Also:
The internet video showing a little old lady hitting a bumper with her purse and setting off an airbag was a hoax. There are protocols built into the airbag system.
1) Vehicle must be moving for the airbag system to arm.
2) The sensors don't detect banging, crunching, or anything like that. They're simple decelerometers.
If the sensor is decelerated more than a preset amount, it makes an electrical connection that tells the airbag controller "We're slowing down faster than the brakes can achieve." The original systems had a centrally mounted arming sensor and three sensors on the front bumper. If the vehicle was in motion and the central sensor and any one of the three bumper sensors saw a too-rapid deceleration, it would fire the airbag. The systems proved so reliable that the second gen systems only had one sensor on the front bumper. That was about the time I left being a dealership technician, but I've heard the newer systems only have the central sensor.