ACR ON DSCNT HAS PRESSURIZATION PROBLEM. INCREASE RATE OF DSCNT; O2 MASKS DROP.

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ON DSNDING INTO TRI WHILE PASSING THROUGH 20100 MSL FELT AND SAW CABIN CLBING FULL SCALE UP (CABIN ALT APPROX 5000 FT). TOLD COPLT WHO WAS FLYING TO INCREASE RATE OF DSCNT TO 6000 FPM TO PREVIOUSLY CLRED ALT OF 7000 FT MSL. CHKED BOTH PACKS ON AND OUTFLOW VALVE SHOWED FULLY CLOSED. AS AIRPLANE PASSED THROUGH 14000+ CABIN REACHED 14000 FT AND O2 MASKS DROPPED IN PAX CABIN. 1 MIN LATER WE WERE BELOW 10000 FT AND MADE PA FOR PAX TO TAKE OFF O2 MASKS. LATER FLT ATTENDANT SAID THEY HEARD LOUD HISSING NOISE AND FELT CABIN FLOOR MOVE UNDER THEIR FT IN AFT CABIN. ALSO SAID THEY HAD TO TELL EACH PAX INDIVIDUALLY TO PUT MASK ON BECAUSE NO PAX PUT THEM ON WHEN THEY DROPPED. POST-FLT LOOKED FOR RUPTURED DUCTS BUT COULD NOT FIND ANY. CALLED MAINT COORDINATOR LATER. THEY SAID THEY HAD REPLACED THE OUTFLOW VALVE. SINCE CABIN CLBED APPROX 5000+ FPM WOULD HAVE BEEN A HANDFUL AT HIGHER ALT WITHOUT DSCNT CLRNC ON BUSY JETWAY OR ON FULL AIRPLANE WITH PAX NOT PUTTING MASKS ON SINCE THERE WAS ONLY 21 PAX. IN THIS CASE MORE OR LESS ROUTINE.