Hearing onset delays
Onset delays occur when air can leak out between the vocal cords more easily than it can entrain them:
- Air pressure may be too low to entrain them even in their normal phonatory, nearly parallel position.
- The gap between the vocal cords may be wider than usual for a given subglottic pressure.
- The vocal cords may be too stiff to entrain at a normal subglottic pressure.
- A swelling touches the opposite side and dampens easy vibration while allowing air to leak anterior and posterior to the swelling(s).
An individual attempts to make a sound at a given pitch. Initially air leaks and there is no sound, but then the individual changes the glottic configuration or the subglottic pressure to initiate entrainment. The vocal cords suddenly entrain and although delayed, begin oscillating, generating sound.
Case study
History
Tammy began to notice her voice giving out while rehearsing. She is active in two different choirs. She is very talkative and rates herself a 7 on a 7-point talkativeness scale.
Vocal Capabilities
Her voice is clear and very energetic on a reading task. Her maximum phonation time at her comfortable pitch is 20 seconds. She has a full, 3-octave vocal range from B2 to C6. For vocal tasks at a soft volume I have her sing “Happy Birthday to you” at several pitches. As I increase her starting pitch, when she attempts to sing the “to” on G4, her sound begins as air only, then harmonic sound production starts. She quickly adds, "When I warm up first, I don’t have this much trouble. She did not have a chance to warm up her voice before today’s examination. When she sings the same passage loudly, she does not have any audible delay. However, upon returning to soft singing, the onset delay returns.
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Her voice is clear at a typical female speaking pitch. Her comfortable speaking pitch is clear at pitch G3, in the range of a typical female’s speaking pitch. As she sang Happy Birthday’s first line softly at higher pitches, some air began to leak. At G4 no sound was momentarily produced. She performed the task on the same notes loudly and there was no impairment. When she returned to singing softly, her voice initially pauses on the same pitch again.Laryngoscopy
To visualize the finding on endoscopy, the patient can glide upward at soft volume and there will be a phonatory stop at approximately the same pitch. This type of impairment that occurs on a fairly repeatable pitch is usually from a swelling and the onset delay represents the tension where the vocal cords are just tight enough for vocal cord swellings to touch and lightly dampen the tendency to oscillate. The reason for the delay, rather than complete aphonia is that the individual usually rapidly adjusts something when no sound is produced as expected. Either slightly more air is passed through the cords, blowing the swellings apart or the vocal processes are opened slightly, pulling the swellings away from each other, allowing the vocal cords to entrain.
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Summary
We can diagram onset delays on a pitch vs. volume plot. We can correlate this with a visual finding of something touching the vibrating vocal cord and dampening the vibrations at a specific pitch during stroboscopy. Since it typically occurs at a soft, high pitch, the impairment will usually be in the right lower area of the plot.
The vocal capability test of soft singing allows the examiner to hear this specific vocal impairment of onset delay and specify the pitch where it is occuring. The onset delay often represents something touching a vocal cord at that pitch and suddenly stopping vibration. The lower the pitch impaired, the larger the swelling. The more precise and repeatable the onset delay, the more likely that it is caused by a swelling.
An onset delay also occurs when the vocal cords are apart, even when there is no swelling causing this configuration. The gap may be from muscle tension or even paresis where one vocal cord cannot reach the other. At low subglottic pressure, air flows between the membranous vocal cords without initially entraining them. A central or posterior glottic gap will be much more variable over the pitch range where an onset delay is audible when compared to a swelling touching and dampening the vocal cords.
Warming up is neither the issue nor does it interefere with the exam. Warming up allows the individual to either deliver a note with more subglottic air pressure (more volume) or with the vocal cords apart (and more airflow, so more breathy). Warming up just prepares for a compensation that has a trade-off. During the vocal capabilities exam, the examiner is trying to remove any compensation in order to establish the actual capacity of the vocal cords under the various permutations of pitch and volume.