#1 Too Noisy- free
http://www.academyapps.net/toonoisyliteonline/
The "Too Noisy" website is in beta and has been designed to assist a teacher keep control of general noise levels in a classroom using a visual stimulus. As the noise level in a classroom increases beyond an acceptable level the noise level meter dynamically indicates the level of noise, and the background graphics within the program change to reflect the noise levels. Teachers are able to control the sensitivity of the app by using the sensitivity slider within the site itself so the tool can be used for all types of lessons.
#2 Bouncy Balls- free
https://bouncyballs.org/
Bouncy Balls is a website that activates your microphone and detects noise level. The more noise in the room, the more the balls bounce. The quieter the room is, the more still the balls remain.
#3 Silent Light app
$2.99
Silent Light is a very colorful stoplight, which changes from green to orange to red, based on the noise level in the classroom. The classroom can earn points by being on-task and quiet while Silent Light is running. The noise sensitivity is fully programmable.