When you were a kid you may have had some fun looking at books with illusions, where you had to try to see the shapes hidden in patterns, sometimes by crossing your eyes. There are some pretty cool optical illusions out there, and you can find a fairly comprehensive set here. But that was then and you’ve seen a fair few in your time, so now it’s time to move on to auditory illusions.
It will be best to listen to the examples below with stereo headphones, so get ‘em on!
Let’s start with a simple one, called a Shepard Tone. Have a listen first and then I’ll explain it to you (assuming you don’t go crazy first).
So, it sounds like an infinitely progressing pitch, right? I’ve listened to it for too long and I think it has altered my sense of time, but it’s actually a very simple collection of three tones, called a tritone paradox. The three tones are simply each one octave apart – which, for the non-musical folk out there, means that they are the same note, but just higher or lower versions. They each progressively get higher at the same rate and restart once they have become inaudible at the high-end, creating this infinite loop! It’s a bit unsettling, I know.
The Risset Rhythm sounds like it’s constantly speeding up, even though it’s not, but is probably the most unsettling of the bunch. However, it is well worth the listen as it is just amazing! Try it:
How does it work? Well, it uses the same principles as the Shepard Tone, but adds in a constant rhythm (drums) that seem as though they are speeding up. But why does it seem like that? It has to do with the way that your brain processes information. The constant looping of the Shepard Tone confuses the brain, which makes mistakes when ‘categorising’ this information. The Risset Rhythm takes advantage of these mistakes, and the brain tries to make sense of the information by subtly merging the looping and the beats. This makes it sound like the beats are increasing in tempo while the pitch is increasing (but isn’t really, as we discovered above). Cool, huh?
Holophonic Sound, better than stereo! This is my personal favourite, and is all to do with you having two ears (assuming you do, of course). Normally when we listen to stereo recordings, the information sent to each earpiece is only slightly different. For example, in a jazz track, the drums might be in the centre, the guitarist slightly off to the right and saxophonist (what a great word) slightly off to the left, and they mostly will stay in those positions for the entire track. But in real life situations, this is not what usually happens. Holophonic sound replicates real-life sound using true stereo recording. Have a listen, and be amazed.
Virtual Haircut - please use headphones for this one!
The process is quite simple: the volume of the sounds in each earpiece is adjusted individually according to where the sound is coming from. So if a noise is coming from our right side and moves around the front and over to the left, it will start louder in the right ear but end up louder in the left ear. Simple, right? There is no real difference between a sound coming from in front of us, or behind us, our brain just chooses to interpret the information this way. This highlights our ability to be spatially aware just through sound. Our brain is recreating the scene in our minds just through interpreting volume levels!
I would strongly recommend visiting the fabulously named website Get High Now for more amazing audio and optical illusions, as well as explanations as to what is going on.