Most people are not familiar with auditory illusions, because they are much more difficult to create and then spread on the Internet. Another reason is that auditory illusions are much easier, and sometimes absolutely necessary, to illustrate on a set of high-quality headphones, rather than simple cell phone speakers. No matter the reason, once you experience them for yourself, you will definitely want to share them with your loved ones and friends. So how do auditory illusions work? In order to better understand this, we must first know how we perceive sounds.
Psychoacoustics
Psychoacoustics is a scientific study that deals with the ways in which we perceive sound, but also with our psychological reactions to it. It encompasses the entire audible spectrum from speech to music to any other sound frequency that travels through the air to our ears. Through this study, we can learn a lot about how people perceive sounds.
Familiar auditory illusions
Now that we know a little more about how we perceive sound, it will be easier for us to understand some of the common ways we misperceive sound. These illusions play with our hearing and exploit our limitations in the ways our auditory perception works. Be prepared to challenge everything you think you know about how our hearing works.
Roundness of Heights - Shepard Scale Illusions
The Shepard scale illusion is created by playing multiple frequencies an octave apart, either in ascending or descending order. As an example, take the ascending Shepard scale, as the higher notes approach the top of the scale and gradually become weaker, the lower notes are lost. The pitch of each note rises throughout the loop, but as long as this is repeated, our ears are tricked into thinking we are hearing a continuously rising note, even though the sound loop is still the same. This great concept has applications in other cases where we can create audible illusions like this by changing aspects other than pitch.
Rising rhythms
They present another illusion that tricks us into believing that the sound we hear is something it really isn't. With rising beats, the repeated looping beat sounds like it's getting faster and faster, when in fact each sound loop remains exactly the same. This is a very similar concept to the Shaperd tone, but instead of the higher or lower notes decaying at the opposite end of the spectrum, in this case the faster part of the beat is constantly fading away while the slower part is getting stronger.
The rhythms in the loop itself actually speed up, but have an end point. When these sound loops are played repeatedly, your brain hears these identical loops as one ever-increasing beat. Many electronic music producers have used this concept very effectively over the past few years.
Binaural illusion
If you've ever heard of binaural illusion or binaural beats, it may have been in the context of wave therapy. It is becoming more and more popular in the world of meditation and many people believe that binaural rhythms can help reduce anxiety and stress, help us relax and lead to better sleep. Specific health uses are still being speculated, but the binaural illusion is a real thing.
Binaural beats are sound illusions in which two tones with slightly different frequencies are played into each ear through headphones, and a third sound is produced by our mind that creates the difference in frequencies between one tone and the other. For example, if the tone in the left ear is played at 100 Hz or 100 beats per second, the tone in the right ear is played at 103 Hz, then the third perceived tone is heard at 3 Hz.
However, this illusion requires the use of headphones, because in almost all other cases, each ear will pick up both tones at once, and this illusion will not work.
The illusion of a talking piano
Another of the great auditory illusions circulating in the world is the illusion of a talking piano. In this illusion, an MP3 file with music or speech is converted to MIDI and played through the piano. The resulting sound is chaotic, but once your ears adjust, you can make out the words of the song being played in the form of selected keys played at the same time.
Since almost all sounds are just the sum of multiple notes, by reproducing the individual notes that make up the sound on, say, a piano or other instrument, you'll hear lyrics even where there aren't any.
The same goes for any musical instrument. However, in the world of sound illusions, the piano is not necessarily the best translator of the human voice. Selecting an instrument preset on your MIDI playback device produces tones that are closer to a perfect sine wave like a flute, amplifying this auditory illusion.
The McGurk effect
One illusion that combines audible and visible stimuli is the McGurk effect. If you've ever heard speech sounds differ depending on how your mouth moves, you've witnessed the McGurk effect. This is because a large part of listening to speech correctly involves looking at shapes, such as the movement of the mouth when speaking. If the sound source is weak, our brain looks for visual clues to determine what exactly the person is saying. So if we are in a noisy room with bad acoustics, we can quite often interpret the things that someone says to us in the wrong way.
In a popular video circulating on the Internet about this illusion, we see an image of a man speaking similar-sounding words with only one of the words sounding off. Every time we see him say one of the other words while we hear a similar word in the same audio track, our brain converts the sound into the word he is saying. This is a very interesting look at the way our senses of sight and hearing are connected.
Learning from auditory illusions
Auditory illusions clearly show us the difference between perception and reality. We humans tend to trust our perceptions and let our instincts dictate our actions, but these perceptions are not infallible. Optical illusions make us question the things we see, while auditory illusions teach us how our hearing can be tricked into hearing sounds that aren't actually there. The next time someone asks you "what do you hear," the question may seem a little trickier than ever.
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