Effects of length and depth of vibration of the vocal folds on the relationship between transglottal pressure and fundamental frequency of phonation in canine larynges

Numerical Analysis of Effects of Transglottal Pressure Change on Fundamental Frequency of Phonation

Objectives:

In humans, a decrease in transglottal pressure ( Pt) causes an increase in the fundamental frequency of phonation ( F0) only at a specific voice pitch within the modal register, the mechanism of which remains unclear.

Methods:

In the present study, numerical analyses were performed to investigate the mechanism of the voice pitch-dependent positive change of F0 due to Pt decrease. The airflow and the airway, including the vocal folds, were modeled in terms of mechanics of fluid and structure.

Results:

Simulations of phonation using the numerical model indicated that Pt affects both the average position and the average amplitude magnitude of vocal fold self-excited oscillation in a non-monotonous manner. This effect results in voice pitch-dependent responses of F0 to Ft decreases, including the positive response of F0 as actually observed in humans.

Conclusions:

The findings of the present study highlight the importance of considering self-excited oscillation of the vocal folds in elucidation of the phonation mechanism.

Experiments on Analysing Voice Production: Excised (Human, Animal) and In Vivo (Animal) Approaches

Experiments on human and on animal excised specimens as well as in vivo animal preparations are so far the most realistic approaches to simulate the in vivo process of human phonation. These experiments do not have the disadvantage of limited space within the neck and enable studies of the actual organ necessary for phonation, i.e., the larynx. The studies additionally allow the analysis of flow, vocal fold dynamics, and resulting acoustics in relation to well-defined laryngeal alterations.

PURPOSE OF REVIEW

This paper provides an overview of the applications and usefulness of excised (human/animal) specimen and in vivo animal experiments in voice research. These experiments have enabled visualization and analysis of dehydration effects, vocal fold scarring, bifurcation and chaotic vibrations, three-dimensional vibrations, aerodynamic effects, and mucosal wave propagation along the medial surface. Quantitative data will be shown to give an overview of measured laryngeal parameter values. As yet, a full understanding of all existing interactions in voice production has not been achieved, and thus, where possible, we try to indicate areas needing further study.

RECENT FINDINGS

A further motivation behind this review is to highlight recent findings and technologies related to the study of vocal fold dynamics and its applications. For example, studies of interactions between vocal tract airflow and generation of acoustics have recently shown that airflow superior to the glottis is governed by not only vocal fold dynamics but also by subglottal and supraglottal structures. In addition, promising new methods to investigate kinematics and dynamics have been reported recently, including dynamic optical coherence tomography, X-ray stroboscopy and three-dimensional reconstruction with laser projection systems. Finally, we touch on the relevance of vocal fold dynamics to clinical laryngology and to clinically-oriented research.

Cooperative regulation of vocal fold morphology and stress by the cricothyroid and thyroarytenoid muscles

Voice is produced by vibrations of vocal folds that consist of multiple layers. The portion of the vocal fold tissue that vibrates varies depending primarily on laryngeal muscle activity. The effective depth of tissue vibration should significantly influence the vibrational behavior of the tissue and resulting voice quality. However, thus far, the effect of the activation of individual muscles on the effective depth is not well understood. In this study, a three-dimensional finite element analysis is performed to investigate the effect of the activation of two major laryngeal muscles, the cricothyroid (CT) and thyroarytenoid (TA) muscles, on vocal fold morphology and stress distribution in the tissue. Because structures that bear less stress can easily be deformed and involved in vibration, information on the morphology and stress distribution may provide a useful estimate of the effective depth. The results of the analyses indicate that the two muscles perform distinct roles, which allow cooperative control of the morphology and stress. When the CT muscle is activated, the tip region of the vocal folds becomes thinner and curves upward, resulting in the elevation of the stress magnitude all over the tissue to a certain degree that depends on the stiffness of each layer. On the other hand, the TA muscle acts to suppress the morphological change and controls the stress magnitude in a position-dependent manner. Thus, the present analyses demonstrate quantitative relationships between the two muscles in their cooperative regulation of vocal fold morphology and stress.

Computer-aided technique for automatic determination of the relationship between transglottal pressure change and voice fundamental frequency

OBJECTIVES

The effect of artificially altered transglottal pressures on the voice fundamental frequency (F0) is known to be associated with vocal fold stiffness. Its measurement, though useful as a potential diagnostic tool for noncontact assessment of vocal fold stiffness, often requires manual and painstaking determination of an unstable F0 of voice. Here, we provide a computer-aided technique that enables one to carry out the determination easily and accurately.

METHODS

Human subjects vocalized in accordance with a series of reference sounds from a speaker controlled by a computer. Transglottal pressures were altered by means of a valve embedded in a mouthpiece. Time-varying vocal F0 was extracted, without manual procedures, from a specific range of the voice spectrum determined on the basis of the controlled reference sounds.

RESULTS

The validity of the proposed technique was assessed for 11 healthy subjects. Fluctuating voice F0 was tracked automatically during experiments, providing the relationship between transglottal pressure change and F0 on the computer.

CONCLUSIONS

The proposed technique overcomes the difficulty in automatic determination of the voice F0, which tends to be transient both in normal voice and in some types of pathological voice.

Magnetoelastic sensors as a new tool for laryngeal research

CONCLUSIONS

The use of custom-built magnetoelastic sensors designed for insertion in the larynx to measure the force of transverse deformation of the thyroarytenoid and cricothyroid muscles was examined.

OBJECTIVES

The availability of sensors that transduce deformation force into an electrical current (measurable in millivolts) suggested that such sensors could be used to study muscle function in the canine larynx. This study aimed to obtain information about the force exercised by larynx muscles on the sensor in an experiment in vivo.

MATERIALS AND METHODS

The results of six surgical interventions in three dogs using four magnetoelastic sensors are described. The sensor and insertion technique were atraumatic. One dog underwent surgery four times to study the morbidity, reliability and reproducibility of the technique. An electromyographic recording was made at the same time that sensor response was registered to compare results with a standardized technique.

RESULTS

We obtained 584 regular responses: 256 deglutitions (150 thyroarytenoid and 106 cricothyroid recordings) and 328 phonations (223 thyroarytenoid and 105 cricothyroid recordings). No signs of injury or functional deficit were observed after any intervention. In the dog that underwent four interventions, results were consistent after each intervention.

On pressure-frequency relations in the excised larynx

The purpose of this study was to find relationships between subglottal pressure (P(s)) and fundamental frequency (F(0)) of phonation in excised larynx models. This included also the relation between F(0) and its rate of change with pressure (dFdP). Canine larynges were prepared and mounted over a tapered tube that supplied pressurized, heated, and humidified air. Glottal adduction was accomplished either by using two-pronged probes to press the arytenoids together or by passing a suture to simulate lateral cricoarytenoid muscle activation. The pressure-frequency relation was obtained through a series of pressure-flow sweep experiments that were conducted for eight excised canine larynges. It was found that, at set adduction and elongation levels, the pressure-frequency relation is nonlinear, and is highly influenced by the adduction and elongation. The results indicated that for the lower phonation mode, the average rate of change of frequency with pressure was 2.9+/-0.7 Hzcm H(2)O, and for the higher mode was 5.3+/-0.5 Hzcm H(2)O for adduction changes and 8.2+/-4.4 Hzcm H(2)O for elongation changes. The results suggest that during speech and singing, the dFdP relationships are taken into account.

Effect of Artificially Lengthened Vocal Tract on Vocal Fold Oscillation's Fundamental Frequency

The fundamental frequency of vocal fold oscillation (F(0)) is controlled by laryngeal mechanics and aerodynamic properties. F(0) change per unit change of transglottal pressure (dF/dP) using a shutter valve has been studied and found to have nonlinear, V-shaped relationship with F(0). On the other hand, the vocal tract is also known to affect vocal fold oscillation. This study examined the effect of artificially lengthened vocal tract length on dF/dP. dF/dP was measured in six men using two mouthpieces of different lengths.

RESULTS

The dF/dP graph for the longer vocal tract was shifted leftward relative to the shorter one.

CONCLUSION

Using the one-mass model, the nadir of the "V" on the dF/dP graph was strongly influenced by the resonance around the first formant frequency. However, a more precise model is needed to account for the effects of viscosity and turbulence.

Aerodynamic profiles of a hemilarynx with a vocal tract

The pressure-flow relationship was examined in excised canine and human larynges with and without a vocal tract. Canine and human larynges were prepared and cut in the midsagittal plane from the top to about 10 mm below the vocal folds. The right half was removed and replaced with an acrylic plate. The vocal tract was simulated initially with a 15-cm plastic tube and later with a vocal tract model with an area function resembling "ah." Simultaneous recordings were made of the glottal pressure, mean subglottal pressure, and average airflow at various levels of adduction. Preliminary data indicated that the pressure-flow relationships were similar to those of a full larynx and were almost linear. The addition of the vocal tract increased the glottal resistance by moving these pressure-flow lines to the lower-flow and higher-pressure region. The human larynx appears to phonate more easily than the canine larynx on the laboratory bench and has lower phonation threshold pressures.