Clinical relevance of the fractal dimension of F-0 perturbations computed by the box-counting method

The box-counting method for determining the fractal dimension (D-fj) was applied to the fundamental frequency (F-0) perturbations in normal and pathological voices in order to assess its clinical value. The upper limit of these D-fj values was similar for both groups, but the distribution for pathological voices extended to lower values than for the normal voices, However, these lower values were most probably the result of one or a few outlying frequency points due to incorrect determination of the vocal period. The D-fj of normal voices were within the range of values found for randomly varying F-0 values. It was concluded, that the vocal perturbations in pathological voices are also more or less randomly distributed. So, the D-fj, at that least determined with the box-counting method, do not contain clinically relevant-information in addition to the traditional measures for the extent of the vocal period perturbations. An exception is special perturbation types like diplophonia. The result of the computation is very sensitive for voice breaks and vibrato and depends on the number of periods.