Double Hydco
Do you think passengers listen to let alone respond to PA's? The only way to get most people in the cabin to comply is with physical intervention by the CC and even then there can be resistance, especially where PED's and personal headphones are concerned. Remember, we live in a world where personal connectivity is seemingly as important as the air we breath these days, and not only amongst the younger generation.
I watched a passenger last week argue with a purser over whether she was using a tablet (hand held electronic device but with detachable keyboard) or a laptop (which would have to be stowed for takeoff and landing). Watch the number of passengers (and often those in premium cabins) who unbuckle and stand up as soon as the aircraft has come to a standstill but before the seat belt signs are extinguished, despite being told to remain seated until the signs are off. A quick tap on the brakes used to do the trick but these days, even though the pax are not compliant, it's not worth the risk of litigation.