitive sound, this interaction can lead to destructive interference of the sound wave, which can negatively impact the acoustic quality of a room. To prevent such issues, architects often use oblique walls to avoid the resonances associated with parallel surfaces.12