Fellow speaker,
If you ever do a group activity, you know that it can get pretty noisy after the activity is over. Often people use the opportunity to start chatting and soon you have lost control over the entire activity, trying in vain to get people's attention so you can continue on with your presentation. Instead, after the activity is over, announce to the noisy room, "Clap once if you can hear me," and keep on repeating that until everyone in the room claps and the noise stops.
Clapping,
Tim
