A unit of frequency interval describing a band of frequencies such that the highest frequency is 21/3 = 1.260 times the lowest. This unit is commonly used in noise control and abatement
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}