Overview
Fluid flow is essential for speech production. Not only is an air medium necessary for sound wave propagation, but air must also be transported en masse through various constrictions along the respiratory tract for sound generation. This chapter examines the transport properties of air through confined spaces such as tubes and ducts, with specific application to the respiratory system.
The chapter begins with fundamental concepts of fluid pressure and its measurement, then progresses to specific anatomical regions including lung (alveolar) pressure, thoracic pressure, pleural pressure, and abdominal pressure. Understanding these pressure relationships is crucial for comprehending how the respiratory system generates and maintains airflow during phonation.
What You’ll Learn
Fluid Mechanics Fundamentals
- Pressure: Definition, measurement, and the distinction between absolute and relative pressure
- Pascal’s Law: How pressure transmits rapidly and uniformly throughout enclosed fluids
- Boyle’s Law: The inverse relationship between pressure and volume in soft-walled enclosures
Respiratory Anatomy and Physiology
- The Pulmonary System: Structure and function of lungs, trachea, and respiratory airways
- Lung Volume: Vital capacity, tidal volume, and reserve volumes during various activities
- Pressure Distribution: Relationships between alveolar, thoracic, pleural, and abdominal pressures
Breathing Mechanics
- Inspiratory Phase: Role of diaphragm and external intercostal muscles
- Expiratory Phase: Elastic recoil, internal intercostals, and abdominal muscle contributions
- Speech vs. Nonspeech Breathing: Asymmetries in the breathing cycle for phonation
Conservation Laws
- Continuity Law: Flow constancy in incompressible fluid systems
- Bernoulli’s Law: Energy conservation and the pressure-velocity relationship
- Flow Resistance: Glottal resistance and its relationship to voice quality
Clinical Applications
- Breath Support: Different approaches to managing subglottal pressure
- Airflow Optimization: Balancing aerodynamic efficiency with vocal fold health
- Common Issues: Breathing faults and corrective strategies
Physiological Context
The respiratory system serves dual functions: life support (gas exchange) and communication (speech and singing). During phonation, the typical breathing cycle is dramatically altered. While quiet breathing involves roughly equal inspiratory and expiratory durations, speech requires rapid inspiration followed by prolonged, controlled expiration.
This asymmetry places unusual demands on the motor control system. The lungs must expand quickly to intake adequate air, then contract slowly and steadily to maintain consistent subglottal pressure throughout a phrase. This control requires precise coordination between:
- Diaphragm and external intercostal muscles (inspiration)
- Internal intercostal and abdominal muscles (expiration)
- Laryngeal muscles (airflow regulation)
Aerodynamic Power
All physical power in voice production originates from aerodynamic power—the energy carried by moving air. This power is proportional to the product of lung pressure and airflow. Maximum aerodynamic power in speech is approximately 1 watt, though typical conversational speech uses considerably less.
Understanding how this aerodynamic power converts to acoustic power requires knowledge of fluid mechanics principles, tissue biomechanics, and acoustic resonance—topics that integrate across multiple chapters of this text.
This chapter provides the foundational understanding of breathing mechanics necessary for studying phonation and resonance. The principles presented here apply broadly across speaking, singing, and clinical voice disorders.
Check Your Understanding
Five short questions on pressure, airflow, and breathing for voice. Each one explains itself as soon as you answer, so treat them as a way to test your reasoning rather than a test to pass.
Correct Not quite Best answer
Abdominal compression is the third phase, needed at low lung volumes when elastic recoil has fallen to zero or gone negative. At the start of a phrase from high lung volume, recoil alone already exceeds the target pressure.
Correct Not quite Best answer
That describes the second phase, at mid lung volumes, when recoil has fallen to roughly the desired pressure and begins to need topping up. Early in the phrase, recoil is still too strong, not too weak.
Correct Not quite Best answer
At high lung volumes the elastic recoil of the lungs and expanded rib cage produces more pressure than 0.7 kPa on its own. To keep the pressure constant, the diaphragm or external intercostals must stay partly active, "braking" the recoil—the chapter's image is "breathing in while breathing out." Only as volume falls do the internal intercostals and then the abdominals take over.
Correct Not quite Best answer
Recoil does supply the pressure early on, but too much of it. Left unchecked it would overshoot the target and dump air, so muscle activity is required from the start—just in the inspiratory direction rather than the expiratory one.
Correct Not quite Best answer
With P + ½ρv² constant along the stream, a hundredfold jump in particle velocity must be paid for by a drop in pressure. The chapter's worked example gives a fall of about 240 Pa (from 1000 Pa to 760 Pa) from this effect alone, with viscous losses adding more. The resulting low pressure on the medial surfaces is the Bernoulli suction that assists closure.
Correct Not quite Best answer
This is the intuition the chapter explicitly corrects. Traffic jams form because cars compress (slow and bunch up). Air at speech pressures is effectively incompressible, so it must speed up through the constriction—and to accelerate it needs a pressure gradient that is high upstream and low in the narrow section.
Correct Not quite Best answer
Continuity conserves flow (v₁A₁ = v₂A₂), not pressure. It is precisely because flow is conserved through a smaller area that velocity must rise—and Bernoulli's law then says pressure must fall.
Correct Not quite Best answer
Faster air carries more kinetic energy, but that energy is drawn from the static pressure against the walls, not added to it. In an energy-conserving stream, higher velocity means lower wall pressure.
Pressure applied to an enclosed fluid at rest is felt almost instantly and equally everywhere in it. Because the alveolar airspace is continuous, this is what lets the chapter speak of a single "lung pressure."
Choose an answer for this row, then check again.
At constant temperature, PV is constant: expanding the thorax lowers lung pressure and draws air in; compressing it raises pressure and drives air out. This is the principle behind the whole breathing cycle.
Choose an answer for this row, then check again.
For incompressible flow in a duct, v₁A₁ = v₂A₂ = U. Flow is the same at every cross-section, so air must speed up wherever the duct narrows—about a hundredfold at the glottis.
Choose an answer for this row, then check again.
P + ½ρv² is constant when no energy is lost. An increase in particle velocity must be accompanied by a decrease in pressure, which is why pressure drops inside the narrow glottis.
Choose an answer for this row, then check again.
R = P/U, the pressure drop needed per unit of flow. The glottis is the largest resistance in the airway, and its value rises steeply as the glottal area shrinks.
Choose an answer for this row, then check again.
Correct Not quite Best answer
Pressed voice goes with very low flow—below about 0.05–0.08 L/s— because excessive adduction chokes the airstream. A flow well above 0.2 L/s points in the opposite direction.
Correct Not quite Best answer
The chapter places normal phonation at roughly 0.08–0.15 L/s and breathy phonation above about 0.20 L/s. A flow of 0.35 L/s with an airy quality indicates incomplete closure: a wide glottal gap lowers resistance (R = P/U), wastes air, adds turbulence noise, and shortens phrases. The cause may be technique, muscle weakness, or pathology, and needs evaluation.
Correct Not quite Best answer
Typical speech flows are far lower—about 0.1–0.2 L/s. At 0.35 L/s the speaker is moving two to three times the normal volume of air per second, which is why the phrases sound airy and run short.
Correct Not quite Best answer
High Reynolds numbers produce turbulent, not laminar, flow—and turbulence adds noise rather than removing harmonics. The excess flow through an incompletely closed glottis is what raises the Reynolds number and the audible turbulence here.
Correct Not quite Best answer
Capacity is not the issue: conversational speech uses only about 15–25% of vital capacity. What changes is the timing of the cycle, not the amount of air.
Correct Not quite Best answer
The opposite is true. Expiratory flow during speech (0.1–0.2 L/s) is lower than in quiet breathing, because the adducted vocal folds throttle it. The demanding part is sustaining a steady pressure over a long, slow expiration.
Correct Not quite Best answer
Speech makes the two phases more unequal, not more equal. Inspiration becomes rapid (about 0.5–1 s at 1–2 L/s) while expiration is stretched out over the phrase.
Correct Not quite Best answer
Quiet breathing runs at roughly 1:1 to 1:2 inspiration-to-expiration; speech runs at 1:5 to 1:10, and singing more still. The motor system must inhale quickly without airway constriction, then hold lung pressure nearly constant across three expiratory phases while phrase length is dictated by the text or score, not by metabolic need. That reorganization is what training addresses.
Want more? The practice section has recall flashcards for this chapter's key terms, plus these questions one at a time.