The rise and fall of sound waves
Traditional rhetorical conventions had allowed for the difference in tone between addressing a large assemblage in a public arena, where strong lungs and broad strokes were required, and pleading a case in a courtroom, which called for careful attention to detail and a more soft-spoken, closely argued, intimate approach. (Eisenstein, "Some Features of Print Culture," Writing Material 129)
We just learned about these different styles of speech in my honors presentation course. A quiet, almost whispered tone of voice leads your audience to believe that you're sharing a secret with them, which immediately makes whatever you're saying more interesting and believable. A booming roar, on the other hand, gets their attention by catching them off guard with a sudden shock and makes your words seem more powerful and important. Alternating between highs and lows like these during a speech or presentation is a surefire way to captivate your audience, no matter how boring your presentation material might be. Delivery is everything.