Auditory Illusions

Kung fu movies on VHS were a rare delight for my friends and me during the late 80s. We enjoyed them not because of their cinematic brilliance, but instead because the translation was particularly bad.

Now, this wasn’t the only reason to watch a third rate kung fu film! There was plenty of martial arts action, but the plot lines tended to be quickly put together and full of easily spotted inconsistencies.

If you’ve ever seen voice dubs like these, you’ve probably experienced The McGurk Effect a few times. This is when a mouth moves around and makes a particular shape, but the sound coming out of the TV is something really different—something that particular shape of mouth could not make.

Sometimes, your brain sort of auto-corrects this.

You hear a word that the person simply did not say, because your brain is trying to make sense of the conflicting accounts from your eyes and ears. In this case, it sides with your eyes and overrides the information coming in from your ears.

Or, as with the kung fu videos, the sound effects also had to be dubbed back in. This was even more disconcerting than hearing the wrong word, because the mismatch often took you out of the moment. I found my brain trying to justify these sounds, sliding into the same sort of effect so that my viewing experience was more enjoyable.

Our ears have a way of fooling our brains like this. One of my favorite examples is called a Shepard Tone, named after a cognitive scientist who discovered how to create a tone that seems to go up or down in tone forever.

You might not know the name, but you’ve probably heard plenty of Shepard Tones in your life. Here’s one:

This sound seems to go up forever, doesn’t it?

This is wild, but you can understand how it works if you know that sound is made of waves. The Shepard Tone creates multiple tones, one octave apart. This is a bit like a bullwhip that has a ripple that starts, and then another starts right behind it, and then another, and so on.

Each tone rises up and then fades out at the top of the wave, and then another tone starts immediately behind it, starting to rise. Our brains put this continuously cycling sound together into a straight line, like watching the bull’s whip.

Our brains interpret this as constantly rising in pitch, kind of like watching how the wave travels along the bullwhip from one side to another. However, the Shepard Tone only seems to move somewhere: it just cycles through the same tone over and over again.

The Shepard Tone is just one example of how understanding sound waves can be useful. If the waves line up just right, they can cancel one another out. That’s exactly how noise-canceling headphones work: they instantaneously meet the incoming waves with their inverses, utterly preventing the sound from reaching your ear.

Waves of sound can work the other way, too: they can amplify one another and become almost weaponized. Sound waves can stack and knock things over in the physical world.

This makes me think of some of the louder punk rock shows I’ve been to. I’ve felt those sound waves pounding in my chest.

If you’ve enjoyed a quick peek at auditory illusions, you might really enjoy this deeper dive into Apophenia, the way we see all sorts of things that aren’t really there: