Franssen effect
Encyclopedia
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.
to the left and right of the listener. Each is about 3 feet in distance from the listener, at approximately 45o angles.
. 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.
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 event
s remain at the position of loudspeaker 1. This mislocalization remains, even if the test supervisor plugs off the cables of loudspeaker 1 demonstratively.
auditory system
to localize sound sources in enclosed rooms:
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.
Auditory illusion
An auditory illusion is an illusion of hearing, the aural equivalent of an optical illusion: the listener hears either sounds which are not present in the stimulus, or "impossible" sounds...
where the listener incorrectly localizes a sound. It was found in 1960 by Nico V. Franssen. There are two classical experiments
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
, which are related to the Franssen effect, called Franssen effect F1 and Franssen effect F2.
Setup
There are two speakersLoudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
to the left and right of the listener. Each is about 3 feet in distance from the listener, at approximately 45o angles.
Producing the Illusion
The left speaker suddenly begins to produce a sharp pure tonePure tone
A pure tone is a tone with a sinusoidal waveshape.A sine wave is characterized by its frequency, the number of cycles per second—or its wavelength, the distance the waveform travels through its medium within a period—and the amplitude, the size of each cycle...
. 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.
Experiment
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 localizeSound localization
Sound localization refers to a listener's ability to identify the location or origin of a detected sound in direction and distance. It may also refer to the methods in acoustical engineering to simulate the placement of an auditory cue in a virtual 3D space .The sound localization mechanisms of the...
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 event
Auditory event
Auditory events describe the subjective perception, when listening to a certain sound situation. This term was introduced by Jens Blauert in the year 1966, in order to distinguish clearly between the physical sound field and the auditory perception of the sound.Auditory events are the central...
s remain at the position of loudspeaker 1. This mislocalization remains, even if the test supervisor plugs off the cables of loudspeaker 1 demonstratively.
Conclusions
This effect gives some information about the capabilities of the humanHuman
Humans are the only living species in the Homo genus...
auditory system
Auditory system
The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...
to localize sound sources in enclosed rooms:
- The human auditory system is able to localize a sound source in reverberant sound fieldsReverberationReverberation is the persistence of sound in a particular space after the original sound is removed. A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air...
, if there are fast signal changes or signal onsets. (Loudspeaker 1 was correctly localized at the beginning of the experiment.) - The human auditory system is not able to localize signals with a constant amplitudeSound pressureSound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
and spectrumHarmonic series (music)Pitched musical instruments are often based on an approximate harmonic oscillator such as a string or a column of air, which oscillates at numerous frequencies simultaneously. At these resonant frequencies, waves travel in both directions along the string or air column, reinforcing and canceling...
in reverberant sound fields. (The fade over to loudspeaker 2 was not recognized by the listeners.) - As long as no sound source can be localized, the direction of the last localized sound source remains as the perceived direction. (The auditory event remained at loudspeaker 1, although loudspeaker 2 emitted all the sound at the end of the experiment.)
When looking at the sound, which arrives at the listener's ears, the following situation appears:
- At the beginning of the experiment, when loudspeaker 1 started to emit sound, there was a short time period, where only the direct sound of loudspeaker 1 arrived at the listener's ears. In this time period the localization of loudspeaker 1 was surely possible, because it was not yet disturbed by wall reflections.
- Some millisecondsMillisecondA millisecond is a thousandth of a second.10 milliseconds are called a centisecond....
later the sound of the wall reflections arrived and disturbed the localization of sound sources. - During the fade over the level and the spectrum of the emitted sound remained constant. This fade over was overlaid by many wall reflections from the sound situation before. Obviously no sound source localization was possible during this phase.
- At the end, when only loudspeaker 2 emitted sound, the situation was quite similar, the sound of the wall reflections, which arrived simultaneously, prevented a localization of this sound source.
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.