Harmonics
Resonant frequencies of a system that are integer multiples of its fundamental frequency.
Governing formula
f_n = n f1 (where n = 1, 2, 3, ...)
SI unit
Hz
In depth
The second harmonic (n=2) is twice the fundamental frequency (one octave higher); the third harmonic (n=3) is three times the fundamental frequency.
Examples in the real world
A trumpet producing different musical notes by using lip tension to select higher harmonics.