Why Are Men's Voices Lower Than Women's?
― When the voice breaks, the throat's "strings" grow longer and heavier
The pitch of your voice is set by two small folds of tissue deep in the throat: the vocal folds. They vibrate on the same principle as a guitar string. The longer the string, and the heavier it is, the lower the note. A boy's vocal folds start out about as long as a girl's. Over the few years of voice change they stretch to about 1.6 times that length, and they get thicker too.
In primary school, boys and girls sang in equally high voices.
Then, in junior high, a boy's voice suddenly starts to crack or flip. Within a year or two, it has become a fully low "adult voice".
On the phone, you can usually tell whether the speaker is a man or a woman. Where in the body does this difference come from?
There are only two reasons the voice gets lower
The longer a string is, the more slowly it vibrates. The vibrating part of an adult man's vocal folds is said to be about 60% longer than a woman's.
Pull with the same force, and a heavy string still vibrates only slowly. That is why the low-note strings on a guitar are thick.
Both changes happen because the male hormones that rise in puberty make the throat's framework (the larynx) grow large. The Adam's apple sticks out because that framework has pushed forward.
Your voice is made by opening and closing over 100 times a second
The vocal folds are a pair of soft folds, one on each side, behind the Adam's apple. When you hold your breath or start to speak, the two folds close tightly together.
Air pushed up from the lungs forces the closed folds open from below. Once the air has slipped through, the elastic folds spring shut again. This opening and closing is said to repeat about 100 to 130 times a second in adult men, and 190 to 230 times a second in women. That number is the pitch of the voice.
Figure 1 shows the link between the length of the vocal folds and how often they vibrate. Use the slider to change the length. The longer you make it, the wider the waves spread, and the pitch moves lower.
Length alone explains most of the difference between men and women
How often a string vibrates is inversely proportional to its length. Double the length and the rate halves, which means a note one octave lower.
A commonly used estimate in voice research puts the vibrating part of the vocal folds at about 1.6 cm in adult men and about 1.0 cm in women. The length ratio is 1.6. The actual pitch of the voice differs by about the same factor: roughly 120 hertz for men and around 200 hertz for women. Length alone accounts for most of the gap.
Thickness and weight add to this. A man's vocal folds have a thicker layer of muscle, so the vibrating part is heavier. A heavy string, pulled with the same force, moves sluggishly and gives a lower note.
During voice change, the larynx grows a lot in just a year or two. The way you use the muscles that tune the tension of the vocal folds was learned on the smaller folds. So you can't hit the pitch you aim for, and the voice flips. Once the body gets used to its new instrument, the cracking settles down on its own.
Voice change is often thought of as something only boys go through. In fact, girls' vocal folds also lengthen a little in puberty, and the voice settles slightly lower. The change is small, so it is hardly noticed.
Summary
The pitch of the voice is set by how many times a second the vocal folds open and close. Like strings, the longer and heavier they are, the more slowly they vibrate. During voice change, a boy's vocal folds stretch to about 1.6 times a woman's length, and they get thicker. That is why the voice drops by about an octave and becomes an adult man's low voice.
A low voice comes down to the length of the "strings" deep in the throat.
Voice change is a few years in which the instrument itself is rebuilt.
For why your own voice sounds like someone else on a recording, see "Why does my recorded voice sound like a different person?". For how a column of air starts to sound, see "Why does blowing across a bottle make a "whoo" sound?".
- Rest your fingers lightly near your Adam's apple and slowly glide from a low "ahh" to a high one. You can feel the throat's framework move up a little and the vibration get finer.
- Stretch a rubber band over two fingers and pluck it. Compare the pitch when you spread your fingers to make the band longer with the pitch when you make it shorter. The tension changes too, so you should notice that the note depends on how tight the band is as well as on its length.
- Use an instrument-tuner app on your smartphone to measure the pitch of your family's speaking voices. Between adult men and women you should find a difference of about 1.5 to 2 times.
Speaking pitch rises and falls even within a single sentence. The trick is to measure several times and look at the value that comes up most often.
For those who want to know more ― terms, formulas and links to textbooksFrom junior high science to university specialist courses, each part is labelled with its level
- Junior highCovered in junior high school science
- High schoolCovered in high school physics and biology
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityNot covered in high school; university specialist courses (speech science, acoustics)
- ResearchNot taught as settled fact even at university; questions researchers are working on now
Junior highTerms: this phenomenon has names
- Vocal folds: a pair of folds of muscle and mucous membrane, one on each side inside the larynx. They vibrate to make the raw sound of the voice.
- Fundamental frequency: the number of times the vocal folds open and close per second. It is heard as the pitch of the voice. Its unit is the hertz.
- Voice change (breaking of the voice): the growth of the larynx and vocal folds in puberty, which changes the pitch and resonance of the voice.
Junior highHigh schoolChecking with a formula: calculating voice pitch from vocal fold length
If we treat the vocal folds as a taut string, we can estimate how often they vibrate with the string formula. Let's actually calculate the pitch of a man's and a woman's voice.
| In symbols | f ＝ ( 1 ÷ 2L ) × √( σ ÷ ρ ) ＝ c ÷ 2L |
| In words | Vibrations per second ＝ speed of a wave along the string ÷ (twice the string length) |
| Where it comes from | For a string fixed at both ends, the time for a wave to travel from end to end and back is one vibration (the string's natural vibration, Mersenne's law). The wave speed is set by the balance between the pulling force and the weight of the string. |
| Symbol | Meaning and unit |
| f | Number of times the vocal folds vibrate per second (fundamental frequency), in hertz |
| L | Length of the vibrating part of the vocal folds, in metres |
| σ | Strength of the force pulling the vocal folds, per square metre of cross-section, in pascals |
| ρ | Density of the vocal fold tissue, in kilograms per cubic metre |
| c | Speed of a wave along the vocal folds (the square root of σ÷ρ), in metres per second |
| Vibrating part of an adult man's vocal folds | About 1.6 cm ＝ 0.016 m, it is said |
| Vibrating part of an adult woman's vocal folds | About 1.0 cm ＝ 0.010 m, it is said |
| Vibrating part of a child's vocal folds | Around 0.7 cm ＝ 0.007 m, it is said |
| Wave speed c along the vocal folds | Estimated at about 4 m per second for the tension of speech |
| Man: twice the length | 2 × 0.016 ＝ 0.032 |
| Man's voice pitch | 4 ÷ 0.032 ＝ 125 hertz |
| Woman: twice the length | 2 × 0.010 ＝ 0.02 |
| Woman's voice pitch | 4 ÷ 0.02 ＝ 200 hertz |
| Child: twice the length | 2 × 0.007 ＝ 0.014 |
| Child's voice pitch | 4 ÷ 0.014 ≒ 286 hertz |
| Ratio of woman to man | 200 ÷ 125 ＝ 1.6 |
Even if we assume the same wave speed for men and women, the calculated values match real speaking voices well (about 120 hertz for men, about 200 hertz for women). From a child to an adult man, the figure falls from about 286 to 125, less than half, which works out to almost one octave lower. A ratio of 1.6 between men and women is a pitch difference of about eight notes on a piano, counting white and black keys together.
High schoolHigh school+Voice pitch and voice "resonance" are different things
High schoolThe natural frequency of a string is taught in physics under "vibration of strings". Pulling harder makes the wave faster and the note higher. We can change our pitch while talking because the muscles that pull on the vocal folds fine-tune their tension.
High school+The sound made by the vocal folds resonates in the spaces of the throat and mouth before it comes out. The peaks of resonance in this space are called "formants". A man's vocal tract is longer too, so not only the pitch but also the resonance is lower. This is why a man speaking in a high falsetto still keeps some of a man's vocal character.
UniversityWhy can the vocal folds keep vibrating?
How the vocal folds keep vibrating on the force of breath alone is explained by the "myoelastic-aerodynamic theory". The direction of the air's push on the folds differs between opening and closing. That mismatch adds a little energy to each vibration, and the motion keeps going. The "body-cover theory" treats the vocal folds as two layers, a soft surface "cover" and an inner muscle "body". It was proposed by the Japanese otolaryngologist Minoru Hirano. For research that breaks down the voice differences between men and women into length, thickness and tension, the analysis by Titze is often cited.
📖 Derivation of the formula and further reading: Vocal folds (Wikipedia) ／ Titze (1989) Physiologic and acoustic differences between male and female voices
ResearchWhat is still not clearly understood
- Individual differences in when voice change starts. Hormone levels alone are said to be insufficient to explain the age it starts or the size of the change.
- The evolutionary meaning of a low voice. One hypothesis is that a low voice worked as a signal of body size to others. How much of it is the result of evolution is still debated.
- How the voice changes with age. In old age, men's voices tend to rise slightly and women's to fall slightly. How this relates to changes in tissue is still being studied.
In other words, what this article says is also "an explanation of what is known so far". The string formula is only a rough simplification of living vocal folds.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: "Properties of sound" | Frequency and pitch, string length and sound |
| High school | Physics: "Vibration of strings"; Biology: "Hormones" | The string formula, growth of the larynx in puberty |
| High school+ | Advanced physics: "Resonance" | Formants and voice resonance |
| University | Speech science, acoustics | Myoelastic-aerodynamic theory, body-cover theory |
| Research | Physiology of the voice, evolutionary biology | Individual differences in voice change, the meaning of a low voice |
| ― | Links to daily life | Understanding children whose voices are changing, vocal health |
- I. R. Titze (1989) Physiologic and acoustic differences between male and female voices, The Journal of the Acoustical Society of America
- I. R. Titze, Principles of Voice Production (a textbook on voice production)
- Wikipedia, "Vocal folds" (声帯)
- Wikipedia, "Voice change" (変声)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. If hoarseness or pain in your voice lasts a long time, please consult an ENT (ear, nose and throat) doctor.