The mike will do it for you, right? Wrong.
Have you ever turned up late for someone else's speech? You can hear the sound of the speech (not the individual words) through the closed doors. Within five seconds, you know if the speech is likely to be boring when you go in and find a seat.
If you have an analytical mind, you may find that statement outrageous. But for the vast majority, it makes perfect sense because the impact of a speech is much more in delivery than content. And in a large audience, the speaker is a distant figure, so it's mostly the sound and rhythm that tells our brain to listen or to shut down.
Okay, the microphone is useful, boosting volume and pushing the sound into the corners. But relying on it is a big mistake, because it does nothing to make our brain want to listen.
Now, imagine you're the speaker. To make your large audience want to listen, it needs to hear three things from you.
Project your voice
Audience ears need to hear you making an effort to reach them. That's not just volume. It's the subtle extra tones that say, I want you to hear this. The best mike in the world won't do that for you. (And yes, bigger body language does help.)
Your pitch needs to rise slightly. What's pitch? Literally it means frequency, and that too has nothing to do with volume. To get the difference, think of softly calling out to someone at a small distance: "Wait, John, you forgot your wallet." Your voice is conveying, I want you to get this. The mike won't do that for you.
Pause significantly during your presentation
Make your pauses obvious. Pausing is a good thing even for small audiences - we all know that - but for large audiences it's vital. It conveys, This is important and interesting. Again, the mike won't do that for you. Anyway, the collective ear of a large audience cannot stand wall-to-wall words. Want to be really effective? Turn all your commas and full stops into pauses in which you deliberately make the audience wait for the next phrase. Yes, it's audience manipulation, and they will love it.
I know, if you're new to large audiences, you won't find it easy to just step onto the stage and do all three. So, find some colleagues, turn them into guinea pigs, and practise on them - getting their feedback. They need to listen to your projection, pitch and pause, and you need them tell you how much is too much, and how much is just right. Then your brain will know it's okay to do it.
Does it seem daunting? Think of the rewards of speaking confidently and competently to a large audience. Terrific.