Sounds files and photos of vocal folds can be found at: http://www.vowelsandconsonants3e.com/chapter_2.html
Chapter 2: Pitch and Loudness
In over half the world's languages, changing the tone of a word changes its meaning. (In English, you can change from a statement to a question to a demand, but it is the same word.)
Tone can be a difference in pitch (high or low) or in the pitch contour (rising, falling, falling then rising) or a combination of both. These differences are relative, not absolute - a high tone from a man may be lower than the low tone of a woman.
Intonation
Intonation is the pattern of pitch across a phrase or sentence, not just an individual word.
Intonation can convey grammatical differences:
When danger threatens your children, call the police
When danger threatens, your children call the police
Some examples of intonation patterns in English:
- Normal, unemphatic statement: falls at the end
- Question: larger fall
- Yes-no question: small fall at the end, followed by a large rise
- Surprise: small fall at the end, followed by a large rise (i.e. yes-no intonation, used for a normal question)
- Alarm: Large fall at the end, followed by a small rise
(Also, intonation is the difference between all the varieties of "I never said she stole my money" - which word the high pitch falls on changes the implications of the sentence.)
Vocal folds
"The pitch of the voice depends mainly on the tension of the vocal folds", which are two small flaps of tissues in the larynx. Below the vocal folds is the trachea (windpipe).
They cause air in the throat and mouth to vibrate by opening and closing rapidly. When you breathe out, air builds up behind the vocal folds until the pressure is high enough to blow them apart. When the air escapes, the pressure drops, and the vocal folds close again, until enough pressure has build up to repeat.
(The air rushing past sucks the vocal folds together faster - like standing on the edge of a platform when an express train goes past. This is called the Bernoulli effect.)
Higher pitch is caused by stretching the vocal folds, so that they are longer and thinner, and can vibrate more rapidly.
Loudness differences
When we push more air out of our lungs, our speech gets louder. A larger push results in the vocal folds being pushed apart more rapidly, and more air sucks them together faster, so the sound also gets higher.
We tend to tense our vocal folds at the same time, which further increases the pitch.
English is a language which has stress-accent: certain syllables in a word will be stressed (louder, higher, longer, more carefully pronounced). Stress in English is mostly predictable, even if the pattern is not simple, but we also use it to distinguish the meaning of some words:
to insult vs an insult
to record vs a record
to permit vs a permit