When my class gets too noisy, I switch off all the classroom lights and fans. Once the children find that the room has suddenly turned dim, they look around them. When they see me at the switch plate with my finger on my lips, which is our quiet signal, they know that they have been making lots of noise and it's time to be quiet and carry on with their task. Don't do it too often or it won't be as effective. I like this method because I don't have to bring in an extra resource such as a bell or chime to keep the class quiet.