As part of our special issue on music, Daniel Levitin has written The Music Illusion, which looks at auditory illusions and how they can help us understand the workings of the human brain. Here we have compiled five of the most striking auditory illusions discovered so far.
We had a big pool to choose from, from the mysterious quintina (fifth voice) heard in some types of throat-singing, to the saxophone break that isn't on Lady Madonna (it's actually the Beatles singing into their cupped hands - not to be confused with the actual sax solo) and the soaring guitar sound of Pink Floyd's Dave Gilmour. Listen to our top 5 below, and read our explanations of the effects involved.
1 Barber's shop illusion (Listen with headphones)
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.
2 Phantom words (Listen through stereo separated loudspeakers, best placed some distance apart)
This illusion was first demonstrated by Diana Deutsch at the University of California, San Diego. Building on the stereo effect described above, 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.
Much of human perception is the result of the brain filling in gaps in the data from our senses. This means that if a part of an audio recording is missing, our brains will often work out what should have been there. In this recording by Richard Warren from the University of Wisconsin in Milwaukee, a spoken sentence is interrupted by a cough. One of the phonemes has actually been completely removed by the cough. But not only do most people hear the complete sentence, they generally find it very difficult to work out which phoneme has been deleted. If the phoneme is replaced by a period of silence, rather than a cough, the deletion is very obvious.
4 Scale illusion (Listen through stereo headphones, or stereo separated loudspeakers, best placed some distance apart)
Another effect first demonstrated by Diana Deutsch, this 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 (see diagram, top right).
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.
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)