Disclosure: the developers of this app are currently advertising on iPadApps4School.com. That said, I would have written about the app even if they were not advertisers.
Too Noisy is an iPad app (free and pro versions available) designed to help students learn to recognize the appropriate volume for conversations. The app measures the volume of the noise in a room and displays a meter indicating whether or not the the room is too noisy. Too Noisy has four situation settings that you can use in your classroom; silent, quiet, group, and class. You can adjust the sensitivity of the meter for each situation.
The pro version of Too Noisy ($2.99) offers additional background themes for the meter display, star awards that you can give to the class for maintaining an appropriate volume, and audible alarms to alert your students when they’re being too noisy. The pro version also removes the advertisements that pop-up when you change screens in the app.
Whether you use the free version or the pro version, projecting the Too Noisy meter for all of your students to see could be a good way to help them understand the appropriate volume for conversations when working on group activities in your classroom.