How your ears let themselves be fooled
Most people are familiar with optical illusions. They are tricks of the eye that make the brain think something is there when in reality there is nothing. But did you know that the ears can be tricked just as easily? Auditory illusions may not be as well-known as their optical cousins, but they are just as fascinating. What’s more, by studying them, we can get a much better idea of how the human auditory system works, helping to formulate better treatments and technology for common hearing problems.
Understanding the McGurk Effect: What you see and hear aren't the same thing
Amazingly, the way we visualize the world has an important impact on the things that we hear. This is neatly illustrated by one of the most famous auditory illusions: the McGurk Effect. Named after one of its discoverers (the British psychologist Harry McGurk), the effect is particularly common in conversation. It turns out that the way words are formed by the speaker is as important in how they are perceived as the sound that they make. So, if you shape your mouth in two different ways, but say the same word, viewers may well hear two separate sounds.
Below you can see an example of the McGurk Effect:
The Shepard Illusion: What goes up must come down
Sometimes, auditory illusions are embedded deep within the music that we adore. Take the famous Shepard Tone, for example. Named after the California-based neuro-scientist Roger Shepard, the tone is a complex mixture of “sine waves” (the curvy audio waves you’ve probably have encountered in geometry class). These waves are constantly being raised in pitch, only to be dropped down an octave, like a kind of waterwheel mechanism.
Below you can see an example of the Shepard Illusion:
The Tritone Paradox: Everyone hears something a little bit different
The Tritone Paradox is one of the most well-known variants of the Shepard Illusion, and it’s an auditory illusion that works best when you try it out on friends. In the normal Shepard Tone, a scale ascends seemingly forever. But with the Triton Paradox, you’ll hear a pair of notes played directly after each other, with a small gap in between called a “tritone.”
But these aren’t ordinary notes. As you might have guessed, they are a pair of Shepard Tones, and have the same magical properties as their ascending compatriots. According to British musicologist Diana Deutsch, these two tones can either be heard as ascending or descending notes. But here’s the interesting part: when you listen to them as a group, you’ll probably come up with different interpretations of which note was high, and which was low. Why is this? It’s still not exactly clear. However, according to Deutsch, the language you speak and the culture you grew up in plays a major role.
Below you can see an example of the Tritone Paradox:
Are auditory illusions relevant to hearing aid design and prescription?
Absolutely. In a way, modern hearing aids are a form of auditory illusion in themselves. Manufacturers are constantly trying to replicate the way the human ear works. They also seek to understand how to create electronic representations of data the human brain can perceive. Many hearing aid units use algorithms to process human speech, picking the sounds that seem to fit speech and amplifying them. The knowledge of how sounds behave and how they can be represented mathematically is a crucial aspect of designing devices that produce a reliable, accurate version of reality. So, we need to know how illusions are created. Even if they are fun to get to grips with, these illusions are more than simply tricks. Aside from helping to create exotic musical sounds and sound effects for movies, they contribute to hearing aid development, too.