I was on this flight, severe vibration and a loud vibrating whooshing sound and then plummeting turbulence came out of no where, many passengers and crew hit their heads on the roof, overhead compartments opened and our bags from under the seat in front of us ended up behind us. There were also smoke alarms going off at the rear of the plane.Crew were initially running down the isles to see if there was any fire but said it was a false alarm due to turbulence, it would turn off and then start again the whole way into landing. It was very scary to say the least. Crew said we hit the wake of another plane, I don't know but I'm thankful we landed safely.