Unless you are doing some type of surprise or big rah rah opening in your public speaking engagement, consciously start out with a slightly lower than normal voice inflection. In the excitement or nervousness that many of us feel at the beginning of a speaking engagement, it is easy to go the other way and start out with a voice inflection that is too high.
The problem with this is that if you are already at a high range when you start, you have nowhere to go as you attempt to crescendo the audience to a big peak of excitement at the end of your program.
When you present in public . . . start low.