Page 62 - Fall2021

PHYSICS OF VOCAL HEALTH
changes in pitch are often accompanied by changes in voice quality. For example, a pitch glide is often accom- panied by changes in vocal registers. Vocal fry, produced often with increased vertical thickness and a long period of glottal closure, occurs at the lower end of the pitch range, whereas the voice at the high end of the pitch range is often in a falsetto register, produced with a reduced vertical thickness and a brief duration of glottal closure. The modal voice, which is used in conversational speech, is produced with an intermediate thickness of the vocal fold at the intermediate pitch range.
Vocal Fold Contact Pressure and Risk of Vocal Fold Injury
During voice production, the vocal folds experience repeated mechanical stress. In particular, the contact
pressure sustained by the vocal folds during repeated collision poses the greatest risk of tissue damage because this pressure acts perpendicular to the load-bearing col- lagen and elastin fibers within the vocal folds (Titze, 1994). For a loud voice such as screaming, the contact pressure can be as high as 20 kPa locally for extreme voicing conditions as reported in recent numerical simu- lations (Zhang, 2020).
Although the vocal folds evolved to withstand the repeated contact pressure during phonation, when the contact pressure exceeds a certain level (e.g., due to talking loudly or screaming) or is sustained over an extended period (e,g., due to excessive talking or sing- ing), it will cause injury to the vocal folds, triggering an initial inflammation response with fluid accumulation.
This often results in degraded voice quality and difficulty in producing or modulating the voice. The threshold contact pressure triggering the inflammation response appears to vary individually depending on the daily vocal load, overall health condition of the speaker, and, pos- sibly, the microstructural composition of the vocal fold tissues. If this hyperfunction behavior (loud voice for a prolonged period) persists, there may be permanent vocal fold lesions such as vocal fold nodules (Figure 2).
The magnitude of the peak contact pressure depends primarily on the subglottal pressure used to produce the voice and, to a lesser degree, the cover layer stiffness of the vocal folds (Zhang, 2020). Soft vocal folds subject to high subglottal pressure will vibrate with a large vibration amplitude and vocal fold speed at contact, and thus a high contact pressure is required to stop the vocal folds during collision. In general, thinner vocal folds (as, e.g., in a fal- setto register) tend to produce lower vocal fold contact pressure (Zhang, 2020). Although the effect of the glottal gap on the contact pressure is generally small, the contact pressure becomes excessively high when the vocal folds are tightly compressed against each other (hyperadduction).
Because the subglottal pressure has a dominant effect on bothvocalfoldcontactpressureandvocalintensity,therisk of vocal fold injury can be significantly reduced by lowering the vocal intensity or completely eliminated by vocal rest. However,vocalrestorreducedloudnessisoftennotsocially practical due to communication needs in everyday life. A more practical strategy is to adopt laryngeal and vocal tract adjustments to minimize the subglottal pressure required
Figure 2. A: vocal hyperfunction can lead to vocal fold nodules on the medial edge of the vocal folds (left), which prevents complete glottal closure during phonation (right). B: vocal fold nodules almost disappear post-voice therapy (left), which significantly improves glottal closure during phonation (right).
62 Acoustics Today • Fall 2021