One interesting trend I have noticed in some tenors I have coached is that while ascending above the passaggio intuitively brings a conscious choice to adjust phonation so as to change registers, sometimes coming down from the upper register into the lower does not always trigger a conscious adjusting into chest register. Some tenors come down from the upper register and continue the same adjustments that mark the upper register.
I will leave it to scientists to describe the differences between the two registers. I will just say that there is a difference between the two. When you descend from the upper register into chest, if you do not shift back into chest register properly you will actually have to push in order to maintain evenness in voice. It is just like going up from Chest to Head registers: if you don’t make an adjustment, you have to push in order to keep evenness. Similarly, when you come down from head, if you don’t make the adjustments to go into Chest, you will have to push.
I have noticed this because tenor have the tendency to learn the adjustments of a thinner phonation as they go up to high notes, but when they come down from the high note they experience a compression at the laryngeal level. The chords are too thin and too long I believe to sing at the lower pitches at that volume. While you can sing, it will require more pressure to do so. The answer to this problem is shifting back to chest.
The guide in this process is finding 1st formant dominance in the middle voice. This lower 3rd layer of the vowel sound produces an harmonic that is exactly an octave higher than the pitch you are singing. The zone between B3 and F4 should start having the cry phonation which allows you to modify the timbre appropriately.
Listen to Caruso in his middle voice and how the 2nd harmonic in his voice emerges (in this case corresponding to the harmonic being enhanced by the 1st formant, and also what I call the lower 3rd layer of the vowel below the passaggio).
This harmonic is enhanced by keeping the tongue out of the path of the breath, keeping the support engaged so that the larynx can stay in a passive low range. The voice pressure seems to expand, a sense of strange hollowness that is still very much linked to the squillo. The trick is to not let the width of this spacious feeling of this harmonic monopolize your phonation and cause you to let go of the adduction. Sometimes we feel the harmonic and we let it dominate our attention. You can’t let go of the anchors in the voice: the squillo and 1st layer.
So as you descend from the higher register, make sure you shift back to this type of expansion and not stay in the same types of adjustments that allow the top to emerge. You have to know how to go back and forth from one to the other.