An octave is the interval between two notes where one vibrates twice as fast as the other.

The longer version

When one vibration completes exactly twice as many cycles per second as another, the two blend so smoothly that listeners treat them as versions of the same note. Every second peak of the faster wave lands on a peak of the slower one, so the combined pattern repeats neatly.

That simple two-to-one ratio is the most consonant interval after unison, and it appears in nearly every musical tradition. Doubling continues: each further doubling moves another step up, which is why scales repeat their pattern in register after register. The ear probably favours it because the overtones of the lower note already contain the upper one.

Where this entry sits

Sound and Music gathers facts about how vibrations move through air and water, how ears turn them into meaning, and how instruments and notation shape what listeners hear. Expect echoes, pitch, volume, acoustics, and the odd silence in between. The full list sits on the Sound and Music page, where every entry is listed in order.