The Franssen effect is an auditory illusion where the listener incorrectly localizes a sound. It was found in 1960 by Nico V. Franssen. There are two classical experiments, which are related to the Franssen effect, called Franssen effect F1 and Franssen effect F2.
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There are two speakers to the left and right of the listener. Each is about 3 feet in distance from the listener, at approximately 45° angles.
The left speaker suddenly begins to produce a sharp pure tone. The two speakers are complementary to each other: i.e., as one increases, the other decreases. The left one is decreased exponentially, and the right speaker becomes the main source of the sound. The interesting illusion achieved here is that the listener perceives the sound as only coming from the left speaker, although the right speaker has been on most of the time.
Inside a room (auditorium) there are 2 loudspeakers at different positions. At the beginning of the presentation, loudspeaker 1 emits a sinusoidal signal with a steep attacking slope. Subsequently the power of this loudspeaker remains constant. The listeners can localize this loudspeaker easily. During the stationary part of the envelope the signal is very smoothly faded over from loudspeaker 1 to loudspeaker 2. Although loudspeaker 2 emits all the sound at the end, the listener's auditory events remain at the position of loudspeaker 1. This mislocalization remains, even if the test supervisor plugs off the cables of loudspeaker 1 demonstratively.
This effect gives some information about the capabilities of the human auditory system to localize sound sources in enclosed rooms[1]:
When looking at the sound, which arrives at the listener's ears, the following situation appears:
As a consequence the auditory system seems only to be able to localize sound sources in reverberant environment at sound onsets or at bigger spectral changes. Then the direct sound of the sound source prevails at least in some frequency ranges and the direction of the sound source can be determined. Some milliseconds later, when the sound of the wall reflections arrives, a sound source localization seems no more to be possible. As long as no new localization is possible, the auditory systems seems to keep the last localized direction as perceived sound source direction.