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Fractional-octave analysis is a technique for analyzing audio and acoustic signals. Fractional-octave analyses, especially 1/3 and 1/12 octave, exhibit characteristics analogous to the response of the human ear. Many industry standards, such as ANSI and IEC, require fractional-octave analysis.

The following steps summarize fractional-octave measurement.

1. Send a time-domain signal through a bank of N bandpass filters.
2. Compute the instantaneous square value for the outputs of the N filters.
3. Average the instantaneous square values according to a selected averaging mode.
4. Display the averaged values in a bar graph.

The type of fractional-octave measurement you perform, 1/1 through 1/24 octave, determines the number of filters to apply, as well as the passband of those filters. The filters used are known as constant Q filters.

Full-Octave Analysis in the 31.5 Hz–16 kHz Band

The following example is an illustration of full-octave analysis. In the example, an input signal acquired by a microphone is sent through 10 bandpass filters centered at the frequencies listed in the following illustration.

The power in each band is computed and displayed in a bar graph with a log scale for the x-axis, as shown in the following front panel.