I posted something that sounds like what you describe. It's a "Classroom Noise Level Chart." Take a look to see if it's something you can use... or maybe it will give you a starting point for something else you'd like to create. It's in MS Word so you can modify it to fit your needs.
I have it on my magnetic white board. I keep an arrow (attached with a magnet) at the level I want the class to be using.
Classroom Noise Control Poster