I like mygolden’s explanation. Certainly we recognize the overtone series in the way he suggested. Notes at each octave will have the same overtone series, so they sound the same except for having a fundamental that is different. If you push down the damper pedal on a piano, and hit a key, each key in the octaves above will resonate most strongly, though the thirds and fifths will also vibrate in sympathy.
On any instrument, when you cut the length of the instrument in half, and play it, you will find the pitch goes up an octave. Place a finger in the middle of a guitar string, and you hear the octave sounding (this is because you are damping the vibration of the entire string, yet allowing the two halves to vibrate freely, but at an octave up. You can also get fifths, thirds and double octaves to sound by putting your finger in the right place.
On any brass instrument, you can play the overtone series by using your lips to emphasize certain overtones. On the trumpet you get the first octave above the fundamental (I’m not sure why), then a fifth up, a fourth above that, a third above that, and so on.
However, even if there are no overtones; even if a synthesizer plays a pure pitch, we recognize the octaves easily. I’m sure it has something to do with the way these frequencies stimulate the cochlea. Octaves must all stimulate the same cochlea, so we perceive them as the same note, except we also recognize they are at a higher or lower pitch. In any case, it’s easy to hear which tones match other tones in the harmonic progression.
Or is it? Is there such a thing as being tone deaf? Many people will tell you they can’t carry a tune. Does this mean they are innately unable to identify separate pitches?
Well, no. The ability to distinguish pitches, and even perfect pitch are learned, just as language is learned. If you aren’t trained, you won’t learn it; and if you are, you will attain perfect pitch. People with tonal languages, such as Chinese, have a much higher proportion of those with perfect pitch, because pitch is necessary to understanding the language. My son, when he first started piano, knew the notes perfectly, but since we haven’t worked on it for a while, he may not have it any more.
Sndfreq started learning music as a youngster (but how young?) and his teacher identified him as having perfect pitch, so they probably worked on it, practicing formally or informally, and that’s what made him able to retain his memory of pitches. I’ll bet there was a lot of music in his house even before he started. Maybe his parents were musicians, too. I could be wrong in all these predictions, but it would be consistent with the history if I’m right.
Of course, the really interesting thing about music is what it does to your brain, and how it can alter your consciousness. Now there’s a question!