TITLE: Why don't we percieve chords like we perceive the mix of two light waves?
QUESTION [25 upvotes]: Why is the combination of two light waves (red, yellow) percieved as the same color as the arithmetic mean of their frequencies (orange) while we percieve two musical notes at the same time as just those two waves stacked on top, and not the mean of those frequencies?

REPLY [5 votes]: The answer to any human perception question is "its complicated."
We evolved for maximum fitness (for some fitness function).  The way we hear sound and the way we hear light are the most effective ways we have found to deal with the signals.
If we try to read into it further, however, its worth noting that its useful to be able to identify "individual" sources of sound.  You can do a lot once you have individual sounds isolated from each other.  Virtually all sounds we hear in the natural world have a harmonic character to them.  You never hear a 440Hz sine wave in the forest.  You always hear 440Hz mixed with others (such as 880Hz and 1760Hz), and the relative amplitudes of these frequencies are reasonably fixed because they're defined by physics.
There's little advantage in being able to hear each frequency and then to do some heavy duty processing deeper in the brain.  Instead what we have found it its most valuable to take sound and basically split it up into a fundamental frequency and a sound "color" which captures the associated overtone amplitudes.  Its this color which is why a trumpet sounds different than a flute, even if they play the same note.
Indeed, if you split up the signal in this way, you can do some remarkably processing.  Our ears "color" the sound based on direction, emphasizing different frequencies depending on which direction the sound approaches the ears.  If our brains can identify the "color" of the sound, we can actually back out this frequency-dependent response and figure out the direction of any given signal (even if there's a bunch of other sounds going on).
For a fun aside, there's a trick in the electronic music community where you leave off the fundamental on your low bass notes.  Leave it off completely, and only have the harmonics above.  Our brain will recognize the "color," and fill in the fundamental for you.  This is great because it can take a lot of subwoofer power to reproduce the fundamental, and quite a lot less power to reproduce the harmonics.  If the musician can rely on the human brain to fill it in, their music will sound better on weaker sound systems!
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