LGT CLEARED TO DESCENT TO 11000'. READBACK AND DESCENDED TO 10000'.

Narrative

F/O FLYING; CAPT HANDLING RADIOS. ZDC CLRED US TO CROSS BUZZY INTXN AT 10000' AND 250 KTS. I READ BACK BUZZY AT 10000'/250 KTS ALONG WITH FLT IDENT. 10000' WAS SET IN THE ALT ALERT AND THE REQUIRED XING AT BUZZY INTXN WAS ACCOMPLISHED. AFTER PASSING BUZZY INTXN; ZDC HANDED US OFF TO RDU APCH CTL. I CHKED IN WITH APPROPRIATE CALL SIGN AND OUR ALT: 10000'. APCH ACKNOWLEDGED OUR XMISSION AND ASKED US TO REPEAT OUR ALT; WHICH I DID. THE CTLR SAID 'ROGER;' AND A MINUTE OR SO LATER CAME BACK AND SAID ZDC HAD ONLY CLRED US TO 11000'; BUT SINCE WE WERE AT 10000' IT WAS OK. THE F/O AND F/E BOTH COMMENTED AT THIS TIME THAT THEY BOTH HEARD 10000' AND THE READ BACK OF 10000' AND NOTED THE 10000' SET IN THE ALT ALERT. THE PROBABLE CAUSE WAS THE CTLR SAYING 10000' AND THINKING HE SAID 11000'; AND/OR THE PLTS HEARING (OR THINKING THEY HEARD) 10000' AND READING BACK 10000' AND IF THERE WAS AN ERROR IT WAS NOT DETECTED BY ZDC; NOR CORRECTED. THE CORRECTIVE SOLUTION TO THIS PROB IS FOR ALL PARTIES TO SPEAK CLEARLY AND LISTEN TO EACH OTHER'S XMISSION AND MAKE SURE THEY ALL AGREE WITH WHAT WAS SAID.