I’ve just been enjoying the 20,000 Hz podcast on beatboxing. This included a focus on the amazing things beatboxers do with their vocal anatomy to mimic the sound of a drum machine. But there is an equally interesting bit of science about how listener’s hear beatboxing. The performer must exploit how the brain processes sound to impress the listener.
Beatboxers need to create the impression that they are creating the sound of different parts of the drum kit. They leap between different drum sounds and yet what the listeners hear are multiple rhythmic lines. The most impressive examples for me are when the performer is singing at the same time. The video below gives a well-known example from Rahzel, whose party piece is a rendition of the song ‘If Your Mother Only Knew’. Before he starts fully beatboxing about 2 minutes in, he tells his audience that he will be doing five things simultaneously, ‘[I’m] going to do the beat, right, the chorus, the bass line, the verse, and the background vocals.’
How does Rahzel manage to be singing at the same time as playing the drums? Some of the time, this is because the sounds come from different parts of the anatomy. As this blog post explains, a drum sound can be created in the front of the mouth. At the same time Rahzel can be creating his singing voice starting with the vocal chords in his larynx. Even so, the two sounds can’t be created completely independently, so when the drum and singing are on the same beat, a compromise sound that is somewhere between singing and a drum is made. Why doesn’t the listener notice this mangling of the sound?
Rahzel is exploiting how the brain takes fragments of sounds entering the ear and pieces them together. Take this simple example below of the continuity illusion. In the video you will hear an intermittent beeping sound, with a short burst of noise in the gaps between the beeps. When the noise burst is loud, as happens in the middle of the video, the beep goes from sounding intermittent and appears to become a constant tone. This is an illusion in the brain, the beeps are exactly the same throughout the video and are always intermittent. Your brain is imagining a constant tone even though it does not exist in reality.
The fact that the brain can hear things that aren’t exactly what the ear picks up can be exploited by beatboxers. At some points Rahzel has to sing at the same time as create a drum sounds. Inevitably the drum sounds he creates get a little distorted. But the listener doesn’t notice because their brain assumes the distorted drum sound landing on the beat will be the same as previously heard. The listener’s brain goes for the simplest representation, that the drum sound is unaltered from previous examples.
One thing Rahzel does to help this illusion, is before he fully launches into his impressive party trick of doing the drums and vocals simultaneously, he spends a long time singing the lines on their own and setting the rhythm up without singing. Doing this primes the audience with lyrics and rhythm, so when he adds the drum sounds the listeners are able to smooth over any mangled sounds. This is an example of top-down cognition influencing how the brain sorts out the sound picked up by the ear. The brain is bringing in memory and expectation, which has been primed by Rahzel, to help work out what is going on.
An ability to piece fragments of sound into something more coherent is a vital skill. For instance, it allows us to join up fragments of speech into a whole discourse when noise interrupts what can be heard. This is all about auditory stream analysis, which is how the brain organises the sound picked up by the ear into meaningful objects. You can find other demonstrations of how the brain processes sound on Al Bregman’s website. And you can read about other ways beatboxers exploit auditory stream analysis in Now You’re Talking.
The Sound Blog 