A new criterion for speech frame classification based on Fuzzy Logic is presented. An algorithm which correctly discriminates between sounds allows a better trade-off between quality and bit rate in the recent generation of vocoders. Extracting from speech waveforms a set of significant parameters for voiced/unvoiced classification, we trained a Fuzzy Voicing Detector (FVD) system, obtaining a fuzzy knowledge base of only 5 rules. The new voicing system has certain advantages over the main Voicing Determination Algorithms (VDAs). With a notoriously low level of complexity, on account of its fuzzy nature, the FVD is extremely robust and maintains good performance even in the presence of background noise.