A TURB ENCOUNTER IN A B757 RESULTS IN MINOR PAX AND CREW INJURY EVEN THOUGH PREPARATIONS WERE MADE FOR ENTRY INTO THE TURBULENT AREA.

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LEVEL CRUISE FL390 OVER CENTRAL ALABAMA. DEVIATING W AROUND AN AREA OF TSTMS. APPROX 80 MI PRIOR TO THE AREA OF STORMS I MADE A PA TO THE PAX THAT I WAS TURNING ON THE SEAT BELT SIGN BECAUSE WE WERE COMING TO AN AREA OF SCATTERED TSTMS; THAT THERE WOULD BE TURB AND THAT THEY SHOULD RETURN TO THEIR SEATS AND FASTEN THEIR SEAT BELTS. I TOLD THEM THEY SHOULD REMAIN SEATED WITH THEIR BELTS FASTENED UNTIL I TOLD THEM IT WOULD BE OK TO MOVE ABOUT AGAIN. I STATED THIS WOULD LAST ABOUT 20-30 MINS. RIDE CONDITIONS WERE CONTINUOUS LIGHT CHOP WITH OCCASIONAL MODERATE CHOP AS WE NEARED THE AREA. I DEVIATED W ABOUT 25 DEGS TO GO AROUND THE WESTERN MOST CELL. AS WE WERE APCHING THE END AND ABOUT TO MAKE MY FIRST TURN BACK TO CGI I DID A RADAR TILT DOWN TO LOOK FOR LOW CELLS AND NOTHING SHOWED ON RADAR. WE WERE 20 MI W OF THE LAST CELL IN HIGH CIRRUS AND I MADE A 10 DEG TURN TO THE N. WE WERE IN AND OUT OF THE TOPS OF THE CIRRUS AND THE CELL WAS POKING THROUGH THE TOP ANOTHER FEW THOUSAND FT. NOTHING AT 12 O'CLOCK POS. ABOUT 3 MINS LATER WE ENCOUNTERED AN AREA OF MODERATE TURB FOR ABOUT 20 SECONDS. IT FELT LIKE WE WERE GOING OVER THE TOP OF AN UNKNOWN CELL. THERE WAS AN INITIAL UPDRAFT WITH AN INCREASE IN TURB. THE AUTOPLT TRIMMED DOWN AND REDUCED THRUST. THEN THE UPDRAFT STOPPED AND THE PLANE DSNDED CAUSING PAX AND FLT ATTENDANTS AND PLTS TO 'FLOAT.' I DISCONNECTED THE AUTOPLT AND HAND FLEW THE PLANE UNTIL WE EXITED THE TURB ABOUT 20 SECONDS LATER. THE PLANE WAS CONTROLLABLE EVEN THOUGH I LET THE ALT VARY SO WE WOULDN'T GET ANOTHER 'FLOAT.' ONE LADY HIT HER HEAD ON THE OVERHEAD. I WAS INFORMED BY THE FLT ATTENDANT ABOUT 10 MINS LATER THAT MAYBE THE PAX NEEDED TO SEE A PARAMEDIC IN ZZZ. ONE MALE PAX IN THE FORWARD LAVATORY; FLT ATTENDANTS ADVISED HIM NOT TO GO IN THE LAVATORY BECAUSE THE SEAT BELT SIGN WAS ON; SUFFERED AN UNKNOWN INJURY. I WAS NOT INFORMED HE NEEDED ASSISTANCE UNTIL AFTER WE LANDED. BOTH PAX WALKED OFF THE PLANE UNDER THEIR OWN PWR. 2 FLT ATTENDANTS SUFFERED MINOR INJURIES AND THEY DECIDED TO REMOVE THEMSELVES FROM THE NEXT FLT. THEY DID NOT NEED PARAMEDIC ASSISTANCE AND PROCEEDED TO INFLT. PARAMEDICS MET THE FLT UPON ARR IN ZZZ. I DON'T KNOW IF EITHER PAX WENT TO THE HOSPITAL. NO LOGBOOK ENTRY WAS MADE AS I DETERMINED THE MAX TURB ENCOUNTERED WAS MODERATE.