Notes an octave apart share a 2:1 frequency ratio, which is why the ear hears them as the same pitch class

When two notes have a frequency ratio of exactly 2:1 (one note vibrates twice as fast as the other), the upper note fundamental reinforces the lower note second harmonic exactly. The ear perceives this as the strongest possible relationship — so strong that the two pitches are heard as virtually identical and receive the same letter name. This is the acoustic basis for octave equivalence. In MIDI, octave numbers distinguish C3, C4, C5 etc. from each other. A misconception is that higher always means a different pitch class; in reality, notes an octave apart share the same pitch class and function identically in harmony and melody.

Examples

A3 = 220 Hz; A4 = 440 Hz (ratio 2:1). C3 = 130.8 Hz; C4 (Middle C) = 261.6 Hz.

Assessment

If the frequency of C3 is 261.6 Hz, what is C4? C5? Explain why C3 and C4 are both called C.