Auditory Illusions: Examples and Explanations
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. In auditory illusions, the human brain thinks that it can hear something that is either not “there” or exists in a very different form to how it is perceived.
Normally, we rely on being able to translate the world around us into accurate sense perceptions. But in certain circumstances, the relationship between stimuli and senses breaks down, leading to some interesting results. The more we learn about how complex everyday hearing is, the more auditory illusions start to make sense. They could just be momentary disconnections in the way our minds and ears interact, as the brain processes fragments of sound into a continuous whole.
The primary reason for auditory illusions is simple: the human ear and brain are capable of some amazing achievements, but it can only do so much. The exterior world is constantly changing and incredibly complex. The ear and brain have a lot of work to do if they want to interpret it and prime the body for action. Evolution has taken plenty of short cuts to ensure that humans can interpret their environment, but the process has left behind plenty of rough edges. These often show up as auditory illusions. And then there’s the fact that our hearing is limited. We can (generally) only hear sounds from around 20Hz to 20,000Hz.
Humans are fairly susceptible to illusions, despite an innate ability to process complex stimuli. Confirmation bias is believed to be largely responsible for the inaccurate judgments that people make when evaluating information, given that humans typically interpret and recall information that appeals to their own biases. Amongst these misinterpretations, known as illusions, falls the category of auditory illusions. spatial information is processed with greater detail and accuracy in vision than in hearing. Auditory illusions highlight areas where the human ear and brain, as organic survival tools, differentiate from perfect audio receptors; this shows that it is possible for a human being to hear something that is not there and be able to react to the sound they supposedly heard. Many auditory illusions, particularly of music and of speech, result from hearing sound patterns that are highly probable, even though they are heard incorrectly. This is due to the influence of our knowledge and experience of many sounds we have heard. In order to prevent hearing echo created by perceiving multiple sounds coming from different spaces, the human auditory system relates the sounds as being from one source. However, that does not prevent people from being fooled by auditory illusions. Sounds that are found in words are called embedded sounds, and these sounds are the cause of some auditory illusions. Composers have long been using the spatial components of music to alter the overall sound experienced by the listener. One of the more common methods of sound synthesis is the use of combination tones. There are a multitude of examples out in the world of auditory illusions.
Examples of Auditory Illusions
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.
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. It’s hard to recreate this in real life. When you say words, your mouth tends to make very similar shapes, but skilled tricksters can use the McGurk Effect to fool people into hearing something they really can’t. The key to what’s actually happening is inside the hearer’s brain. We don’t just absorb sound waves and turn them instantly into sound, like a radio microphone might do. Instead, our brains anticipate and interpret what they see and hear, taking short cuts to produce the perception of sound.
The Shepard Tone
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). When you first hear the sound this creates, all you hear is a basic scale, with the pitch rising steadily. But if you replay that scale, something amazing happens. Instead of starting from the base note, it sounds as if the scale is constantly rising in pitch, which it isn’t actually doing, because the sine waves constantly cycle back downwards. What we hear is a scale ascending forever, a little like the stripes on a barber pole. But it’s a function of how our brain fails to assimilate complex audio information.
You may well have heard the Shepard scale in action without realizing it. For instance, it’s commonly used to build a kind of oppressive atmosphere in movies (the 2017 war film “Dunkirk” used it throughout to create the feeling of peril and dread).
The Tritone Paradox
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 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.
Pareidolia and "Phantom Words"
There are plenty of other remarkable auditory illusions beside the Tritone Paradox, the McGurk Effect and the Shepard Tone, and more are being discovered all the time. For example, Diana Deutsch also looked into what is known as “pareidolia” - the perception of words or images which make sense from a chaotic, disorganized backdrop. As illustrated here, we ‘see’ visual objects’ that correspond to those that are familiar to us, even if our perceptions are quite incorrect. Deutsch rigged up a pair of stereo speakers and played a two-word phrase through them, with one offset so that the first word is overlaid constantly by the second. What results shouldn’t make sense, but in experiments, people have heard “phantom” words emerging as their brains struggle to understand what they are hearing.
Some years ago I discovered a way to produce a large number of ‘phantom words’ and phrases within a short period of time. To obtain the best effect, find a time when you will not be disturbed, and sit in front of two loudspeakers, with one to your left and the other to your right. (Headphones don’t work so well for this illusion.) Make sure that your sound system is set for stereo, and that the two loudspeakers are balanced for loudness. In a ‘phantom words’ demonstration, each track contains two words, or a single word composed of two syllables, and these are repeated over and over again. The same sequence is presented through both loudspeakers, but the tracks are offset in time so that when the first sound (word or syllable) is coming from the speaker on the left the second sound is coming from the speaker on the right; and vice versa. It works well to have a pen and paper in front of you, so that you can write down the words and phrases that you hear. Often people initially hear a jumble of meaningless sounds, but after a while distinct words and phrases suddenly emerge. It often seems that the left and right loudspeakers are producing different words, which sometimes appear to be spoken by different voices. After a while, you will probably find that new words and phrases appear to be coming from one or both of the loudspeakers. When this happens, write down the new ones also. In addition, it’s not unusual to hear a third stream of words or phrases, apparently coming from some location between the loudspeakers. Nonsense words, and musical sounds such as percussive sounds or tones, sometimes appear to be mixed in with the meaningful words.
If English is your second language, you may find that you hear some words and phrases in your native language. In courses on illusions that I taught at UCSD, I generally played some ‘phantom words’ to my class. The students at our university are linguistically very diverse, and taken together I’ve received reports of ‘phantom words’ in Spanish, Mandarin, Cantonese, Korean, Japanese, Vietnamese, Tagalog, French, German, Italian, Hebrew, and Russian - to name just a few. People appear to hear words and phrases that reflect what is on their minds - rather as in a Rorschach test, though it’s my impression that the present effect is stronger. I can guess who is likely to be on a diet, as they report words like ‘I’m hungry’. ‘diet coke’ or ‘feel fat’. And students who are stressed tend to report words that are related to stress - if I play these sounds close to exam time, students may well hear phrases like ‘I’m tired’, ‘no brain’, or ‘no time’.
The "Speech to Song" Effect
Another fascinating illusion is the “Speech to Song” effect. In this case, the brain seems to turn ordinary speech into a form of song if a phrase is repeated often enough. But not just any phrase will do. Apparently, it needs to be a phrase that has been taken from a longer sentence, and then immediately repeated.
Auditory Illusions and Hearing Aids
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.
Dr. Emily Russell is a lead audiologist at hear.com, a global leader in hearing care and the largest online retailer of medical-grade hearing aids. Dr. Russell graduated from the University of Oklahoma Health Sciences Center and has performed hundreds of Teleaudiology appointments to help people find greater access to hearing healthcare. Outside of audiology, Dr.