The Stevens method of loudness summation requires a knowledge of the SPL of the sound source over 8 octave bands, or alternatively in the 16 half-octave
or 24 third-octave bands. Using the graph below or a chart, the loudness index for each band is found. The total
loudness of the source in sones *S** _{t}* is given by:

S_{t} = S_{m} + F (S - S_{m})

where *S** _{m}* is the maximum loudness index,

An analogous procedure is used to calculate the total noisiness for the Perceived Noise Level by replacing the loudness index with that of the noy and using the Kryter curves. See: Perceived Noise Level.

The Stevens method applies when the spectrum of the sound is fairly regular, with no pure tone components, and the measurement is made under diffuse sound field
conditions, i.e. when the acoustic energy is evenly distributed throughout the space. For other types of spectra,
the Zwicker method may be used (Reference: *Acoustic Noise Measurements,* Brüel and Kjaer, 1969, pp. A6 - A15).

Contours of equal loudness index.