A robust vocalization, not a scream, but a well-supported yell on the word “Hey” assesses the ability of a patient to maintain or recruit additional closure with increased subglottic pressure.
The additional energy from increased pressure beneath the vocal cords can cause weak vocal cords to flutter. The task may allow stiff vocal cords to actually produce sound, when quiet sounds were almost impossible. Psychogenic problems often show up on this test when the patient hesitates or exaggerates performing this task, perhaps subconsciously worrying that the voice will create sound in an unexpected way where there previously was no sound. (In fact, watch a patient's face when you ask them to perform this task. If they delay performing the task, ask you, "Do you really want me to yell?" and smile at the same time, then you are witnessing la belle indifferance, an inappropriate response to a task.)
Vocal effort and vocal quality during high vocal intensity (volume) may be assessed at both low and high pitch. Notation is made whether the volume seems normal, reduced (typical of paralysis) or better than expected (typical of bowing and termed vocal recruitment). Notation is made of volume relative to the pitch. A loud sound that can only be produced at high pitch is suggestive of weakness such as a recurrent laryngeal nerve paresis. Notation is made of quality. A loud phonation that is clear at high pitch and causes flutter at low pitch is suggestive of an anterior branch, recurrent laryngeal nerve paresis. Notation is made of patient effort. A patient with a nonorganic voice issue will typically defer on this task or be surprised when their voice is suddenly normal or there may be facial signs such as la belle indifference.
Given the large dynamic range of the human voice, this task may saturate the digital signal and lead to clipping, an artifact on the recording. One technique for getting around this problem is to use two channel (often called stereo recording). My microphone has only one output, but I run the signal through a preamplifier and split it into two, setting amplification lower on one of the channels. If there is saturation and clipping of the signal as might be present where the red is at the top of the image above, then I can listen to only the lower amplification channel. The reverse works as well, lower signals are best heard on the more amplified channel.
In a female with normal vocal recruitment on increased subglottic presure, her voice could be diagrammed as follows.