LESS THAN STANDARD SEPARATION BETWEEN MLT AND ACR. OPERATIONAL ERROR.

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I WAS GIVING AN OJT EVALUATION ON SECTOR R51. THE DEVELOPMENTAL WAS WORKING A HEAVY VOL OF TFC WITH THE ASSISTANCE OF A HDOF MAN AND D-SIDE. MLT X CHKED ON AT FL180 AND WAS DSNDED TO 16000'. AT THE SAME TIME WE HAD A SECOND FREQ UP ON THE OVERHEAD SPEAKER THAT WAS EMITTING STATIC AND AN OCCASIONALLY ACFT XMISSION. THE READBACK WAS COVERED BY BACKGROUND NOISE (SECOND FREQ; HDOF AND D-SIDE COORD AND SUPVR AND FLOW CTLS TALKING ABOUT SECTOR). THE CLRNC WAS ISSUED A SECOND TIME AND ACFT WAS ISSUED AND FREQ CHANGEOVER TO SECTOR R53. ABOUT 1 MIN LATER THE C/A WENT OFF BTWN MIL X AND ACR Y WITH THE 2 CONVERGING. I THOUGHT THAT R53 WAS RESOLVING THE SITUATION; BUT AFTER A FEW RADAR UPDATES I TOLD THE SUPVR TO CHK ON THE SITUATION IN THE OTHER SECTOR. I CANNOT BELIEVE THAT THE READBACK OF MLT X DSNDING TO 13000' COULD HAVE BEEN MISSED BY THE RADAR DEVELOPMENTAL; THE HDOF PERSON; THE D-SIDE AND MYSELF W/O THERE BEING SOME ADDITIONAL FACTOR; WHICH COULD HAVE BEEN EITHER THE SPLIT FREQ OVERLAP; STATIC OR ADDITIONAL BACKGROUND NOISE. SUPPLEMENTAL INFO FROM ACN 99420: MLT X ENRTE TO ABE CHKED ON MY FREQ AT FL180. THE ACFT WAS DSNDED TO 16000' AND READ BACK 13000'. AGAIN THE CLRNC WAS ISSUED TO DSND TO 16000'. THE ACFT AGAIN READ BACK 13000'--THE READBACK WAS MISSED. THE ADJACENT SECTOR ACCEPTED THE HDOF BELIEVING THE ACFT WAS DSNDING TO 16000'. MLT X WAS SWITCHED TO FREQ 120.95; BUT NEVER ACKNOWLEDGED THE CHANGE. THE RECEIVING SECTOR HAD TFC AT 14000' AND TOOK EVASIVE ACTION WITH THAT ACFT. MLT X HAD TO BE CALLED 2 MORE TIMES BEFORE HE ACCEPTED THE FREQ CHANGE.