Intonation

variation. An oscilloscope will give an oscillograph of speech. The frequency will be shown by the closeness of the waves (high frequency will be shown by waves which are closer together).
2. The volume (strength of signal) will be shown by the height of the waves. The height of the note depends on the speed of opening and closing of the vocal cords. More vibrations of the larynx (up to 800 per sec) show up more compact waves.

another person or taking your turn in a conversation: rise and fall are used as a signal for when to speak and when not. Remain at a high pitch if you want to continue talking. A fall shows completion. (See Brazil)
INFORMATION STRUCTURE (See O'Connor): Major stress items pick out the most important words in the sentence: they point to the new/unknown information in the sentence. Michael Halliday has done most work on this.
Note that one function of intonation is stress. The tonic (stressed item) is the item which has the greatest amount of pitch movement on it.
Implications for teaching English pronunciation

STRESS rather than RISE & FALL since there is a massive difference between how one person and another perceives an utterance. You need a machine to determine whether it's a rise or a fall.
At higher levels - for example, pronunciation sessions for learners involved in the language of negotiation or presentation in fields such as business or education, emphasis should also be given to TOPIC STRUCTURE - also related to turn-taking. Topic Switching: Start high. When people switch tack, they mark it with their voice.
[a] CONCLUSION: Teachable items are
Sentence STRESS
Contrastive STRESS

the end of the sentence. This means that the speaker’s voice gets just a little higher as they finish the sentence. Below are some example sentences to practice.
Are you ready to go?
Could you please repeat that?
Is it cold today?
Am I interrupting?
Are they cheaper in Japan?
Are you a girl?
Should we sell the fish?
Is it time to go?

rise/fall intonation at the end of the sentence. This means that the speaker's voice goes higher for a moment and then drops lower to end the sentence.
What time is it?
What’s your name?
Where do you live?
How do you get to school?
When should we go?
Who ate the apple pie?
Where can I find a bank?
How old is Eric?
How did they win?