One-third octave
A one-third octave is a logarithmic unit of frequency ratio equal to either one third of an octave or one tenth of a decade. An alternative term for one tenth of a decade is a decidecade.
Definitions
Base 2
ISO 18405:2017 defines a "one-third octave" as one third of an octave, corresponding to a frequency ratio of.A one-third octave is precisely 400 cents.
Base 10
IEC 61260-1:2014 and ANSI S1.6-2016 define a "one-third octave" as one tenth of a decade, corresponding to a frequency ratio of. This unit is referred to by ISO 18405 as a "decidecade" or "one-third octave ".One decidecade is equal to 100 savarts.