one-third octave

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(n.) Frequencies spaced one-third of an octave apart. One-third of an octave represents a frequency 126 percent above a reference point, or 79.4 percent below the same point. A frequency 1/3-octave above a 1 kHz reference equals 1.26 kHz, which is usually rounded-off to 1.25 kHz for equalizers. A frequency 1/3-octave below 1 kHz equals 794 Hz, rounded off to 800 Hz. The term is most often used to express the bandwidth of equalizers and filters that are 1/3-octave wide at their -3 dB (half-power) points. This bandwidth is about the smallest region in which human hearing is able to reliably detect change.