OBJECTIVE The aim of the study was to clarify the mechanisms underlying the singing voice. METHODS Forty-eight professional opera singers underwent flexible and rigid endoscopy, spectrographic analysis and perceptual evaluation. RESULTS The data provided by voice analysis were not as clear and relevant to the aim of our study as those commonly obtained for speech evaluation. Laryngoscopy with rigid and flexible fiber optics and the singing power ratio (SPR) measurement provided more applicable data. Indeed, the former allowed us to assess laryngeal position, the glottic pattern and vocal tract modifications during the actual singing performance. The latter, already recommended by other authors as a reliable vocal emission index, also yielded more relevant information in comparison with the assessment of voice quality. Specifically, SPR provided data directly correlated to both the years of singing activity and the vocal category of each singer (the higher the singing pitch, the wider the vocal extension). More importantly, the data fully reflected the subjective assessment of each phoniatrician. CONCLUSIONS We suggest that the SPR indices can be used as the electroacoustic equivalent of the subjective judgment of vocal focus.