The science of phonology and the science of phonetics know various approaches which connect them together. In this work we examine the Standard Feature Theory and Element Theory with the focus on the voicing of fricatives. In the traditional view, voicing is defined with respect to articulation, however, Element Theory focuses on the acoustics. The latter has been the subject of our research. Our focus has been the lenght and periodicity of the sound as the acoustic reflection of voicing in slovenian. The contribution of each variable to the acoustic expression of the voicing on the fricatives has been measured in the empirical part of the thesis.