A one-third octave is a logarithmic unit of frequency ratio equal to either one third of an octave (1200/3 = 400 cents: major third) or one tenth of a decade (3986.31/10 = 398.631 cents: M3 Play (help·info)). An alternative (unambiguous) term for one tenth of a decade is a decidecade.
One octave is a factor of 2, so decades per octave, while a third would be .
ISO 18405:2017 defines a "one-third octave" (or "one-third octave (base 2)") as one third of an octave, corresponding to a frequency ratio of . A one-third octave (base 2) is precisely 400 cents.
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 (base 10)".
One decidecade is equal to 100 savarts (398.63 cents).
- Malcolm J. Crocker, Handbook of Acoustics (1997)
- von Benda-Beckmann, A. M., Aarts, G., Sertlek, H. Ö., Lucke, K., Verboom, W. C., Kastelein, R. A., ... & Ainslie, M. A. (2015). Assessing the impact of underwater clearance of unexploded ordnance on harbour porpoises (Phocoena phocoena) in the Southern North Sea. Aquatic Mammals, 41(4), 503.
- ISO 18405 Underwater Acoustics - Terminology (International Organization for Standardization, Geneva, 2017)