Over the past week, the internet has been consumed by a raging debate about whether a sound clip is saying “laurel” or “yanny.” The clip comes from a vocabulary site page for the word “laurel.” Yet, about half of people hear “yanny.”

So, how can people listening to the exact same audio clip hear two completely different things? The answer lies in how our brains process sound and variations in the frequencies different people are able to hear.

Spoken language is sound waves propagated through the air at different frequencies, which our brains then interprets into words. “yanny” and “laurel” have a similar length and cadence. But, depending on the frequency, the same clip will sound more like one or the other. In higher frequencies, it sounds more like “yanny”, while in lower frequencies, sounds more like “laurel.” What you will hear depends on what frequencies your brain emphasizes and hears best.

The clip is a recording of the audio pronunciation of the word “laurel” from Vocabulary.com played through computer speakers. This has the effect of accentuating the higher frequencies making it possible for some to hear “yanny” rather than “laurel.”

This is because some, especially younger people, are able to hear higher frequencies better than others. As we get older, our ability to hear higher frequencies tends to diminish. So, older people will be less likely to hear “yanny” and more likely to hear “laurel.”

Another factor: our brains filter out frequencies to allow us to better focus on certain sounds. This is useful for things like hearing what someone is saying to us in a noisy room. If your brains is filtering out higher frequencies, you’ll be more likely to hear “laurel” rather than “yanny.”

We can simulate this by using audio processing software. When we shift the pitch lower, bringing the high frequencies down into the lower frequency range, the clip sounds more like “yanny.” When we shift the pitch higher, it sounds like “laurel.”

Isolating the frequencies makes easier to hear why this works. If we isolate the high frequencies, it’s possible to make out “yanny.” But, if we isolate the low frequencies, all we hear is “laurel.”

What you hear is purely a function of what frequencies your brain is processing and how it interprets them. It’s a debate that will never be settled. Some people will hear “laurel” and others will hear “yanny.” And a few will hear something else entirely — President Donald Trump says “I hear ‘covefe.'”