With me, meltdowns are something that only happens in my room. I'll be frustrated and stressed out at the same time, then all of a sudden I'll feel I have to go, sit in the back of my room, and meltdown. This is the ONLY place it will happen. (Though place didn't matter when I was younger.) I will drop to the floor, cry, sometimes hit things etc. If ANYONE is there, weather they mean well or not, it will only make it worse, and end up making me angry (Or more angry, if that person was the cause of the meltdown) at that person. It is the only time I can even conceive causing physical harm to a person. Often, if I've been yelled at by my parents and it proves to be too much, I will go into meltdown at the back of my room. But then, say 10 mins later, Mum might come in (Dad knows to leave me alone) and ask me to do something, or even yell at me more. Worst, she may even ask me to speak, which is near impossible at times like that. In these cases, the frustration becomes too much to handle, and I end up whacking my head against the wall particularly hard until the person has left and I calm down a little.
It's usually extremely rare that meltdowns happen for me. Perhaps once every 3 months, then I'll have 3-4 in one week, then nothing for months again.
My parents found it extremely dificult when I was a child. I would scream and cry for almost an hour on end when I was upset and they'd have NO idea what was going on, as I'd refuse to speak. Meltdowns have really changed since then. Now, meltdowns happen with relatively little noise, as I usually like to be quiet so nobody decides to walk in.