system
Given the following post, respond as if you're commenting on Reddit:
human
How can our brain distinguish the sound of a single note vs. a chord?

I wondered this when I was listening to piano chords on my computer. My headphones produced a singular sound, yet I knew it was a chord compared to when only a single note is played. How can we tell the difference if they are both just single sounds?
gpt
What will blow your mind more is when you realize that literally *everything you hear* is a "single sound" in the sense that there is only a single waveform being received by your ear. Think of listening to a song... the singer's voice is essentially a continuously modulated "chord" in the sense that it is comprised of a fundamental frequency (the note they are singing on) layered with "Formant frequencies" (which form vowels and which are influenced by consonants to create speech). Every instrument, even if it is only playing a single note, similarly has layers of resonant frequencies that are responsible for its timbre (how you can tell the difference between instruments playing the same note).

Our brains effortlessly group together those frequencies that belong to a single source (an instrument or voice) and separate those sources into different "auditory objects" in a process called "auditory scene segregation". It's a phenomenally difficult task and we don't really understand how it works in the brain; there is some evidence that the "phase" of the component sound waves contributes.

Our perception of sound is so fascinating and surprising.

Even something as fundamental as our musical scale is not as immutable as we believe... check out the work of "William Sethares". Basically, he demonstrated that our scale is an artifact of the kind of musical instruments we play. All of them vibrate in such a way to create predictably linear resonances that are essentially the notes in our scale. However it need not be that way: you can create scales with arbitrarily many consonant notes and create strange alien-sounding music from it. The thing is that an octave, which we would normally define as the doubling of frequency, turns out to be a perception. In Sethares' artificially constructed scales, you absolutely can perceive an octave, the same way you can recognize that "C" is the same note in different octaves, even though the scale is no longer based on that doubling of frequency.