How we change what others think, feel, believe and do
Modulating your voice means moving the pitch up and down. The alternative is to keep an even pitch throughout the sentence, which at best will sound odd and at worst dull, boring or confusing.
A change in frequency in speaking tone provides the listener with a signal that something is happening. Modulation can thus be used to enhance emphasis in words, with rising and lowering tones adding subtle meaning.
Going down for authority
A downwards pitch adds authority to a sentence. It gives imperative that seeks unquestioning acceptance. To go down, you need to be up, so gradually increase the pitch of your voice beforehand to give yourself the ability to suddenly drop the pitch on an important point. The pitch of the sentence may thus go up and down several times as key words are emphasized.
You know, it has now become important that you work harder so you can support your family.
Looking back at this sentence, the emphasized, low-pitched words stand out and combine together to form the key message that you want to get across.
Going up for engagement
An upwards pitch adds interest to a sentence. It questions and provokes engagement and response. Going up starts from a medium or low pitch. You thus cannot stay at this involving level for long. Sometimes when people want to invite the other person they will use a fragmented sentence that contains several rising-pitch words to repeated the invitation.
I was just wondering...whether you would like...maybe to come today?
A regular up and down modulation creates a pattern of repetition that is not initially recognized and can thus be used to 'creep up on' the other person.
Pounding home a point
When someone is making a point about something, their pitch will go up and down as they repeatedly pound the point of authority. This may well be done in a rhythmic way, accompanied with pounding of a finger or fist to exaggerate further. The repetition may also emphasized further by repeating a single word or phrase.
I was never there, I would never do a thing like that and I have never been so insulted in my entire life.
Repetition and modulation is a common element of hypnotic speech. The modulation pulls people in with the impression of something worth listening to, then the repetition sends them into a trance. This can be a deliberate method for public speakers that can be used to lull a whole audience into a suggestible state before the real message is slipped past the weakened cognitive defenses.
Listen to a beginner actor or salesperson: as they concentrate on the words, the modulation goes out of the window and they sound odd and wooden. Modulation makes meaning.
Listen to the TV presenter introducing a program or reading the news. Notice how their voice goes up and down very deliberately. They know very well what they are doing.
Notice also how the TV presenter moves their head and their in time with the modulation. Their body is often obscured by a table or even off-camera. They must thus use just their voice and their head to convey meaning.