1. Changes in Vocal Intensity
Two distinct mechanisms:
respiratory
laryngeal
Respiratory control of intensity. This is one straightforward and intuitive: The respiratory system supplies the aerodynamic power for speech production. An increase in lung pressure will produce an increase in the intensity of the voice. It’s pretty much that simple.

2. This mechanism can be demonstrated easily by giving your chest a little push while phonating. The loudness/intensity of the voice increases immediately.
As we saw earlier, vocal intensity is not the only thing that changes when lung pressure is increased. If you don’t recall how/why this works, take a look at the respiration.
Push on chest

3. Laryngeal control of intensity
Laryngeal control of intensity. This is not quite as obvious as the respiratory mechanism.
The basic idea is that the laryngeal muscles increase medial compression (squeezing force) on the vocal folds. This increases subglottal pressure (Psg) for two related reasons:
The resistance offered by the glottis increases – the way it does when you put your thumb over the open end of a hose. Psg has increase for the same reason that water pressure increases when you put your thumb over the open end of a hose.

4. The increase in medial compression means that the VFs will stayed closed longer.
Why? Remember that there is a tug of war between the muscular force (which keeps for VS at midline) and the respiratory force (which is acting to blow them away from midline.
An increase in medial compression keeps the VFs together longer.
But: When the VFs finally are forced away from midline, they do so with greater force. The result is an increase in vocal intensity.

5. One of the spectra below is high intensity, the other is low intensity (moderate now shown). Which is which? Why?

6. One of the lessons here is that high and low intensity glottal signals are not just scaled versions of one another; i.e., a high intensity glottal signal is not just a scaled-up version of a low intensity signal. The shapes of the pulses are different. We have seen this before.
This fact is relevant because the quality (timbre) of a sound is affected by its spectrum envelope.
Listeners can tell the difference between a voice that was originally spoken at high intensity and one that was spoken at low intensity – even when the intensities are matched.

7. You’ll learn the details in your voice course.
Differences in the shape of the spectrum envelope (which result from the different pulse shapes) in part convey these sound quality/timbre differences.
Last point. There are many neurological conditions that can make it difficult to produce the medial compression that’s needed to generate higher intensity voices, including:
Parkinson’s disease
ALS
Vocal cord paralysis
Myasthenia Gravis
Multiple Sclerosis
Strokes of various kinds
You’ll learn the details in your voice course.