The Haas effect, also known as precedence effect, is a psychoacoustic phenomenon that describes how human hearing localises sound sources. When a sound is followed closely by another sound (typically within 20-40 milliseconds), the listener perceives a single fused auditory event coming from the direction of the leading sound source.
This allows for some interesting audio techniques and illusions. For example, if you play a sound from both the left and right speakers but delay the right channel by about 20-40 ms, the listener will perceive the sound as coming only from the left speaker. Their brain “ignores” the slightly delayed right channel sound.
Likewise, you can simulate a wider stereo image by duplicating a monophonic sound and delaying one copy ever so slightly. Even though it is the same sound played twice, our hearing localizes the two delayed versions separately and we perceive a wider sound. This technique is commonly used in music production and sound design.
The Haas effect demonstrates how our auditory system localizes sounds more by the first arriving sound wave rather than the overall combination of waves entering our ears. It allows sound engineers to manipulate localization and widen the stereo field. However, if the delay exceeds about 40ms, a distinct echo may be perceived.