Third-Octave Band (1/3 Octave)
Definition
Third-Octave Band (1/3 Octave)
A 1/3 octave band is a frequency range spanning one-third of an octave, defined by IEC 61260, yielding 31 bands across the audible spectrum. With approximately 23% bandwidth relative to center frequency, 1/3 octave analysis provides three times the resolution of full-octave analysis and is the standard for environmental noise surveys, HVAC assessment, and sound insulation testing.
One-third octave bands offer a practical balance between the coarse resolution of full-octave analysis and the excessive detail of narrow-band FFT. The 31 standard center frequencies from 20 Hz to 20 kHz match the frequency discrimination ability of the human auditory system at moderate sound levels, making 1/3-octave analysis perceptually meaningful.
The bandwidth of each 1/3-octave band is approximately 23% of its center frequency. For example, the 1 kHz band spans from 891 Hz to 1122 Hz (a width of 231 Hz), while the 100 Hz band spans from 89.1 Hz to 112.2 Hz (a width of 23.1 Hz). This proportional bandwidth means that each band captures a roughly equal number of perceptual "critical bands," the frequency resolution units of the human cochlea.
Noise regulations and standards predominantly use 1/3 octave analysis. ISO 1996 (environmental noise) specifies 1/3 octave band measurements for tonal detection. ISO 16283 (sound insulation) requires 1/3 octave bands from 100 Hz to 3150 Hz. ASHRAE guidelines for HVAC noise use 1/3 octave data. NR and NC curves are defined at octave band centers but are often evaluated from 1/3 octave data.
In audio system measurement, 1/3 octave smoothing applied to FFT data approximates what a hardware 1/3-octave RTA would display. This is useful for evaluating broad spectral trends and matches the resolution of graphic equalizers (31-band GEQs have 1/3-octave center frequencies). However, for identifying narrow-band problems such as feedback frequencies or resonances, finer resolution is needed.
SonaVyx computes 1/3-octave band levels from the FFT power spectrum using energy summation, and displays them as a bar chart overlay alongside the fine-resolution spectrum.
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