An EMB145 dispatched with the automatic pressurization controller MEL'ed; was unable to control the cabin in manual mode. An emergency was declared but because of oxygen mask communications; ATC did not clearly understand as the aircraft descended to 10;000 FT.

Narrative

The aircraft we took possession of on Dec/08 had an MEL that stated the automatic cabin pressure controller was inoperative and we would have to manually operate the cabin pressure during the flight. During the initial climb; we were able to operate the cabin normally. Passing through FL180 the cabin pressure began to rise and quickly climbed above 10;000 FT cabin altitude; we were unable to control it so we closed the outflow valve completely and the pressure stabilized momentarily. We donned our oxygen masks and began a descent to a lower altitude. ATC cleared us to 13;000 FT. We descended and had not yet declared an emergency. We had requested 10;000 FT; but were not given it by ATC. I was then told by the Captain to declare an emergency. I did so; and we descended to 10;000 FT. Apparently; ATC did not realize we had declared an emergency and that is what caused the confusion. The confusion most likely resulted with ATC because we were wearing oxygen masks and it may have been hard to understand one another. Supplemental information from ACN 817421: At 15;000 FT we noticed that the cabin pressure wasn't keeping up at the book recommended 600 FPM. We increased cabin manual control to control differential to no avail. Cabin altitude increased; leveled at FL180. Cabin continued to climb. When cabin altitude reached 10;400 FT; declared emergency memory items and descended to 10;000 FT.