To determine what measures are appropriate, you must do what is called a frequency analysis. The signal to be measured is electronically separated into various frequency bands, for example octave bands, each of which covers a 2:1 range of frequencies. The analysis yields a series of levels, one for each band. These are referred to as "band levels", or for octave bands "octave-band levels". In order to make this meaningful the band in which a reading of level is obtained must be specified if the information is to be of value.

Octave Bands
The preferred series of octave bands for acoustic measurements divides the audible range into ten bands. The centre frequencies of these bands are 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 and 16000 Hz. The actual nominal frequency range of any one of these bands is 2:1. This means that the effective band for the 1000-Hz octave extends from 707 to 1414 Hz.

In a frequency analysis, a graph is made of the results of octave-band sound pressure level measurements. The frequency scale is commonly divided into equal intervals, between the position designated for each band and the position for the band adjacent to it in frequency. The pressure level in each band is normally presented as horizontal lines centered on the band at the measured level. Below is an example obtained with the Norsonic Nor-121 analyzer.

Example of how frequency analysis often is presented. The level vs. frequency (here in one-third-octave bands) is presented using a logarithmic frequency axis The illustration is a screenshot taken from the Nor-121 analyzer.

One-Third-Octave Bands
A more detailed analysis can be obtained by using bands more narrow than octaves. If the octave is split into three parts, each of these three parts are referred to as a one-third-octave band, or often just a third-octave band.

The choice of using one third of an octave is partly based on the fact that ten such filters can be arranged effectively to cover a 10:1 frequency range.
The preferred series of centre frequencies are multiples of 100, 125, 160, 200, 250, 315, 400, 500, 630 and 800 Hz. This means that the next 10:1 set of centre frequencies are 1000, 1250, 1600, 2000 etc. In other words the rest of the total series is obtained by multiplying the basic 100-800 series by 0.01, 0.1, 10, 100 etc. For practical reasons the centre frequency range used with most sound measuring instruments spans 20 Hz to 12.5k Hz (applies to type 1 instruments). This range is likely to expand upwards to 16k Hz in the future, but many commercially available instruments come with a 20Hz to 20kHz one-third-octave band capability as standard already.

The actual nominal frequency range for a one-third-octave band filter at 1000Hz extends from about 891 to 1122 Hz. This means that the bandwidth of the filter is about 23% of the centre frequency.

When one-third-octave filters are used in other applications than sound measurements they are often referred to as 23% relative bandwidth filters.

Example of how frequency analysis often is presented. The level vs. frequency (here in one-third-octave bands) is presented using a logarithmic frequency axis. The illustration is a screenshot taken from the Nor-121 analyzer.

Comparing Octaves and Third-Octaves
Suppose you make measurements in both octaves and third-octaves and want to compare the obtained results. The recommended way to do this is to combine the one-third octave values in groups of three to get the equivalent octaves. Be sure to pick the right three third-octave bands. For example, to find the equivalent 1000-Hz octave-band level, combine the third octave band levels of the 800, 1000 and 1250-Hz bands.

Reference: Norsonic.com