In the upper range of the male operatic singing voice, typically ca. 300-400 Hz, there is a widely recognized point at which production changes from chest register to what can be called robust head register, or voce piena in testa. At the top of the chest register in an open (high-F1) vowel the dominant resonance is typically that of the first formant (F1) resonating the second harmonic. At the next ascending scale step, the second harmonic is beyond the effective range of F1. A classic compensation for this loss of resonance is for the second formant to resonate the third or fourth harmonic, although this is by no means always what happens, even with quite skilled singers. The fundamental frequency at which passaggio occurs is a function of both the physical build of the vocal organs and the production employed (since a lower larynx will result in a lower F1 and earlier passaggio).
The protocol used in collecting this data calls for ascending scales through passaggio on open back and front vowels ([a] and [E]). One of the points of interest is to see just how, and at what point, the various singers cope with this maneuver. In the figure we see a tenor subject singing an octave scale beginning at A3-flat (208 Hz). The power spectra in the upper and lower panels, respectively, are taken from the F4 and G4, and they show how the dominant resonance shifts abruptly between these two notes from F1 on the second harmonic to F2 on the third harmonic. Further signs of passaggio are the slight, temporary dip in the SPL visible in the audio envelope, and a perceptual "darkening" of the sound that suggests why singers often label this effect "cover."