Hearing and Believing – The Most Interesting Auditory Illusions
Sensory illusions are interesting concepts. They cause us to perceive the world in different ways than we are used to and even question our judgment on what seemingly straightforward things represent. That’s because sensory illusions are created by stimuli that can be easily misinterpreted by the senses. We are presented with a limited amount of data, and our brains fill in the rest.
All of our senses are prone to being tricked. Optical, tactile, olfactory(smell), taste, and auditory illusions are all possible. In optical illusions, our vision is the sense being tricked by misleading information. Maybe we see a dress as being a different color than it actually is. Tactile illusions affect our sense of touch. In auditory illusions, it’s our ears. You get the idea.
Understanding illusions gives us an insight into the way our brains work and are just plain fun to experiment with.
The other sensory illusions
The type of illusion most of us are familiar with is the optical illusion. We’ve seen them on our friends’ social media posts and on the walls in our high school math classes. In an optical illusion, we are presented with misleading information that makes us see two things, question the size of something, see a still image as a moving one, or come to a number of other possible misinterpretations of the image. They teach us important lessons about not believing everything we see.
Though we may be less familiar with the other types of sensory illusions, they can each offer us valuable information about the way we perceive and interact with the world around us. For the purposes of this article, we will focus on auditory illusions.
Auditory illusions
Most people aren’t incredibly familiar with auditory illusions, possibly because it’s much harder to make memes out of them and mass disseminate them across the internet. Another reason for the disconnect may be that it’s much easier, and sometimes absolutely essential, to illustrate auditory illusions on a set of quality headphones rather than our readily available phone speakers. No matter the reason, once you experience them for yourself, you’re sure to share them with your friends and family.
So, how do auditory illusions work? To understand, we first need to know a little bit about how we perceive sounds.
Psychoacoustics
Psychoacoustics is the scientific study that relates to the way we perceive sound, as well as our psychological responses to it. It includes the entire audible spectrum, from speech to music to any other sound frequency that travels through the air and into our ears. Through this study, we have learned a lot about how humans perceive sounds.
When we hear sounds, we are almost always hearing a combination of different frequencies. Think of it like a chord on a piano or guitar. We are playing multiple harmonic notes or frequencies, but they combine together to create the audible experience of a single tone. If the source of that sound is poor, it opens up the possibility for our brains to play all kinds of tricks on us.
In addition to simply recognizing and processing sound, our brains use a bit of inference to determine what exactly we should be hearing. If your brain expects a certain note or word to be part of a musical or speech pattern, it may insert that note or word, whether we actually hear it or not. Your brain will take over and create the sound it wants to hear, regardless of the actual sounds you are experiencing. This inference is actually the basis of many aural illusions. To see this phenomenon in action, let’s look at some of the more common illusions.
Some well known auditory illusions
Now that we know a little more about how we perceive sound, it will be easier to understand some of the common ways we can incorrectly perceive sound. These illusions play on known limitations in the way our auditory perception works. Get ready to question everything you think you know about how your sense of sound works.
Pitch circularity – Shepard scale illusions
The Shepard scale illusion is created by playing multiple frequencies, one octave apart and either ascending or descending in pitch in tandem. Let’s take an ascending Shepard scale for our example. As the higher pitched tones approach the top of the scale and fade out, lower pitched tones are faded in. The pitch of each note rises throughout the loop, but when it’s repeated, our ears trick us into believing we are hearing a constantly rising tone, even though the loop is exactly the same every time. It’s a cool concept that can be applied elsewhere. We can create other audible illusions like this one by changing aspects other than pitch.
Risset rhythm
Here is another auditory illusion that makes us believe that the sound we’re hearing is doing something it’s not. In a Risset rhythm, a repeated, looped beat sounds like it is getting faster and faster when in reality, each loop remains exactly the same. It’s a very similar concept to the way the Shepard tone works, but instead of the higher or lower pitches fading out as the tones on the opposite end of the spectrum are fading in, the faster part of the rhythm continually fades out while the slower part fades in.
The beats in the loop itself are in fact speeding up, but they do have an end point. When these loops are played back to back, your brain hears these identical loops as one, continually increasing beat. Many producers of electronic music have exploited this concept to great effect over the years.
Binaural illusion
If you’ve heard of the binaural illusion or binaural beats, it may have been in the context of self help or wave therapy. It’s becoming increasingly popular in the meditation world, and many people believe binaural beats can help reduce anxiety and stress and may help us relax and get better sleep. The specific health benefits are up for speculation, but the binaural illusion is a very real thing.
Binaural beats are sound illusions wherein two tones at slightly different frequencies are played into each ear through headphones, and a third sound is produced by our minds, beating at the difference in frequencies from one tone to the other. For example, if the tone in the left ear is played at 100Hz, or 100 beats per second, and the tone in the right ear is played at 103Hz, or 103 beats per second, then the third perceived tone will occur at three beats per second.
This illusion does require headphones, though, since in almost every other case, each ear will pick up on both tones, and the illusion won’t work.
The talking piano illusion
Another great illusion making the rounds in the world of auditory illusions is that of the talking piano. In this illusion, an MP3 file of a song or speech is converted to MIDI and played back through a piano. The resulting sound is chaotic, but once your ears adjust, you can make out the words to the song played back in the form of selected keys played simultaneously.
Since almost all sounds are just the aggregate of multiple notes, reproducing the individual notes that create the sound of the voice on a piano or other instrument leaves you hearing lyrics where there are none.
The same would hold true for any instrument. In fact, in the world of auditory illusions, piano isn’t necessarily the best translation of the human voice. Choosing an instrument preset on your MIDI playback device that produces tones closer to a perfect sine wave, like a flute or a tuning fork, will only enhance the illusion.
McGurk effect
One illusion that combines both audible and visible stimuli is the McGurk effect. If you’ve ever heard speech sounds differently due to the way the mouth is moving, you’ve had a brush with the McGurk effect. That’s because a big part of hearing speech correctly involves seeing the shapes a mouth makes when the words are spoken. If the audio source is poor, our brains look for visual clues to determine what exactly is being said. When we are in a loud room with poor acoustics, we can easily misinterpret the things someone is saying.
In a popular video circulating on this illusion, we see the image of a man mouthing words that all sound similar, along with an audio track of only one of the words. Each time we see him mouthing one of the other words while we hear the similar word in the audio track, our brains transform the sound into the word being mouthed. It’s a very interesting insight into the ways our senses of sight and sound are interconnected.
Learning from auditory illusions
Illusions demonstrate the difference between perception and reality. We have a tendency to trust our perception and instincts to dictate our actions, but those perceptions aren’t infallible. Optical illusions make us question the things we see. Auditory illusions teach us how our ears can be tricked into hearing things that aren’t really there. Next time someone asks, “what do you hear” the question may seem a little more loaded than it used to.
