FLC MISTAKENLY BELIEVES THEY LOST CABIN PRESSURIZATION.

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I FELT A RAPID CLB IN CABIN PRESSURIZATION AND LOOKED BACK TO ENGINEER'S PANEL. HE EXCLAIMED 'I'VE LOST THE CABIN' WHICH I TOOK TO MEAN A COMPLETE DEPRESSURIZATION. THE CAPT TOLD ME TO BEGIN A DSCNT. I PUT ON MY OXYGEN MASK AND NOSED THE ACFT OVER WHILE LOOKING FOR TFC ON TCASII THINKING WE HAD A COMPLETE DEPRESSURIZATION; I ANNOUNCED ON THE PA FOR PAX TO PUT ON OXYGEN MASKS. THE MASKS HAD NOT DEPLOYED BECAUSE WE HADN'T 'LOST THE CABIN;' IT WAS JUST CLBING RAPIDLY. THE #1 FLT ATTENDANT INFORMED THE ENGINEER THE MASKS WERE NOT DOWN SO HE DEPLOYED THEM MANUALLY WITHOUT REALIZING THE CABIN WASN'T OVER 10000 FT. HE HAD TRIED TO ASK THE CAPT IF HE SHOULD; BUT IT WAS IMPOSSIBLE TO COM WITH MASKS ON AND HOT MIKES. THE PROC TO PUT MIKES TO EMER CAUSED THE PROB BECAUSE OF THE INABILITY FOR THE CAPT TO HEAR EITHER CTR WHEN HE REQUESTED EMER DSCNT OR THE ENGINEER. WE DSNDED FROM FL240 TO 18000 FT WITHOUT KNOWING IF CTR ACKNOWLEDGED OUR INTENTIONS. WE CONTINUED FLT AT 10000 FT WITH NO FURTHER INCIDENT.