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Octaves, register and frequency
Why do two C's sound 'the same' yet different? Meet the octave, labels like C4, and the magic number 440 Hz.
What an octave is
An octave is the distance between a tone and the next tone of the same name - for example from one C to the next C. These two tones sound strikingly similar because the higher one vibrates exactly twice as fast as the lower one.
Within one octave there are twelve semitones - all twelve tones of the chromatic scale we met earlier.
Frequency: how we measure pitch
Physically, pitch is measured in hertz (Hz) - the number of vibrations per second. The agreed reference is the A above middle C, vibrating at 440 Hz. An octave higher is 880 Hz, an octave lower is 220 Hz - always double or half.
Numbering the octaves
So we know which C we mean, octaves are numbered. The most famous is middle C, written C4 - it sits roughly in the middle of the piano. Tones above it are C5, C6 ...; below it C3, C2 ...
A standard guitar sounds one octave lower than written - which is what keeps its notation friendly to read.
You now know three foundations: tone names, semitones and octaves. In the next article we leave pitch behind and turn to time - rhythm and note durations.
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