A virtual bell is all you need when you want your groups attention. You might be a teacher assembling your students, a child playing a game or a host for a friendly bingo or quiz night, as long as your phone is loud enough, you can just wave it around (tap the bell) and indicate that times up.
We also implemented a little game to pass the time: Every ring gets you points, furthermore if you can tap it without making a sound you get extra points.
Enjoy.