Page 65 - Fall2021

relative positions between laryngeal cartilages, which often require compensation by increased activity of the intrinsic laryngeal muscles to maintain pitch or adduc- tory positions. This may change the relative balance between the intrinsic laryngeal muscles, resulting in increased laryngeal effort.
Involvement of the Respiratory System
Adaptive behavior to tighten the larynx may also result from laryngeal-respiratory compensation. The respira- tory system is responsible for providing and maintaining the subglottal pressure desired for speech production. In breathing at rest, the respiratory muscles are actively engaged during inspiration, whereas expiration often relies on a passive elastic recoil of the lungs and thorax, known as the relaxation pressure. The amount of relaxation pres- sure increases with the lung volume and is positive (i.e., pushes air out of the lungs) at a high lung volume and becomes negative (draws air into the lungs) at a very low lung volume. Speech production occurs during the expiration phase of breathing and takes advantage of the relaxation pressure in supplying and maintaining the desired subglottal pressure. By taking a breath to start speech at the appropriate lung volume, the desired sub- glottal pressure can be mostly supplied and maintained by the relaxation pressure for the entire breath group dura- tion, without much extra respiratory muscle effort. In this sense, speech is often considered “effortless.”
However, when starting speech at either too high or too low lung volumes, extra expiratory muscle effort would be required to either overcome or supplement the relaxation pressure. This additional muscle activation increases rap- idly as the lung volume approaches the lower or upper end of the lung capacity. In the extreme case of starting speech at a very low lung volume, in addition to this extra expi- ratory muscle activation required to maintain the desired subglottal pressure, the level of vocal fold adduction must also be increased to conserve airflow and prevent running out of air before completing an utterance. Thus, speakers who habitually start their speech at a low lung volume often produce a voice with hyperadducted vocal folds and pos- sibly adduction of supraglottal structures (Desjardins et al., 2021), leading to vocal fatigue and undesired voice changes.
A tight laryngeal configuration at a low lung volume may also result from a reduced tracheal pull effect. Tracheal
pull is a downward force exerted by the trachea and the respiratory system on the larynx. This force applies to the cricoid cartilage and tends to reduce the degree of vocal fold adduction. Tracheal pull increases as the diaphragm descends. That is, the tracheal pull is strong when speaking at a high lung volume and decreases as the lung volume decreases (Sundberg, 1993). Thus, when speaking at a very low lung volume, vocal fold adduction may increase naturally due to reduced tracheal pull.
Hydration and Environmental
Acoustic Support
Hydration is another important factor in maintaining vocal health. The vocal fold surface is lined by a mucous
layer that functions as lubrication to reduce the contact pressure during vocal fold collision. When the speaker is dehydrated, the mucus becomes thick and sticky instead of thin and watery, a condition that deteriorates the lubrication effect in reducing vocal fold contact pressure (Colton et al., 2011). Dehydration may also increase vocal fold stiffness and viscosity, thus increasing the lung pres- sure required to produce voice. Thus, maintaining good systemic hydration is essential to voice professionals who use their voice extensively in their daily life.
Voice production is mediated through auditory feedback and thus is subject to changes in the speaker’s acoustic environment. For example, with increasing background noise, we often increase vocal intensity to maintain the sufficient speech-to-noise ratio desired for communica- tion. The increase in vocal intensity is often accompanied by a boost of high-frequency harmonic energy with respect to lower-frequency harmonic energy, indicating increased vocal fold adduction.
Similar voice changes are also observed when speaking in rooms with different reverberation characteristics. Speakers produce voice with a higher vocal intensity in rooms with a shorter reverberation time compared with rooms with a longer reverberation time in which acoustic reflections of their own voice provide strong auditory feedback and acoustic support (Brunskog et al., 2009). Thus, speaking for an extended period in a noisy environ- ment or an acoustically “dead” environment with a very short reverberation time is likely to require an increased vocal effort and the speaker is prone to vocal fatigue and risk of vocal fold injury.
Fall 2021 • Acoustics Today 65