A very interesting phenomenon became very clear to me over the past few weeks, or rather I have discovered with clarity how to put into words what I have always aurally experienced relative to tension (i.e pressed voice, glottal squeeze) as distinctly different from singing “too heavily”, (as many teachers and singers often misrepresent). When we consider the vertical dimension of fold vibration as it relates to pitch, it becomes very clear that glottal squeeze, which is often mischaracterized as heavy singing actually occurs for the exact opposite reason, namely “singing too thin.” Let us consider the following picture from the website of the University of Stuttgart!
Imagine that the vertical plane where the folds meet (fold depth, fold thickness) represents the fold mass necessary for the pitch A4 (440 Hertz or 440 vibrations per second. The pitch is arbitrarily picked for easy mathematics)! This means that every 1/440 of a second a puff of air will travel through that specific thickness, which requires exactly 1/440 of a second to open and close thereby allowing one puff of air to come through. The repetition of this process normally occurs 440 times a second for the pitch A4, whereby 440 waves traveling at the speed of sound strike the eardrum 440 times within a second, which is translated by the brain as said pitch. In fact it is the desire of the singer to produce that pitch that sets up the mechanism to produce it.
Under normal circumstances, the appropriate thickness would be set up appropriately to produce an even oscillation whereby adequate pressure is build up and released depending on the desired level of volume. It can be assumed that fold oscillation would be nearly 50/50 ratio between the open and close phase.
However, not all circumstances are normal. The singer desires not only pitch but also timbre (sound quality) which may be or may not be in keeping with the singer’s natural vocal function. This is where the pertinent information comes in: Imagine that the singer desires to sing that same pitch A440 but has a timbre in mind that sets up the folds at a different thickness, a thinner one to be exact. By the explanation above, a thinner production would cause an immediate rise in pitch because it would take the folds less time to open and close when the vertical dimension is shallower. However a singer can increase the close phase by pressing the folds together, thereby slowing down the release of air for each cycle. This then causes a great increase in sub-glottal pressure that would build up with each cycle since less air is released than the air that pressurizes beneath the folds. This pressure causes the larynx to climb. The resultant sound is strident and brittle, lacking in warmth (low partials) since the natural depth of the larynx would be compromised.
I have found that the great majority of female singers that I hear is particularly prone to pressed voice in the middle range because of inadequate fold depth. This stems from a misunderstanding of the issues relating to the first passaggio between Eb4 to Gb4. There is an acoustic change in this area, which leads many ill-advised singers to change their mode of phonation, often attempting to access what they believe is head voice, when in fact they end up producing either a slightly breathy sound remedied by pressing the folds together or by a thinner production that must be pressed to maintain pitch level, as explained above.
The either/or approach to head voice and chest voice is the cause of this. A good broadway belter, compared to the average operatic soprano, is often more exciting to listen to in the middle range at a visceral level because she is usually applying proper fold depth to accomplish the sound. The result is a firm more present sound. The acoustic element (first formant dominance beyond the passaggio point resulting in a high larynx) is what distinguishes a healthy belt. In this case the high larynx is not caused by sub-glottal pressure but rather by the singer’s desire to sing a speech-like quality. The fold depth is appropriate and the pressure/flow balance also correct. In fact the successful classical singer needs the phonation balance of a healthy belter but with a different acoustic strategy that like a man’s passaggio at the same fundamental frequency levels (Eb4-Gb4) accesses second formant dominance by maintaining a low larynx, which prevents the lower formant to follow the rising pitch. The “and” approach requires what is often called “chest voice”, which in fact is appropriate fold depth. This is the speaky quality. What is associated with “head voice” is adequate pressure/flow controlled by how the the folds come together medially. Both elements are necessary: the grounded feeling produced by adequate fold depth and the floating sensation produced by balanced pressure/flow. One without the other produces an unbalanced sound.
In addition, it would seem logical that an appropriately low larynx is achieved when the correct fold depth has been accomplished. Since a longer vocal tract is associated with the conditions for inertial reactance of the vocal tract (which induces flow phonation), it would follow that adequate fold depth is of prime importance to vocal production as a whole. It will have an effect of the nature of breath management by creating the desired glottal obstruction. It will by definition correct phonation-related dysfunction and as it relates to inertial reactance will effect resonance in a positive way as well. It is my opinion that greater harm is done to the voice by singing a thin sound than by the opposite.