An acoustic illusion is analogous to an optical illusion, an acoustic perception that has no real equivalent outside of our body in the form of a physical process. Psychoacoustics deals with the question of how people hear and process sounds . Acoustic illusions can provide clues as to how the human ear processes sound signals.
Due to the anatomical peculiarities of the inner ear , a person perceives only the louder of two tones that are close to each other in terms of their frequency , but differ greatly in volume . The quieter one is covered . Among other things, this effect is used in MPEG audio coding.
The effect becomes stronger as the level of the loud sound increases. Above the frequency of the louder tone, the quieter tone is more obscured than below. In extreme cases, the effect can extend over several octaves.
The Shepard scale is an acoustic illusion in which the listener has the impression that a scale continues to rise or fall, although only the same sequence of notes is repeated. It was first described in 1964 by the psychology professor Roger Shepard .
The tritone paradox was first discovered in 1986 by the English music psychologist Diana Deutsch . It is a paradox of perception in which two Shepard tones sounding one after the other are perceived by different listeners in different directions at a distance of a tritone . For example, while one listener perceives the tone pair C and F sharp as an upward-pointing interval, another listener always perceives this pair as a downward step. This does not depend on whether the listeners are musical or not. A reliable explanation for this phenomenon has not yet been provided, but there are indications that the socialization and in particular the melody of the mother tongue could have an influence on the reception .
This acoustic illusion was also discovered and published by Diana Deutsch in 1972. It shows how knowing a melody can influence the perception of the melody. For this, a well-known melody is played with the correct tones, although these tones are randomly distributed over several octaves. If the listener does not know what tune it is, they will have difficulty recognizing it. Once they know what melody it is, it will be easier for them to follow the melody.
Melody with octave jumps
Both examples at the same time
With stereophony , phantom sound sources are generated.
This takes advantage of the fact that people can hear with two ears and reconstructs the location of the original signal from the acoustic signal via stereo loudspeakers in the stereo triangle . This makes it possible to achieve spatial sound with just two channels. By increasing the base width you can get the impression that the sound is coming from outside the area in which the loudspeakers are set up. Runtime stereophony and intensity stereophony are used to achieve the effect . By increasing the number of channels, the stereo impression can be improved, among other things by the associated enlargement of the so-called sweet spot, the location where you can hear the optimal ambient sound.
The Franssen effect states that a person in reverberant rooms can only determine the direction of a sound source if the volume or the sound changes significantly. If the volume and sound remain constant, it is no longer possible to determine the direction and the direction initially perceived is retained.
This can lead to acoustic illusions. If a sound starts in a loudspeaker in a somewhat reverberant room, a listener who is further back in this room (outside the reverberation radius ) can correctly determine the direction of the sound. If this tone is then faded very gently to a second loudspeaker, the direction perceived by this listener remains with the first loudspeaker, although the other loudspeaker is now active. To the amazement of the listener, you can even unplug the speaker cable, the perceived direction remains with this speaker.
The explanation for this is that when the sound is used, the direct sound of the sound outweighs the listener for a short time so that the direction can be determined. A short time later, wall reflections arrive at the listener from all possible directions, which outweigh the direct sound. From this point on, it is no longer possible to determine the direction. The slow change of direction of the direct sound to the second loudspeaker can no longer be heard, and the auditory event remains at the location of the first loudspeaker. Only when there is a major change in volume or sound is there a short period of time again in which (at least in some frequency ranges ) the direct sound is stronger than the reflections, so that the direction of the sound can be determined.
Synesthetes combine different sensations. For example, these people perceive sounds as color impressions.
Visual information can also influence the perception of hearing, first described under the McGurk effect and at that time a novelty in perceptual psychology.
When one suffers from tinnitus , one hears noises or single tones that have no external source and can be very disturbing.
- Examples of acoustic illusions
- Acoustic illusions , Neanderthal acoustics
- Demonstration of the Franssen Effect ( Memento from July 4, 2008 in the Internet Archive )
- Diana Deutsch: Octave generalization and tune recognition , Perception & Psychophysics (1972), issue 11, pages 411-412