ZOB CTLR RPTS LOSS OF SEPARATION DUE TO DSCNT CLRNC MISUNDERSTANDING.

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ACFT Y WAS ON FREQ INBOUND TO AGC ARPT AT FL210. ACFT X WAS ON THE SAME RTE INBOUND TO PITTSBURGH AT 150. I GAVE ACFT Y A DSCNT TO 170. HE READ BACK 120 AND WAS CORRECTED IMMEDIATELY. A FEW MINS LATER; ACFT Y WAS GIVEN A DSCNT TO 160. HE READ BACK AN ALT OF 60 AND THE READBACK WAS NOT CAUGHT. MEANWHILE; ACFT X WAS GIVEN A DSCNT OUT OF 150 TO 90. ACFT X WAS GIVEN THE TFC CALL OF THE CITATION DSNDING TO 160. ACFT X RPTED ACFT Y IN SIGHT PASSING OFF HIS R SIDE. ACFT Y WAS NOTICED DSNDING OUT OF 156. I ASKED ACFT Y HIS ALT. HE REPLIED THAT HE WAS DSNDING TO 60. I IMMEDIATELY CLBED HIM BACK UP TO 160 AND EXPEDITED ACFT X OUT OF 140 TO SEPARATE THE 2 PLANES. THE CONFLICT ALERT HAD GONE OFF. ACFT Y WAS IN FRONT OF ACFT X BY 3.96 MI 150 KTS FASTER BUT TOO LATE TO ESTABLISH REQUIRED SEPARATION. BOTH ACFT WERE ON FREQ 128.15 WHICH HAS BEEN DOCUMENTED FOR SOME TIME TO HAVE PROBS OF 'DEAD SPOTS' IN THE ACO AREA WHICH COULD HAVE BEEN A POSSIBLE FACTOR FOR THE ACFT Y HEARING THE WRONG ALT 2 TIMES. OR THE PLT JUST WASN'T PAYING ATTN. IN MY OPINION; THE PLT'S MISSED READBACK OF 60 SOUNDED AMBIGUOUS ON THE TAPES.