Fractional octave and fractional decade frequency bands in acoustics: Historical review and recommendations

Frequency bands are used in acoustical analysis to group sound energy into meaningful bands that simplify frequency-dependent behavior or characterize the spectrum in a manner relevant to the perception of sound. Early frequency bands were typically based on the octave (oct) and submultiples, such as one-third of an octave (1/3 oct), and such base 2 bands were first standardized in the 1950s. Ten 1/3 oct bands span a frequency range close to 1 decade (dec), which made it convenient to divide each decade into ten equal parts, resulting in the specification of 1/10 dec (decidecade) bands by modern standards, replacing the previous 1/3 oct bands. Because 1/10 dec is approximately equal to 1/3 oct, these decidecade bands are sometimes referred to as “one-third octave” bands, even in international standards, leading to ambiguity. The historical evolution of frequency bands in acoustics is reviewed and it is proposed to distinguish clearly between 1 oct and 3/10 dec (or equivalently between one-third of an octave and one-tenth of a decade).