Precedence effect

Figure 11.7: Due to the precedence effect, an auditory illusion occurs if the head is placed between stereo speakers so that one is much closer than the other. If they output the same sound at the same time, then the person perceives the sound arriving from the closer speaker, rather than perceiving an echo.

A more common auditory illusion is the precedence effect, in which only one sound is perceived if two nearly identical sounds arrive at slightly different times; see Figure 11.7. Sounds often reflect from surfaces, causing reverberation, which is the delayed arrival at the ears of many ``copies'' of the sound due to the different propagation paths that were taken from reflections, transmissions, and diffraction. Rather than hearing a jumble, people perceive a single sound. This is based on the first arrival, which usually has the largest amplitude. An echo is perceived if the timing difference is larger than the echo threshold (in one study it ranged from $3$ to $61$ms [366]). Other auditory illusions involve incorrect localization (Franssen effect and Glissando illusion [61]), illusory continuity of tones [347], and forever increasing tones (Shepard tone illusion [290]).