Unless you are doing some type of surprise or big rah rah opening in your art of 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 an art of public 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 or volume 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.
Remember in the art of public speaking you always and in all ways plan ahead.
When you present in public . . . start low.