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.
In short, auditory illusions highlight areas where the human ear and brain, as organic, makeshift tools, differ from perfect audio receptors (for better or for worse).
Here are the examples of the main ones:
Top 10 Incredible Sound Illusions
In these illusions, your mind is tricked in to thinking it is hearing something when, in fact, it is not. To get the most out of this list you should have a set of stereo headphones and a stereo sound system. Unless otherwise noted (before each audio file), you should listen to these sounds with your stereo. Laptop speakers are not suitable for most of these illusions.
10 - Scale Illusion
This illusion was discovered by Diana Deutsch, and is an example of our brains “grouping” similar notes together. Two major scales are played: one ascending, one descending. However, the notes alternate from ear to ear – for instance, the right ear hears the first note of one scale, and then the second note of the other.
There are several ways in which people perceive these sounds, but the most common is to group the high and low notes together. Rather than hearing the two scales, people hear a descending and re-ascending melody in one ear, and an ascending and descending melody in the other. In other words, the brain reassigns some of the notes to a different ear in order to make a coherent melody. Right-handed people tend to hear the high melody in the right ear, and the low one in the left, while left-handers show a more diverse response.
NOTE: Listen through stereo headphones, or stereo separated loudspeakers, best placed some distance apart.
Some pieces of music consist of high-speed arpeggios or other repeating patterns, which change only subtly. If they’re played fast enough, the brain picks up on the occasional notes that change, and links them together to form a melody. The melody disappears if the piece is played slowly.
Compare these recordings of Christian Sinding’s Frühlingsrauschen (“Rustle of Spring”). At the higher speed, the changing notes linger in your perception long enough to be linked into a melody, but at the lower speeds they’re too widely separated. (original recording: www.classicalmidi.co.uk / Slow recording courtesy of Karle-Philip Zamor).
This is a recording of Shepard’s paradox synthesized by Jean-Claude Risset. Pairs of chords sound as if they are advancing up the scale, but in fact the starting pair of chords is the same as the finishing pair. If you loop this sample seamlessly then it should be impossible to tell where the sample begins and ends.
This is a recording of a paradox where bells sound as if they are falling through space. As they fall their pitch seems to be getting lower, but in fact the pitch gets higher. If you loop this sample you will clearly see the pitch jump back down when the sample repeats. This reveals that the start pitch is obviously much lower than the finishing pitch.
This is a demonstration of the stereo effect. Listening to it, you feel as though you are in a barber’s chair, with the barber moving around you, clipping away at your hair. As the barber “moves” to your right, the volume increases slightly in the right channel and decreases in the left. Similarly, increases in the volume of sound from the clippers give the impression that he is bringing them closer and closer to each ear. The illusion demonstrates our ability to locate sounds in space; by comparing the inputs to the two ears, we can work out where a sound is coming from.
NOTE: Listen through stereo headphones.
This sound was created by QSound Labs (www.QSound.com), which used the sound/technology in games like those found at www.EarGames.com.
This, like the barbershop above, is another stereo illusion. In this illusion a man shakes a matchbox all around your body and lights matches occasionally.
NOTE: Listen through stereo headphones.
3 - Tritone Paradox
This illusion was also discovered by Diana Deutsch. In this recording, some people hear the two notes going from low to high, while others hear them going from high to low. The is a good one to listen to in a group so you can compare notes afterwards. The notes being played are called the tritone – it falls exactly in the middle of a standard musical scale. This note was once considered to be evil and was not used in music until modern times.
Wait! Do not play the clip above until you have read this text. When you play this clip for the first time, play it with your eyes closed. Listen to what the man is saying. Now, play it again with your eyes open. Do you hear BA-BA, GA-GA, or DA-DA? Most adults (98%) think they are hearing “DA” – a so called “fused response” – where the “D” is a result of an audio-visual illusion. In reality you are hearing the sound “BA”, while you are seeing the lip movements “GA”.
1 - Phantom Words
This illusion was first demonstrated by Diana Deutsch at the University of California, San Diego. The recording features overlapping sequences of repeating words or phrases, located in different regions of stereo space. As you listen to it, you’ll start to pick out specific phrases. However, none of the phrases are really there. Your brain is constructing them, in a bid to make sense of a meaningless noise. Indeed, you may find that the phrases you hear are related to what’s on your mind – for example, people who are dieting often hear phrases associated with food. This can be a very eerie experience.
NOTE: Listen through stereo separated loudspeakers, best placed some distance apart.
This is a sound that can only be heard by people under 20 (some over 20 can hear it but not many) – it is a sine wave at 18,000 Hz (by comparison, a dog whistle sounds at 16,000 – 22,000 HZ – meaning your dog can hear this “under 20s” sound as well). This sound is used by some teenagers as a ring tone on their mobile phone so that only they (and others of their age group) can tell when the phone is ringing. It is also occasionally used in England to play very loud in areas that authorities don’t want teens to congregate in, as the noise annoys them. As people get older they lose the ability to hear higher pitched sounds – that is the reason that only young people can hear this sound – it is too high for most people over the age of 20.