ACR Y NON ADHERENCE TO ATC CLRNC UNAUTHORIZED DSCNT FROM ASSIGNED ALT. PLTDEV. CLRNC REPLY TO WRONG CALL SIGN SAME COMPANY.

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I WAS WORKING APCH WITH 8-10 ACFT ON MY FREQ. TFC WAS MODERATE TO HVY. MY JOB IS TO SEQUENCE ACFT ON DOWNWIND AND BASE LEGS SO AS THE FINAL CTLR MAY VECTOR ACFT TO THE FINAL. I TURNED ACR X HVY 'LEFT TO 265 DEGS; DSND TO 7000 FT.' ACR Y; SAME COMPANY; READ BACK THE INSTRUCTIONS AS 'Y HDG 260 DEGS; DOWN TO 7.' THE XMISSION WAS QUICK AND NOT CLR. I THEN OBSERVED ACR Y ALT AT 10700 FT. I XMITTED TO ACR Y 'YOUR ALT SHOULD BE 12000 FT.' THE ACFT RESPONDED BY SAYING 'I READ BACK 7500 FT.' NO OTHER ACFT WERE INVOLVED. MY AIRSPACE LOWER LIMITS STOP AT 11000 FT. NO EVASIVE ACTION WAS REQUIRED. THE FACTORS THAT CONTRIBUTED TO THIS WERE CTLR AND LACK OF PLT AWARENESS. THE SITUATION OCCURRED BECAUSE OF PLT AWARENESS AND IMPROPER RADIO COM BY THE ACFT. MY SUGGESTION IS FOR PLT AS WELL AS CTLRS SPEAK SLOWER AND CLRER SO AS LESS READBACK ERRORS OCCUR. SUPPLEMENTAL INFO FROM ACN 225541: IN THE APCH PHASE; DOWNWIND FOR LNDG ON RWY 9R AT ATL; WAS GIVEN A TURN TO 265 DEGS AND DSCNT FROM 12000 FT TO 7000 FT. I READ BACK 260 DEGS AND 7000 FT. THE CTLR CAME BACK; TURN TO 265 DEGS. I 'ROGERED' IT. CAPT WAS ON RADIO #2 TALKING TO COMPANY AT THIS TIME. AT 10300 FT; THE CTLR SAID 'ACR Y; YOUR ASSIGNED ALT WAS 12000 FT; CLB TO 12000 FT.' I IMMEDIATELY CLBED TO 11000 FT AS THE CTLR TOLD US TO LEVEL AT 11000 FT. TCASII NEVER 'ISSUED' AN ALERT AND AT NO TIME DID I FEEL A CONFLICT WAS IMMINENT. CONTRIBUTING FURTHER WAS 'CLUTTER' ON THE RADIO FROM OTHER ACFT AND ME FLYING AND TALKING AT THE SAME TIME; EVEN THOUGH THE AUTOPLT WAS ON.