The crescendo and diminuendo of a tone should not be attempted until the sotto voce comes easily. The diminishing of a tone is something that is acquired gradually. It should be cultivated thruout all the exercises in the degree that the tone should be allowed to finish easily and naturally. The student will very soon appreciate this artistic idea.
In diminishing a forte tone into the mezza voce by decreasing the intensity so that the farthest listener can just hear clearly the full, round sound, the tone, if still further dimingished, must begin to pass into the sotto voce. The intensity of tone and the fullness of tone are two different things. The life, or amplitude, of a sound wave depends on the intensity with which the sound is created. The fullness of a sound wave depends on the form. The intensity gives the sound wave power to reach the listener. The greater or less fullness of a wave form gives the wave a bigger or finer structure. A very fine spinning tone on the lips may have the same intensity that is given to a full, round sound.
These two, then, have the same intensity but differ in fullness. The fine, thin sound, however, altho it has the same intensity as a mezza voce tone, is not accepted by the ear as a mezza voce tone. For a mezza voce tone is a tone in its full, round form with an intensity just great enough to make it heard clearly by the farthest listener. In the mezza voce we have the least loud sound of the full, round tone—in the diminishing of the sotto voce we are constantly approaching closer and closer to the element of tone.
When the intensity is lessened more and more after the mezza voce tone passes into the sotto voce, it will have power only to travel a less and less distance, so that the listeners farthest away will soon no longer hear the sound. - Therefore, in order to have a diminishing tone heard by the farthest listener, the intensity of the tone must be kept sufficiently strong to create a wave amplitude that will carry the sound vibration of the full, rounded tone of the mezza voce to the most distant part of the resonant space. By bringing the lips closer together, however, the artist can make his tones less full, and, hence, diminish them without diminishing the intensity, or the wave amplitude. The degree of loudness, i. e., intensity, must always be in proportion to the resonant space. By bringing the lips gradually closer together, the tone can be diminished to the very last thread of its fineness. If the lips are kept open as they are for a full, round tone, the sound, after it has passed from the mezza voce into the sotto voce, with the lips still in the same open position, will no longer be audible to the listeners farther away, and finally will be lost even to those nearest the singer. The singer, when he keeps his lips open, diminishes only the intensity and not the fullness of the tone. To diminish the intensity of a forte tone into a mezza voce is well and good as long as the tone in the mezza voce can be heard by the farthest listener—but when the artist passes from the mezza voce into the sotto voce, the intensity may no longer be diminished; for if the intensity, or wave amplitude, is diminished, the tone will die out before it reaches the most distant listener. A spinning tone is a very fine tone and is made with the lips close together. In order to diminish a mezza voce tone gradually into the finest of spinning tones, the lips must be brought dexterously closer and closer—this lessens the fulness of the tone more and more until it finally reaches the last thread of its sound.
The crescendo and diminuendo should be practiced on the tones which can be created most easily on the lips. Not until the crescendo and diminuendo can be accomplished on these notes should the student try to do the same on the higher and lower ones, else he will be apt to misplace them. A crescendo or diminuendo, or both, should be used to a greater or less degree on the same tone and surely in successive tones in the same rhythmic beat. Especially in dramatic work should this crescendo and diminuendo be felt very strongly. This must be left to the development of the artistic comprehension of the student. Crescendo and diminuendo must not be confused with vibrato. Even tho tones are expressed very dramatically with close vibrato it does not say that there may be no crescendo and diminuendo. The vibrato belongs to the body and form of the wave. The vibrato is the constant increase and decrease of amplitude or intensity in one wave — the crescendo and diminuendo is an increase and decrease in amplitude or intensity of successive waves. The crescendo and diminuendo in its greatest form passes from the finest tone to the roundest, fullest sound and back again into the fine, thread-like tone.
In the stolid work, where the notes are held a long time and are given with great intensity, the beginner will find that he can vocalize every particle of breath and increase he duration of his tone by bringing the lips closer together as his breath gives out. Finally, of course, there is never any want of breath, for a little breath, if vocalized, makes a tone of very long duration. It is not in reality a question of breath capacity, but it is a question of vocalizing breath. There is really no control of breath, for if the syllables are produced on the lips, the breath cannot help but be vocalized. The composer must leave to the imagination of the artist the use of the crescendo and diminuendo in the interpretation of the vocal setting. In order to properly express the emotions the singer must increase and diminish his tones, for in this he will find one of the greatest aids to interpretation and dramatic effect.
Preetorious, Carl. The Tone Placed and Developed (1907): 77-81. Student of Mrs. P. J. Brown, herself a student of Vincenzo Cirillo.