ACR DESCENDED BELOW ALT EXPECTED BY ATC.

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CENTER CLRD US TO DES SO AS TO CROSS 30 MI S OF CVG AT 10000' AND 250 KTS. CLRNC WAS READ BACK TO CENTER AT TIME OF RECEIPT AND AGAIN WHEN WE BEGAN OUR DES. CENTER HANDED US OFF TO CVG APCH WHEN WE WERE PASSING APPROX 14000'. WE CHKED IN WITH APCH SAYING; '(CALL SIGN) OUT OF 14000' FOR 10000';' AND APCH ROGERED OUR CALL. WE THEN REPEATED ASSIGNED ALT SAYING; '...ONE ZERO THOUSAND.' APCH SAID NOTHING UNTIL WE WERE LEVEL AT 10000'/30 MI S OF CVG. THEN APCH SAID; '(CALL SIGN) DES TO 8000' AND YOUR ASSIGNED ALT SHOULD HAVE BEEN 12000'; 11000'; NO; 12000'.' WE TOLD APCH WE HAD READ BACK 10000' 3 TIMES; TWICE TO HIM. HE SAID NOTHING FURTHER; AND WE DESCENDED TO 8000'. WE NEVER DID FIND OUT IF HE MEANT THAT IND CENTER SHOULD HAVE ASSIGNED US 12000' (INSTEAD OF 10000') OR IF HE MEANT WE WERE ASSIGNED 12000' AND WE DESCENDED TO WRONG ALT. ONE EASY WAY TO SOLVE PROBS SUCH AS THIS WOULD BE TO HAVE CTLRS READ BACK ALT ASSIGNMENTS INSTEAD OF ROGERING READBACKS: IE; ATC: '(CALL SIGN) CROSS 30 S OF CVG AT 10000'.' ACFT: '(CALL SIGN) CROSS 30 S OF CVG AT 10000'.' ATC: 'ROGER; 10000'.' THIS ADDS ONE MORE READING SO IF CTLR GIVES AN ASSIGNED ALT; ACFT READS BACK WRONG ALT; CTLR THINKS HE HEARD RIGHT ALT; WHEN HE GIVES ASSIGNED ALT AGAIN; AIR CREW MIGHT PICK UP MISTAKE. SUPPLEMENTAL INFORMATION FROM ACN 80346: PERHAPS 2 MORE DIGITS ADDED TO OUR TRANSPONDER CODE WHICH MUST COINCIDE WITH CTLR BLIP WOULD PROVIDE A MECHANICAL BACKUP. THIS OF COURSE WOULD PROVIDE MORE COCKPIT WORKLOAD AND MORE CLUTTERING UP OF THE RADAR SCREEN; BUT MIGHT SHORTCUT HUMAN FAILURE WITH MECHANICAL BACKUP.