"Passaggio" is an Italian term used in classical singing to describe the transition between a singer's lower and upper register. Some singers and voice coaches characterize the lower register as the "chest voice" and the upper register as the "head voice." The passaggio is the series of notes that fall between the two ranges, and without training, the singer's voice might break, he or she might have difficulty holding notes or might be unable to form certain vowel sounds.
The lower register is where vocal cords are short and thick, and the resonance of a singer's voice is felt through the chest, which is why it is known as the "chest voice." This is a powerful range and is used when speaking normally. The upper register is known as the "head voice" because in this series of notes, the vocal cords are thinner and stretched, and the sound resonates in the cheekbones and teeth. Different sets of muscles control the vocal cords in each register.
Each singer will find their passaggio in a different range of notes. Both male and female singers encounter the same type of difficulty singing through the transition, but at different points on the scale, depending on gender and singing range. In general, the passaggio is found between B-flat and the F-sharp above, and it extends three to seven semitones. Some singers find that they have two areas of transition; the lower is called the primo passaggio, and the upper is called the secondo passaggio.
When singing in his or her passaggio, the vocalist will encounter a change in note tone and quality. There might be a sudden shift in vocal registration, from head to chest or vice versa. He or she might drop a note or have trouble enunciating. Some professional singers find it difficult to sing for any length of time in the transition area, which affect the songs that they can perform or the roles that they can play.
With training, it is possible for a singer to move smoothly through the entire passaggio range without a loss of tone quality, volume or clarity. Techniques include learning to control throat, jaw and breath placement. Practicing gliding up and down the registers allows a vocalist to spot the breaks and smooth over the difficult notes. Familiarity with the problem allows for more relaxed singing, which avoids the muscle tension that contributes to the difficulty in bridging the passaggio.
Another training method aims to strengthen the weaker register. Some singers naturally fall into their upper or lower registers, and the muscles that control the vocal cords in the other register are relatively undeveloped. Singing exercises strengthen the lesser-used muscles, so the upper and lower registers are more equal in strength, allowing the vocalist to successfully blend the sound between upper and lower in the passaggio range.