For notes at the low end of their frequency range, most female opera singers at least occasionally use the chest register, characterized by the large closed quotient (CQ) evident in the electroglottograph (EGG) signal. The smoothing of the transition to the middle register, which basically uses a falsetto pattern of vocal-fold vibration, typically gets attention in training the female voice. The realization of this register transition is one of the points of interest in documenting the individual voice.
In this figure the EGG signals of two adjacent notes in an octave arpeggio beginning on A3 (220 Hz), sung by a mezzo-soprano, are compared. The A3 -- lower panel, with a CQ of 61% -- is clearly in chest register. The C4-sharp has a considerably smaller CQ (35%), but the EGG waveform retains something of the bulging characteristic of the chest register on its opening slope, suggesting what is often called a "chest mixture" in singers' parlance. The transition is in any case not abrupt perceptually.