In ▒Chapter 1░, we defined phonation as the aerodynamic and muscular influences acting on the tissue of the vocal folds, setting them into vibration and creating acoustic energy that we call “voice.” In the context of clinical voice science, the word “aerodynamic” refers to the physical properties of the air stream which act on the vocal fold tissue to drive phonation. When a voice is perceived as dysphonic, the underlying aerodynamic forces driving phonation are typically altered. Aerodynamic assessments of vocal function have a historic and robust research base supporting their use as a clinical modality and have been recommended as a standard component of voice assessment. ¹

Objective measurement of aerodynamic forces driving phonation, which can include one or more parameters of volume, flow, pressure, and/or vocal efficiency. These measures provide valuable clinical evidence which, when combined with the knowledge and skill of the speech–language pathologist, can facilitate important clinical processes including ²

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  Impairment detection.

  
  
  
  
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    Aerodynamic assessments allow clinicians to measure respiratory function, laryngeal function, respiratory–laryngeal coordination, and whether impairment exists in any of those three domains.

  
  
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  Objective characterization (measurement) of an impairment.

  
  
  
  
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    Aerodynamic assessments allow clinicians to objectively quantify physiological substrates of phonation.

  
  
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  Support the process of differential diagnosis.

  
  
  
  
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    Aerodynamic assessments can elucidate the underlying physiological causes of dysphonia, providing evidence that informs differential diagnosis.

  
  
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  Provide a modality for biofeedback during voice treatment.

  
  
  
  
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    Aerodynamic assessments are simple to employ as part of the clinical process and can serve as a visual biofeedback tool.

  
  
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  Provide objective measurement of clinical outcome.

  
  
  
  
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    Aerodynamic assessments can be repeated over time to objectively measure changes in phonation physiology subsequent to voice treatment.

In clinical contexts, volume and flow have been measured using direct or indirect estimates of (1) vital capacity, (2) phonation volume, (3) average and peak flow rates, and the ratio of vital capacity to MPT, called (4) phonation quotient. ^{1,​ 3,​ 4,​ 5,​ 6} Estimates of subglottal pressure may be measured during habitual pitch and loudness phonation or during soft phonation—when measured during soft/quiet phonation, the measure of subglottal pressure is referred to as phonation threshold pressure. ^{7,​ 8} Clinical measurement of vocal efficiency has included glottal efficiency, laryngeal resistance, maximum phonation time (MPT), and s/z ratio. ^{1,​ 9,​ 10,​ 11} Advantages and limitations exist for all aerodynamic measures, and to date no individual or set of aerodynamic measures has been demonstrated to be clinically more important or more cost-effective than others. Definitions for the more commonly applied aerodynamic measures are given in ▶ Table 5.1.

5.3 Aerodynamics and Phonation

Voice production is accomplished by converting aerodynamic energy (pressure and flow) into acoustic energy (sound). The aerodynamic energy of voice production originates from differential pressures between the lower vocal tract (lungs and trachea) and upper vocal tract (larynx and supraglottal spaces), resulting in airflow. The larynx acts as a valve that applies varying degrees of resistance to this airflow. Pressure, airflow, and resistance are related to each other. This relationship can be described by applying Ohm’s law to fluids (air can be thought of as a type of fluid), as expressed in the formula U = P/R, where U = flow, P = pressure, and R = resistance. ⁷ As you can deduce, increasing or decreasing the value of any one variable on the right side of this equation will influence the result of the equation on the left. As an example, when resistance to the expiratory airstream is low (e.g., in hypofunction), airflow will tend to be increased—we typically perceive a voice produced with increased airflow as breathy. In contrast, if resistance is increased (as in hyperfunctional voice types), airflow will tend to be decreased—we typically perceive this type of voice production as strained. In phonation, airflow may be influenced by modulating pressures below, within, and above the glottis through varying degrees of resistance at the level of the larynx and supraglottal spaces.

In ▒Table 1.5░, we have listed a number of aerodynamic parameters that drive vocal fold oscillation during phonation. Among these include subglottal pressure and transglottal airflow, which along with assessments of lung capacity and vocal efficiency constitute the primary clinical aerodynamic measurements obtained to identify, characterize, and differentially diagnose respiratory and laryngeal impairments underlying voice disorders. A number of physical impairments can cause inefficiencies or ineffectiveness of phonation and the accompanying aerodynamic characteristics. Impairments that result in measurable changes to aerodynamic forces in phonation include, among other things:

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  Restricted lung capacity due to disease or muscular weakness.

  
  
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  Hypokinesia or hyperkinesia of the respiratory and/or intrinsic laryngeal muscles resulting from neurological or functional etiologies.

  
  
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  Lesions affecting the vocal fold tissue.

  
  
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  Ineffective posturing of the supraglottal structures and spaces.

These conditions may result in ineffective respiratory support for phonation, altered vibratory dynamics creating irregularities and inefficiencies in vocal fold oscillation, or both. In addition to altered aerodynamic measurements, clinical signs and symptoms related to these impairments include

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  Short phrase lengths (few words in one breath).

  
  
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  Fatigue with prolonged speaking.

  
  
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  Perceptions of excessive effort when speaking.

  
  
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  Dysphonia.

5.4 Lung Capacity

Lung capacity refers to quantities of air exchanged within the lungs during respiration. The most common lung capacity measurement used in clinical voice practice is vital capacity (VC), a measurement used to assess the lung capacity available to support phonation during connected speech. Specifically, VC is defined as the maximum quantity of air that can be exhaled after a maximum inhalation—its relationship with other lung capacity measurements is illustrated in ▶ Fig. 5.1. VC is typically measured using spirometry and reported in either liters (L) or milliliters (mL). While high-tech spirometry systems with exceedingly accurate measurement precision are available to measure VC, acceptable measurements of VC are also easily obtained from low-tech and inexpensive hand-held spirometers.

Within the context of phonation, the lungs can be thought of as a gas tank and the air that fills them as the gasoline which drives the engine—the vocal folds. Phonation will occur only as long as air is available to support the process, just as a car engine will only operate as long as gas is available to power it. That is to say, respiratory support is critical for phonation, and measures of VC are one method of quantifying this support. While it would be extremely rare that the entire vital capacity would be used during a single sustained vowel or continuous speech utterance, initiating speech/voice with a limited respiratory capacity can lead to undesirable imbalances in phonatory function, vocal effort, and eventual fatigue. As an example, initiating voice with a very low driving capacity often leads to compensatory hyperfunction as a means of conserving air and may lead to fatigue and impaired voice characteristics as the day progresses. Awan has suggested that VC, as a measure of maximum performance, provides evidence that informs the clinician of a speaker’s potential ability to produce voice in physically stressful conditions (e.g., extended durations of phonation in connected speech). ⁶ When combined with measurements of pressure, flow, and phonation efficiency, assessment of VC allows the clinician to rule in or out underlying respiratory etiologies related to a voice disorder. It is typically a good idea to measure VC when dysphonia is present, as respiratory muscle incoordination, weakness, or restricted lung capacity can be contributing factors or underlying causes of voice impairments.

Calculation of VC using commercially available hand-held spirometers is a simple and efficient process. A typical method is described later. This method of eliciting and measuring VC is a form of expiratory vital capacity and has been referred to as slow vital capacity, which does not require a patient to blow out as forcibly and quickly as possible (forced exhalation is a method of obtaining VC known as “forced vital capacity”—unless a patient suffers from an obstructive lung disease, slow and forced vital capacity are typically very similar). ¹²

5.4.1 Measuring Vital Capacity with a Hand-Held Spirometer

• 
  

  Calibrate the spirometer as per manufacturer’s specifications or using a known volume of air.

  
  
• 
  

  Attach a new flow tube onto the spirometer.

  
  
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  Place a nose clip on the speaker’s nares.

  
  
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  Instruct the speaker as follows: “Take two comfortable breaths, inhaling and exhaling. On the third breath, inhale as deeply as you can, place your mouth and lips completely around the flow tube, and exhale forcefully into the tube. Keep exhaling until you completely run out of air.”

  
  
  
  
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    Some speakers will need coaching or encouragement during the performance of this task in order to elicit a true maximum performance.

    
    
  • 
    

    The clinician should make sure the speaker’s lips are sealed around the flow tube and that air does not leak around the tube during the task.

A number of factors can influence measures of VC, including the speaker’s sex, age, body mass, and height. These factors have been accounted for in equations which can be used to estimate VC in a normal, healthy speaker against which a patient’s VC can be compared. Many different reference formulas have been generated based on various population samples. As an example, the following formulas have been used as estimates of predicted VC and referenced in a number of publications associated with voice production ¹³:

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  VC (mL) Males: [27.63 – (0.112 × Age)] × Height (cm).

  
  
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  VC (mL) Females: [27.78 – (0.101 × Age)] × Height (cm).

Why Are Measures of Vital Capacity Sensitive to Respiratory Changes/Impairments?

Respiration requires muscular action to modify lung volumes. Therefore, any condition that causes weakness in the respiratory muscles (e.g., amyotrophic lateral sclerosis, Parkinson’s disease) or restricts lung expansion for respiration (e.g., radiation fibrosis) has the potential to affect measures of VC. In all cases, the clinical effect is lowering of VC. Measures of VC also typically decrease with advancing age. ^{14,​ 15} ▶ Fig. 5.2 illustrates data from an investigation demonstrating that VC in healthy females was significantly greater for those below 50 years compared to those older than 50 years. ¹⁵ Thus, clinical measures of VC from dysphonic patients must be considered within the context of biological factors associated with an individual’s overall health, age, sex, and height.

It is important to also consider additional sources of variability in clinical measurements of VC. The American Thoracic Society identified at least three sources impacting lung function measurements, including normal biological sources of variation, the presence of disease states, and variability associated with technique or methodology. ¹⁶ Within this technical category includes the instrument (spirometer) you will be using and the instructions that you give to a patient. As with methods for acoustic analyses and other aerodynamic measurements; it is crucial that clinicians develop consistent procedures, including consistency of equipment used, when measuring and comparing VC within and between patients.

▶ Table 5.2 shows VC measurements from a selection of published studies. Inspection of this table suggests that healthy, young adult male and female speakers typically produce measures of VC within a range of 3 to 5 L, with males exhibiting greater measures of VC than females. With advancing age, VC tends to lower such that measures may be recorded below 3 L for healthy adults older than 65 years. In addition, it should be expected that young children would exhibit much lower VC than adults due to inherently smaller lung structure and available volume.

5.5 Subglottal Pressure

In physics, pressure (P) is defined as the ratio of force applied over a given area, as in the formula: P = F/A (where P = pressure, F = force, and A = area). A common unit of measurement for P is the pascal, although in voice science and clinical voice practice pressure is most often measured in centimeters of water (cm H₂O; 1 cm H₂O = 98.06 pascals). Subglottal pressure can be understood as the tracheal air pressure immediately below the glottis that acts as a force distributed over the inferior surface of the vocal folds (▶ Fig. 5.3). This pressure is responsible for initiating oscillation, is the primary determinant of vocal intensity, and plays an important role in sustained oscillation during phonation. It acts on vocal fold tissue during volitional or involuntary voice production, coughing, and other laryngeal behaviors.

During phonation, subglottal pressure (which we will identify by the symbol P_s) is the aerodynamic force that sets vocal folds into motion once they have been approximated or completely adducted by the intrinsic laryngeal muscles. It does this by separating the lower vocal fold edges, allowing a pressurized pulse of air to flow through the glottis at high velocity. As long as respiratory muscles continue to send air from the lungs and the vocal folds are maintained in the adducted position, P_s will continue to build/release during the closed and opening phases of vocal fold vibration, respectively, and continue to modulate pressure differentials below, above, and within the vocal fold tissue to sustain oscillation.

While P_s can be measured directly by inserting a probe into the trachea below the vocal folds or indirectly by measuring pressure in the esophagus, contemporary clinical applications utilize less invasive and more efficient techniques that provide indirect measures of P_s. This can be accomplished by measuring intraoral (oral cavity) pressure during the production of unvoiced plosive-vowel syllable trains, such as /pa-pa-pa/ or /pi-pi-pi/. ²⁶ This is achieved by either inserting a plastic or rubber tube between the lips into the oral cavity connected in line with a pressure transducer (outside the oral cavity), or by routing the intraoral pressure tube through a facemask that is held firmly against the face (examples of masks used in measurement of intraoral pressure are illustrated in ▶ Fig. 5.4). ²⁷ Syllable trains are produced with equal stress, on a single breath, at habitual pitch and loudness. When produced with adequate velopharyngeal and lip seals, it has been demonstrated that peak intraoral pressure measured from the stop-plosive immediately preceding a vowel is a good estimate of the subglottal pressure used during vowel production in speech. ²⁸ This phenomenon can be explained by the fact that, when produced with a sealed supraglottal cavity, the pressure in the vocal tract equalizes from behind the point of articulatory obstruction continuing through the trachea and lungs. Therefore, the peak intraoral pressure measured during /p/ will be the same amount of pressure used to initiate vocal fold oscillation at voicing onset of the subsequent vowel.

Clinical measurements of P_s are typically acquired while a speaker is producing sound at a comfortable or “habitual” F_o and intensity during syllable productions. However, it has also been observed that measuring P_s while a speaker is producing voice at his or her lowest possible intensity (e.g., as soft as possible without whispering) is very sensitive to the presence of vocal fold impairments. ^{29,​ 30} This measurement is called phonation threshold pressure (PtP), and can be defined as the minimum amount of air pressure required to set the vocal folds into vibration. ³¹ In healthy speakers, we would expect measures of P_s and PtP to be low, and in speakers with vocal fold impairments we would expect these measurements to be elevated (e.g., speakers with vocal fold impairments require greater pressure to set the vocal folds into vibration, either at their lowest possible intensity or during comfortable speaking intensities). Alternatively, speakers with impaired respiratory support may be unable to generate a sufficient amount of sustained P_s to maintain vocal fold oscillation, and thus exhibit severely breathy voices with marked aphonia during speech. ³²

5.5.1 Why Are Measures of Subglottal Pressure Sensitive to Laryngeal Impairments?

Subglottal pressure is influenced by a number of factors, which include respiratory drive (e.g., activity in the respiratory muscles), glottal configuration (e.g., completely closed vs. posterior gap vs. slightly abducted), medial compression force, and tissue stiffness. Different vocal pathologies can affect these factors and subsequently influence P_s. For example, unilateral vocal fold paralysis (UVFP) results in glottal insufficiency such that many speakers have difficulty closing the glottis completely along the midline. When compared to normal speakers or to posttreatment measures (e.g., when glottal insufficiency has been improved), studies generally find that P_s is increased in the presence of glottal insufficiency secondary to UVFP. ^{33,​ 34,​ 35} Significant glottal gaps present less surface area at midline against which subglottal pressure can exert force and also result in pressure leaks. While some may expect that glottal gaps would result in low subglottal pressures (due to low resistance), in actual fact this condition requires speakers to increase respiratory drive and residual glottal and supraglottal muscular force to achieve phonation, translating to increased P_s.

Membranous lesions which add mass to the vocal fold tissue can influence P_s because of their influence on vocal fold adduction (e.g., they can cause varying levels of glottal insufficiency along the length of the vocal folds), but in addition can present increased stiffness against the driving pressure. It has been reported that lesions creating higher degrees of stiffness (e.g., polyps and cysts) require greater P_s than those creating less stiffness (e.g., nodules). ³⁶ Benign mid-membranous lesions (e.g., nodules, polyps, cysts, and pseudocysts) typically result in elevated measures of P_s. In general, speakers with dysphonia typically exhibit measures of Ps greater than 10 cm H₂O at comfortable pitch and loudness. In contrast, the literature collectively demonstrates that most healthy speakers without dysphonia exhibit measures of Ps between 5 and 10 cm H₂O when phonating at comfortable pitch and loudness. ▶ Table 5.3 summarizes P_s data from selected research published since 1988.

It is important for clinicians to consider that patterns of aerodynamic measurements do not always agree across research studies, and do not always move in the direction of “normal” after treatment. This is certainly true for measurements of P_s. For example, while a majority of studies find that surgical improvement of glottal insufficiency lowers P_s, a few studies have reported an opposite pattern or reported that the material/technique used to improve glottal insufficiency influenced the measurements of P_s. ^{33,​ 35} An important variable to contemplate when assessing the evidence from clinical studies is the methodology utilized to acquire the aerodynamic measurements, which likely accounts for a large degree of the variability in values across different published investigations. Many methodological factors can influence aerodynamic measurements and P_s measures in particular, and must be accounted for whenever they are obtained in the voice clinic. These will be addressed in the next section.

5.6 Methodological Considerations for Measuring Subglottal Pressure

Among the aerodynamic measurements a clinician might obtain, measurement of P_s requires the most specialized technology. Measurements of P_s will be affected by many methodological factors, among which include

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  Equipment.

  
  
  
  
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    Transducer sensitivity, facemask design, properties of intraoral tubing, and signal processing methods are among the equipment-related factors that will influence measurements of P_s.

  
  
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  Vocal intensity.

  
  
  
  
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    P_s is a primary determinant of vocal intensity, which will vary as a function of P_s. In general, the greater the vocal intensity, the greater the P_s.

  
  
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  Vocal F_o.

  
  
  
  
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    The influence of P_s on vocal F_o varies as a function of speaking task. During vowel production increases in P_s will result in increases of F_o—for example, as a speaker moves from modal to falsetto register or chest voice to head voice, P_s will increase. During connected speech, P_s also influences F_o but in less predictable ways. Speakers often maintain a relatively stable P_s at habitual speaking F_o, even though F_o will dynamically vary due to intonation. Exceptions to this phenomenon include words or syllables that are stressed during speech, which are characterized by increased F_o, increased vocal intensity, and greater P_s compared to unstressed words and syllables.

  
  
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  Recording environment.

  
  
  
  
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    Environmental temperature and humidity can potentially influence the properties of a facemask and tubing when collecting measures of P_s.

  
  
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  Instructions

  
  
  
  
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    As previously mentioned, relationships exist between P_s and the intensity and F_o characteristics of the voice. Both of these factors are modulated in prosody, such that stress patterns of sounds, syllables, and words can also influence P_s. Additionally, lung volume (how deeply a speaker inhales prior to sound production), respiration pattern while speaking (all sounds on one exhalation vs. multiple breaths), speech stimulus, speaking rate, and the degree of coupling (seal) between a facemask and face are all factors that can potentially influence measures of P_s and should be considered when developing a methodology for acquiring clinical measurements.

When developing a clinical voice laboratory for the acquisition of aerodynamic and acoustic analyses, the clinician must decide on what level of control to exert upon the aforementioned factors. Of utmost importance is that equipment choice, environmental factors, and instructions remain consistent within and between patient recordings. The clinician should choose a setup and recording methodology and stick with it. The authors understand that these decisions can be daunting for the novice or inexperienced clinician, and for this reason they provide some recommendations below, which are based on more than six decades of published research evidence and a combined 40 years of clinical experience of the authors.

5.7 Methods for Measuring Subglottal Pressure

Subglottal pressure generation is dependent on the activation of respiratory muscles which cause air to move through the lower respiratory tract to the level of the vocal folds. For this reason, it is very important to remember that clinical measurements of P_s can be influenced by not only laryngeal impairments but also respiratory impairments. Hixon et al developed a very simple, low-cost device that can be used to determine if speakers manifest sufficient respiratory pressure generation for connected speech purposes. ⁵¹ This device, illustrated in ▶ Fig. 5.5, utilizes a plastic straw clipped to a glass onto which a ruler is taped. With the straw submerged, the air pressure required to blow a bubble into the water is directly proportional to the depth of the straw. For example, if the end of the straw is at 5 cm, the pressure required to blow a bubble into the water which rises to the surface will be 5 cm H₂O. The authors suggested that speakers with sufficient respiratory function for speech should be able to generate bubbles at 5 cm H₂O for at least 5 seconds. This rule of thumb corresponds well to PtP measures required to initiate and sustain phonation, which range between 2 and 5 cm H₂O in healthy speakers. This simple low-tech device can be used to assess respiratory support for phonation, and will further inform your interpretations of subsequent aerodynamic measurements.

A number of commercial equipment applications incorporate pressure tubing into pneumotachograph-based masks (▶ Fig. 5.6) which cover the mouth and nose to allow for simultaneous measurement of pressure and flow. When acquiring pressure measurements using a facemask, it is critical that the patient be instructed to hold the mask “firmly” against their face throughout breathing or speaking trials, and that the intraoral tube be placed between the lips and into the front of the oral cavity above or in front of the tongue. The pressure transducers of commercial applications must also be calibrated on a regular basis.

The literature is consistent in using /p/ as the bilabial plosive stimulus, but studies vary in their choice of the following vowel and inclusion of other speech sounds. The most commonly used stimuli for measuring P_s include slow (e.g., rate of 1.5 per second) repetitions of either /pa/ or /pi/. It should be noted that the specific vowel and the phonetic environment surrounding the unvoiced plosive may influence measurements of P_s. High vowels following the /p/ release, such as /i/, may result in higher measures of P_s than when using a low vowel such as /a/. ^{52,​ 53} Some investigations have also attempted to measure P_s during continuous speech samples such as the Rainbow passage, though much less control over the elicited pressure and increased variability may be expected. ⁵⁴

The following are methodological recommendations for acquiring measurements of P_s. These are aligned with literature reporting P_s measurements from different commercial systems, with the primary difference being the vowel stimulus used. Current options for equipment capable of measuring P_s are listed in Appendix 5.1.

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  Ensure system is calibrated to manufacturer’s specifications.

  
  
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  Utilize a new intraoral pressure tube for each patient.

  
  
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  The tube should be positioned such that it fits between the speaker’s lips, with the tip sitting inside the anterior oral cavity in front of or above the tongue.

  
  
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  A practice recording is recommended to ensure that the intraoral tube is placed correctly (i.e., not loose outside of the lips; not being blocked by the tongue or saliva).

  
  
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  Instruct the speaker as follows: “Take a comfortable breath, place the tube between your lips inside the front of your mouth, and repeat the syllables /pa-pa-pa/ [or /pi-pi-pi/] at a comfortable pitch and loudness, with a slow and steady rate like this (model a rate of 1.5 syll/sec for the patient). You will say /pa/ [or /pi/] five to seven times on a single breath, and I will tell you when to stop.”

  
  
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  When the speaker indicates they are ready, begin recording and tell them to “go.”

Some clinicians might find that a metronome (easily displayed via a smartphone application) can help a speaker achieve the target syllable rate. When using a facemask (for acquiring synchronous measures of flow), instructions also typically include placing the mask “firmly” against the face. Speakers with respiratory impairment and/or significant glottal insufficiency may not be able to produce five to seven repetitions on one breath at the target syllable rate. The authors’ clinical experiences suggest that as long as you have a minimum of three repetitions from multiple trials (e.g., x3) which to measure mean data, it will be clinically acceptable. It is important that clinicians monitor the patient’s behavior when recording. If they perceive the patient is not producing syllables at a “comfortable” pitch and loudness, they should query the patient on their perception of effort—in the authors’ experience, even speakers with typical voice will often push against the mask, resulting in increased intraoral pressures. Re-recording multiple trials will not add significant amounts of time to the clinical process.

Ideally, clinicians should acquire three separate trials (of five to seven syllables each) and use the middle trial or trials which show the best pressure peak characteristics to acquire measurements. The authors’ preference is to avoid the first and last syllables (e.g., selection is from the middle syllable trains) when considering from which peaks to measure P_s.

Data from recent studies that have measured P_s using similar methodology as above in normal and dysphonic populations is illustrated in ▶ Table 5.3. This is not an exhaustive listing of the literature but will provide some guidance to the clinicians looking to compare their patients’ measures to evidence-based reports using similar methodology.

5.7.1 What to Expect from Normal and Dysphonic Speakers?

Efforts to extract absolute values for what should be expected from normal (nondysphonic) and dysphonic speakers (▶ Table 5.3) can be an exercise in frustration. Published aerodynamic data vary widely from speaker to speaker and study to study, for many different reasons. Among these include (1) physiological characteristics and habitual patterns of speakers, (2) different instrumentation, (3) different speaker instructions, (4) different stimuli, and (5) different analysis procedures.

Careful analyses of published investigations, in addition to the clinical and laboratory experience of the authors, do allow for some generalizations based on speaker trends for both normal and dysphonic populations. As clinicians consider these “clinical nuggets,” we are obliged to again point out that aerodynamic measurements are known to vary widely from speaker to speaker, any mean must be considered along with the associated variability around the mean for a given population (e.g., the typical range of measures in normal speakers or dysphonic speakers), and not all published studies agree with the generalizations below. Clinicians must also accept that some dysphonic speakers will produce P_s within normal ranges, and some normal speakers will produce P_s outside of normal ranges. With this in mind, general trends of P_s measurements in different populations include the following:

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  Normal male speakers tend to exhibit greater P_s than normal female speakers (although this trend may not hold for all vowel stimuli or older speakers, e.g., > 60 years of age). ^{11,​ 39}

  
  
• 
  

  Normal young, middle age, and older speakers do not tend to exhibit significantly different measures of P_s. ¹¹

  
  
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  Normal speakers tend to produce P_s between 5 and 10 cm H₂O when producing slow repeated /p/+vowel syllables at comfortable levels. ^{32,​ 39,​ 40,​ 42,​ 45,​ 46}

  
  
• 
  

  Normal speakers tend to consistently exhibit lower P_s than speakers with dysphonia of various etiologies. ^{37,​ 40,​ 41,​ 42,​ 44,​ 45,​ 55}

  
  
• 
  

  Speakers with dysphonia due to nonorganic and organic etiologies tend to produce P_s greater than 10 cm H₂O when producing slow repeated /p/+vowel syllables at comfortable levels. ^{37,​ 40,​ 41,​ 42,​ 44,​ 47,​ 48,​ 50}

  
  
• 
  

  As vocal intensity increases, measures of P_s will also increase in both normal and dysphonic speakers. ^{38,​ 50}

  
  
• 
  

  When voice therapy or surgery improves laryngeal physiology (e.g., improves glottal closure), measures of P_s tend to improve in the direction of lowering (exception: rigid Silastic implants used during thyroplasty can add additional stiffness and P_s may not lower). ^{33,​ 35,​ 37,​ 48}

5.8 Transglottal Airflow

We have previously stated that flow is intimately linked with pressure and resistance as in the formula U = P/R (where U = flow, P = pressure, R = resistance). ⁵⁶ The formula shows that airflow requires a driving pressure. The primary source of driving pressure during phonation is lung pressure, which varies dynamically during breathing and sound production. In accordance with Boyle’s law, during expiration, as the volume of air in the lungs decreases, lung pressure will increase, and air will move from the lungs toward the mouth. Vocal fold vibration will occur only when transglottal airflow (i.e., airflow moving between the vocal folds from subglottal to supraglottal regions) is present.

Flow can be defined as the quantity of fluid that moves across a unit of area over a specific unit of time (i.e., volume/time). In voice production, transglottal airflow can then be understood as the volume of air moving through the glottis each second during phonation. It is typically measured in liters per second (L/s) or milliliters per second (mL/s). The unit of measurement for airflow during phonation reflects a rate of flow, and thus some authors refer to airflow rate as a volume velocity. ⁷ ▒Fig. 1.20░ in ▒Chapter 1░ illustrated characteristics of airflow through the glottis during phonation.

5.8.1 Measurement of Airflow Using a Differential Pressure Pneumotach

The formula U = P/R is a physical phenomenon critical for understanding how airflow is typically measured in clinical voice practice. It can be understood as airflow being the function of pressure and resistance. If the properties of resistance are known, and pressure on either side of the resistance (the pressure differential) can be measured, then airflow rate can be accurately calculated. Most commercial voice analysis systems which measure airflow take advantage of this phenomenon by utilizing a pneumotach. As illustrated in ▶ Fig. 5.7, a pneumotach is a device that can measure the drop in air pressure at the point of resistance to airflow (e.g., the change in pressure on each side of the resistance), from which calculations of transglottal airflow rate can be obtained using transducers (e.g., a transducer converts one type of energy into another, such as pressure into electrical energy that can be digitized, stored, and analyzed by a computer). ⁵⁷ As in measures of P_s, transglottal airflow measures acquired with a pneumotach typically place the device in line with a facemask. The mask allows a speaker to produce voice and speech with limited interference on natural articulation and acts as a sealed chamber to capture aerodynamic energy. Together the pneumotach and associated equipment are referred to as a pneumotachograph. Different pneumotach-based facemasks are shown in ▶ Fig. 5.4.

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