In the event where you trigger the wave, you are going to add another action. Go to System, and then Set Value. For the variable, choose your timer variable, and then for the value, you would enter 20. If you have your timer set up so that it is constantly counting down, this should be all you need to do.
Based on the information you provided, I think this should work, but if it doesn't, let me know more about how you set up your events.