Your vocal cords (more accurately called vocal folds) produce sound by vibrating rapidly as air from your lungs passes between them. This vibration chops a steady stream of air into tiny puffs, creating pressure waves that you hear as sound. The entire process depends on a precise interaction between air pressure, tissue elasticity, and muscle control, and it happens hundreds of times per second during normal speech.
What Vocal Folds Are Made Of
Each vocal fold is a small, layered structure stretched horizontally across your airway. From the outside in, it has five distinct layers: a thin outer lining of tissue (squamous epithelium), a gel-like layer just beneath the surface called the superficial lamina propria, two deeper connective tissue layers that together form the vocal ligament, and finally the muscle itself. This layered design is essential to how they work. The soft, jelly-like outer layer slides freely over the stiffer layers beneath it, creating a rippling wave across the surface during vibration. Without that loose, pliable cover, the folds couldn’t oscillate the way they do.
The Vibration Cycle, Step by Step
Before you speak, muscles in your larynx pull the vocal folds together so they meet at the midline. Air from your lungs builds up below them. When that pressure gets high enough, it forces the folds apart, and air rushes through the narrow gap between them.
Here’s where physics takes over. As air accelerates through that tight space, the pressure within the gap drops (the same principle that lets airplane wings generate lift). That drop in pressure, combined with the natural springiness of the vocal fold tissue, pulls the folds back together. Once closed, subglottal pressure builds again, the folds blow open, and the whole cycle repeats.
This is known as the glottal cycle, and it has four distinct phases: the opening phase (folds pushed apart), the open phase (air flowing through), the closing phase (folds snapping back together), and the closed phase (pressure rebuilding below). During normal conversation, this cycle repeats roughly 100 to 300 times every second.
How Pitch Is Controlled
Pitch depends on how fast the vocal folds vibrate, measured in cycles per second (Hertz). Thinner, tighter, longer folds vibrate faster and produce a higher pitch, just like a tighter guitar string plays a higher note. Adult men typically have a speaking fundamental frequency around 100 to 130 Hz, while adult women average around 190 to 220 Hz. Children between ages 6 and 10 average roughly 262 Hz for boys and 281 Hz for girls, with little difference between sexes at that age. The gap widens dramatically during puberty as testosterone thickens and lengthens the vocal folds in males.
To raise your pitch, muscles in the larynx tilt or rock the cartilage structures that anchor the vocal folds, stretching them longer and making them thinner. To lower pitch, other muscles shorten and thicken the folds. This constant adjustment happens so automatically that you rarely think about it, yet it’s what allows you to ask a question (pitch rises) versus make a statement (pitch falls).
What Controls Volume
Loudness comes down to one primary factor: how much air pressure builds up below the vocal folds before they open. Higher subglottal pressure blows the folds apart more forcefully, which makes the opening phase more sudden. The closing phase is also more abrupt, and the folds snap back together with greater impact. This creates stronger pressure waves in the air above, which your ear perceives as louder sound.
Research in computational voice models confirms that subglottal pressure is the dominant factor in vocal intensity. Pushing more air pressure also tends to raise the pitch slightly and can increase noise in the sound, which is why shouting often sounds both louder and slightly higher and rougher than normal speech.
How Raw Sound Becomes Speech
The buzzing tone the vocal folds produce on their own is a raw, harsh sound. It only becomes recognizable speech after it passes through the spaces above: the throat (pharynx), mouth (oral cavity), and nasal cavity. These spaces act as a filter, amplifying some frequencies and dampening others based on their shape at any given moment.
This concept is called the source-filter model. The vocal folds are the source, producing a basic tone rich in many frequencies. The vocal tract is the filter, shaping which of those frequencies come through strongest. When you change the position of your tongue, jaw, lips, or soft palate, you reshape the filter and produce different vowels and consonants. A boost in the 2 to 4 kHz range, for instance, happens when certain structures near the vocal folds narrow, contributing to the “ring” or carrying power in a trained singer’s voice. The efficiency of your voice depends more on how well you shape these resonant spaces than on how hard your vocal folds work.
Why Hydration Matters for Vibration
The gel-like surface layer of the vocal folds needs to stay moist to vibrate freely. When the tissue dries out, its stiffness and viscosity increase. That means more energy gets lost to internal friction during each vibration cycle, and it takes higher air pressure just to get the folds moving. Researchers measure this as “phonation threshold pressure,” essentially how hard your lungs have to work to start producing sound. Dehydrated vocal folds require more of it.
Dehydration also reduces the amplitude and frequency of the mucosal wave, the ripple that travels across the surface of each fold during vibration. Smaller, slower waves translate to a voice that sounds thinner, rougher, or more effortful. Prolonged exposure to dry air compounds the problem by thickening the mucus layer on the vocal fold surfaces, further impeding smooth vibration. This is why singers, teachers, and others who rely on their voices are often advised to stay well hydrated and avoid prolonged exposure to dry or air-conditioned environments.
The Muscles That Make It All Work
Several small muscles inside the larynx coordinate to position the vocal folds for breathing, speaking, and protecting your airway:
- Posterior cricoarytenoid muscles are the only muscles that open the vocal folds. They pull the back ends of the small cartilages (arytenoids) that anchor the folds together, which swings the front ends apart, opening the airway.
- Lateral cricoarytenoid muscles do the opposite, pulling the back ends of the arytenoid cartilages apart so the front ends (and the vocal folds) come together.
- Interarytenoid muscles squeeze the two arytenoid cartilages toward each other, pressing the vocal folds more tightly together so they can resist the air pressure from the lungs.
These muscles work in rapid, coordinated patterns you never consciously control. Breathing keeps the folds open. Speaking snaps them together. Swallowing closes them tightly to protect your lungs. The transition between these states happens in milliseconds, thousands of times a day.