My teacher uses a Noise Meter similar to this one. The Noise Meter is a cool way to remind students the correct noise level that they should be using! You just use the paper clip and move it to the appropriate noise level the students should be using.
This voice levels poster is used in most preschools during my placesment, I think it is an effective way for teachers to control the voices in the kinder. Child also remember the colors and related voice levels that they should use inside and outside.