One of the strangest viral music phenoms of the year is the Quebecois duo Angine de Poitrine. In February a 27-minute-long YouTube video of the pair exploded. In it, they wore outfits covered in black-and-white polka dots and strange masks that reflected the conceit they are aliens. Playing music that sounds like nothing else in Western pop music, the duo has, at press time, racked up more than 15 million views on that video alone. Even Google took notice, giving the group their very own tribute in search. Physics and neurobiology can help explain why a band whose music uses notes that no one else does—literally—went so viral.

Let’s start with the physics. At its core, a musical note is a repeating vibration, says Mark van Raamsdonk, a physics professor at the University of British Columbia and an amateur jazz musician.

“A piano string or whatever is oscillating at that frequency, and then that makes the air oscillate at that frequency, and then that makes your eardrum oscillate at that frequency, and then your ear converts that into a signal to your brain,” he says.

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In Western music, the vast majority of sounds are made up of 12 notes. Imagine a piano keyboard: the white keys ascend from A to G and back to A, with the black keys representing a few sharps and flats along the way. The distance from a lower A to a higher A is one octave—the higher A vibrates exactly twice as fast as the lower A.

The concept of musical intervals based on frequency dates back more than 2,500 years, and music historians often attribute it to either the Greek philosopher Pythagoras or the ancient Mesopotamians. Either way, what ancient musicians realized is that changing the length of a string also changed its vibration when plucked, which in turn changed the musical pitch. The relationship between notes depends on the ratio of their vibrations to each other.

Western music also relies heavily on harmonic overtones—when you play a note on an instrument, you hear a fundamental frequency but also higher frequencies that are quieter than that “main” note. They are what gives an instrument what might be called its character: a violin produces different harmonic overtones than an acoustic guitar, despite both being stringed instruments. They are also what makes a plucked guitar string sound nice, says Joseph Pechkis, an associate professor of physics at California Polytechnic State University.

How the harmonic overtones of different notes line up with each other can define whether they’re pleasing or not. A musical interval known as a perfect fifth—five notes away from the root on the major scale—vibrates exactly 1.5 times faster than the root. In this case, the two notes’ harmonic overtones overlap, and most people like listening to the sound as a result.

Angine de Poitrine don’t abide by these rules.

The unique sound of Angine de Poitrine

Instead the duo use a distinctive guitar built by one of the duo, known as Khn. They added extra frets, which allow them to effectively play notes between notes. This isn’t dissimilar to Indian classical music, where notes are divided by 22 instead of 12. But almost all of Western pop, rock, hip-hop and jazz is based off the system of 12 notes and their specific overtones—as a result, our brains have become attuned to them, and anything outside that system can just sound weird. And that’s especially true for people who grow up with that 12-note foundation in their music.

“Starting in infancy, in fact, in the first few weeks of life, the infant brain, even though it's not fully developed, starts to pick up on regularities. That could be the speech patterns that it hears; it could be the music that it hears,” says Robert Zatorre, a cognitive neuroscientist at McGill University and author of From Perception to Pleasure: The Neuroscience of Music and Why We Love It. “Their little brain is already forming expectations.”

The interplay of expectation and surprise plays a key role in how the human brain processes music. A 2019 study by Zatorre, his then Ph.D. student Benjamin Gold and their colleagues found that the brain’s enjoyment centers were most excited when an expected musical phrase suddenly shifted. Other research has repeatedly come to the same conclusion—we like a little unpredictability in music.

“If it’s too predictable, then it’s boring, and your brain shuts off,” Zatorre says. “If it’s too unpredictable, if you hear complete randomness, your brain also shuts off because there’s nothing to follow. What you need is a sweet spot where you have some level of complexity but also some surprise.”

Setting up and dashing expectations may help explain Angine de Poitrine’s virality. Their music is different, but it is not a mess of competing frequencies: riffs and musical phrases repeat but over different rhythms. There are strange intervals between vibrations that might sound almost entirely alien to anyone raised on Taylor Swift, the Rolling Stones or Jay-Z. Listeners’ brains can pick up a pattern, only to get a pleasant little shiver when the pattern changes just enough or an oddball note gets tossed in.

“That’s sort of the point, that the unusual elements of it are kind of balanced out by some actually kind of traditional elements, like the repetition of certain phrases,” Zatorre says.