Frequency

Frequency, in sound, is the rate at which air particles vibrate. A higher frequency gives a higher pitch, and a lower frequency a lower pitch.

When several frequencies sound at once, they add together into a new waveform. How complex that waveform is determines the perceived consonance or dissonance of the sound. Because those qualities shape music's effect on us far more than any single pitch does, the relationships between frequencies matter more than the frequencies themselves. This is why relative pitch is more useful than absolute pitch, and why NoSheet trains relative pitch first.