Using your voice for self defense is a great art to learn. It doesn’t take much and can save you a huge amount of grief. A couple weeks ago I was able to use my voice to keep myself out of trouble when someone looked at attacking me (story here). One of the first things new cops learn is how to use their voices to get something done without using violence. In law enforcement the use of force continuum starts with your presence and then goes up to verbal commands, then goes to actually fighting.
You should learn how to use verbal self defense before anything else. It can keep you out of trouble. Any fight you avoid is a good fight. Learning to use your voice is easy, just think of how your parents use to yell at you when you did something wrong. That is the kind of voice you need to find. It sounds crazy, but you should practice in front of a mirror. Does it sound crazy and embarrassing? That is what you will feel like the first time you want to use your voice against someone as a kind of force. So you need to practice.
Start by standing in front of the mirror and say “No, go away!” Do it three or four times until you find that confidence. That voice that makes you cringe. It may take you more then four times, I suggest you try it when no one else is home. Once you can do it without being embarrassed you are ready. It really is that easy.
Now when you are out and about, act quickly when something doesn’t look right. Just use that same voice and same words. Stand up straight and be confident. This won’t stop every attack but it does show that person that you aren’t the easy target they are looking for. Just do it.