Aerodynamic Analysis of Voice Notes

  1. Introduction to Aerodynamic Analysis of Voice - Topic Overview

    • Aerodynamic analysis plays a critical role in understanding voice production, particularly regarding the function of the vocal folds and the respiratory system.

    • Links perceptual acoustic and aerodynamic information to visual representations in the larynx during phonation.

    • Framework set for instrumental examination of the larynx, key for clinical voice evaluation.

  2. Key Concepts in Aerodynamics

    • Aerodynamics Defined

      • The study of gas motion, particularly with respect to objects in motion.

      • Important components:

      • flow

      • pressure

      • resistance

    • Definitions of Key Terms

      • Flow

      • Defined as the volume of fluid or air passing through a cross-sectional area of a tube per second.

      • Measured in milliliters per second or cubic centimeters (cc).

      • Pressure

      • Defined as the force applied perpendicular to a surface area.

      • Measured in pascals (Pa), with variations including hectopascals (hPa) and kilopascals (kPa).

      • Example:

        • 1\text{ Pa \approx pressure of a dollar bill resting on a table}.

        • Normal usage would involve higher values, e.g., kilopascals (1 kPa=1000 Pa1\text{ kPa} = 1000\text{ Pa}).

      • Subglottic Pressure

      • Pressure below the vocal folds during phonation, measured in centimeters of water (cm H2O\text{cm H}_2\text{O}).

      • Conversion:

        • 1 cm H2O=0.098 kPa or 98 Pa1\text{ cm H}_2\text{O} = 0.098\text{ kPa or }98\text{ Pa}.

      • Subglottic pressure is the driving force for vocal fold oscillation, reflecting the pulmonary effort to initiate and sustain phonation.

      • Resistance

      • Defined as the ratio of pressure to flow.

      • Requires consideration of driving pulmonary pressure and airflow rate.

      • Conceptual links include:

        • Driving air pressure versus rate of airflow.

      • Glottal resistance specifically refers to the resistance offered by the vocal folds to the airflow during phonation. It is influenced by the degree of vocal fold adduction, vibratory characteristics, and vocal fold mass/tension.

  3. Measuring Aerodynamic Aspects of Phonation

    • Phonatory Aerodynamic System (PASS)

      • A widely used device for assessing aerodynamic characteristics in voice clinics.

      • Designed by K Pentax; important for non-physicists in clinical environments. The system typically integrates a flow sensor (e.g., pneumotachograph) and pressure transducers for simultaneous measurement of aerodynamic parameters.

      • Primary features measured include:

      • Average phonatory flow rate

      • Sound pressure level

      • Fundamental frequency

      • Vital capacity of the lungs

      • Glottal resistance and subglottic air pressure

      • Voicing efficiency measures

      • Calibration:

      • Utilizes a standard airflow (1 liter at a set rate1\text{ liter at a set rate}) for the calibration process before data collection.

      • Requires airflow to be tight for accuracy during measurements.

    • Major Measures from PASS

      • Average Airflow Rate

      • Defined as the rate at which air flows between the vocal folds during phonation.

      • Calculation:

        • Airflow Rate=Volume of air (liters)Duration (seconds)\text{Airflow Rate} = \frac{\text{Volume of air (liters)}}{\text{Duration (seconds)}}

      • Measured in liters/second, milliliters/second, or cc/second.

      • Influenced by fundamental frequency and phonation intensity. Variations can also depend on specific vowels produced, vocal effort, and individual laryngeal valving mechanisms.

      • Normal airflow rate:

        • 50−200 ml/s50-200\text{ ml/s} based on individual variability and tasks.

        • Male: 40−320 ml/s // Average: 119 ml/s40-320\text{ ml/s // Average: }119\text{ ml/s}

        • Female: 50−220 ml/s50-220\text{ ml/s}

      • Abnormally High Flow Rate

      • Indicative of:

        • Increased subglottic air pressure

        • Decreased glottal resistance

      • Potential scenarios include glottic incompetence (e.g., vocal fold paralysis, vocal fold bowing) where the vocal folds do not close completely.

      • Airflow Volume

      • Volume of air in the lungs available for driving the vocal folds.

      • Measurement affected by:

        • Age, sex, size, health status (e.g., COPD).

      • Maximum Phonation Time

      • Defined as the maximum duration a vowel can be sustained using the maximum airflow volume.

      • Also influenced by lung capacity, age, health, and sex. It is a simple, non-invasive measure that provides insight into laryngeal valving efficiency and respiratory support.

    • Ratio Measures

      • S to Z Ratio

      • A simple technique to estimate glottal resistance:

        • Produce voiced (z) and voiceless (s) sounds until air is depleted. The 's' sound is a voiceless fricative, requiring airflow but no vocal fold vibration, while 'z' is its voiced counterpart, requiring both airflow and vocal fold vibration.

      • Normal ratio for healthy speakers: 1.01.0 (equal duration).

      • Pathological levels indicated by ratios >1.41.4 (s sustained longer than z), typically suggesting incomplete glottal closure during voicing.

  4. Subglottal Air Pressure (PsubP_{sub}) Measurement

    • Measurement techniques provided via proxy measurement due to the invasive nature of direct measurement.

    • Proxy Measurement:

      • Intraoral pressure measured during a production of a plosive consonant reflects subglottal pressure.

      • Oral cavity pressures equalize during oral closures, used in tasks involving repeating voiceless bilabial plosive sounds like 'pa' or 'pi'. During the oral closure phase of the plosive, if the glottis is completely closed, the intraoral pressure equilibrates with the subglottal pressure. A small tube (pressure transducer) is placed in the oral cavity for this measurement.

      • Normal values: About 6−10 cm H2O6-10\text{ cm H}_2\text{O}, influenced by vocal fold conditions, stiffness, and closure efficiency.

  5. Clinical Relevance of Aerodynamic Measures

    • Understanding the relationship between airflow measure and subglottal pressure illustrates:

      • Hyperfunctional Voice:

      • Decreased airflow rate

      • Increased subglottal air pressure

      • Possible voice strain due to excessive vocal fold closure, involving conditions like muscle tension dysphonia.

      • Hypofunctional Voice:

      • Increased airflow rate

      • Increased subglottal air pressure

      • Indicative of incomplete closure/increased need for pulmonary effort