Preferred Frequencies – Octave and one third octave band center frequencies are the geometric means of their respective lower and upper limits as defined by ISO and ANSI standards. The upper and lower nominal octave band limits are arrived at by multiplying the center frequency by 1.4142 and 0.7071 respectively. One-third octave band centre frequencies are given by 10n/10 where n is the one third octave band number. For instance, for 125 Hz the band number is 21 and 102.1 = 125.89. Lower and upper frequency limits are a function of 2-1/6 or 0.89 and 21/6 or (1.12) respectively.
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