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I'll be the first to admit that I take an almost macho pride in my ability to project my voice. I was born this way. Even when I wasn't trying, I could be heard talking a mile away. And yes, I got in trouble a lot in school for being too loud.
However, I've learned my lesson about voice overuse, and I'm happy to say that if a microphone is available and a group is over about 40 people, I will use it. Here's why.
Even if you can be heard in the last row without shouting, you will be straining your voice to some degree if you have to give an hour-long or longer presentation to a large group. In order to be heard, you will have to make the effort to project your voice the whole time. In addition, you will be close to your maximum range in terms of volume, without yelling, and you won't have a lot of options with vocal variety. For example, if you want to make a point by bringing your voice down low, the last row may not hear you.
If you use a microphone, however, you have much more vocal control and the ability to play with vocal variety. You still have to project, even with a microphone, but not to the degree that you have to without it.
You can also hear yourself better and get more of a sense of what the audience is hearing when you use a microphone, because your voice is playing over a PA system. This allows you more vocal creativity and control, as I mentioned above, because you don't have to work as hard to project. And if you want to bring your voice down low, you can -- and still be heard.
Even if you, like me, have a naturally projecting voice, please consider not only the health of your vocal cords, but the ears of the audience when you decide whether or not to use a microphone. The last row should not be struggling to hear you, and you should be able to use the full range and abilities of your voice to make your message more compelling.
Don't be macho. Use a mic.
And here's a handout with more info on using a microphone.