How do we detect register changes?

The duration and extent of vocal fold closure has a pronounced effect on vocal timbre. Any sudden large shift in timbre will tend to be perceived as a register change, commonly called a ‘break’.

Why do register changes occur, and why do they tend to occur in the same two pitch ranges for most singers?
This is a difficult question to answer definitively. Ideally, in order to study this question, one would want to have high-speed video of the vocal folds from various angles while they were vibrating, and data about how all the various muscles are being used. But with the whole singing apparatus tucked away inside the body, this data is hard to gather. Nevertheless, there are two major theories on the physical causes of register changes, and they appear to be supported by the experimental data available at this time:

1. Subglottal Resonance: We know from prior tutorials that when the vocal folds vibrate, they create sound waves that radiate upward into the vocal tract and then out of the body into the surrounding environment. The folds also send, or radiate, sound waves downward, into the air in the trachea, below the glottis. These sound waves are trapped; they cannot readily escape the body like those sent upward. Thus, they are forced to reflect around inside the body until they dissipate. Some of these subglottal waves bounce back up and impact in varying strengths on the underside of the vocal folds, and this can interfere with the vibration of the vocal folds to varying degrees, depending on the strength, but more importantly, the timing of the subglottal waves. The major variable affecting the strength of these subglottal waves is resonance; like all tubes (including the vocal tract itself), the trachea has a natural resonance frequency, at which it can reinforce the strength of vibrations within itself. Research published since 1988 indicates that most human tracheas tend to reinforce subglottal waves best in the pitch ranges from D₄to F#₄, and D₅ to F#₅, which just happen to be the same pitch ranges in which singers tend to encounter register shifts.
2. Maximum stress on the thyroarytenoid muscle: The TA muscle is very active in chest register; a large part of it is in vibration, and it is also contracting to varying degrees to accomplish pitch control. As a singer goes higher and higher within the chest register, the stress on the TA muscle increases, and it is theorized that when this stress reaches the maximum amount that the muscle can sustain, the voice is forced to break into another type of vibration, causing a sudden change in timbre, which is in turn perceived as a register break.

How do singers ‘control’ or ‘smooth out’ register changes?
For opera/classical singers, the goal of voice training is to eliminate any obvious register breaks, and achieve the illusion of having only one seamless register, from the bottom of the range to the top. This is only an illusion; the voice quality still changes, but it changes slowly, in small even increments from pitch to pitch, and so no single transition sticks out as a register ‘break’ to the listener.

These singers learn to do this by gradually releasing some muscles, while gradually activating (contracting) other muscles. For instance, in the transition from the chest to the head register, the TA muscle is relaxed incrementally while the CT is being used more and more. In this way, any sudden register break is averted; since the muscles are continuously adjusted to ‘even out’ any excessive stresses before they can cause a break.