ZLA CTLR DESCRIBED NEAR LOSS OF SEPARATION WHEN FLT CREW RESPONDED TO WRONG ALT CLRNC; CTLR FAILED TO HEAR INCORRECT READBACK.

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I HAD RECEIVED A BRIEFING TO TAKE THE SECTOR FROM THE PREVIOUS R-SIDE CTLR. HE DSNDED ACFT X; AN LJ60; TO FL340. THE PLT CORRECTLY RESPONDED TO THIS CLRNC. I THEN TOOK OVER THE SECTOR AND HAD ACFT Y; A B752 CLBING TO FL310. WHEN ACFT Y CROSSED THE SECTOR 38 BOUNDARY I CLBED HIM TO FL330 AND THE PLT RESPONDED CORRECTLY. I THEN DSNDED ACFT Z FROM FL310 TO FL240; CALLING THE ACFT 'ACFT Z;' AND I HEARD THE PLT RESPOND CORRECTLY. WHEN ACFT X AND ACFT Y WERE 20 MI APART I CALLED THE TFC TO ACFT X AS A B757 CLBING TO FL330 AND TOLD HIM TO EXPECT LOWER WHEN CLR. THE PLT SAID HE WAS LOOKING. I WAS ABOUT TO CALL THE TFC TO ACFT Y WHEN I NOTICED ACFT X'S MODE C INDICATE FL335. I ASKED ACFT X TO CONFIRM LEVEL AT FL340; BUT I FELT THAT THESE 2 ACFT COULD COLLIDE AND WITHOUT WAITING FOR A RESPONSE I INSTRUCTED ACFT X TO TURN 90 DEGS R IMMEDIATELY AND ACKNOWLEDGE WITH AN IDENT. IN THE SAME BREATH I INSTRUCTED ACFT Y TO TURN 90 DEGS R IMMEDIATELY. WHEN I UN-KEYED I GOT A BLOCKED RESPONSE AND REPEATED THE CLRNC TO ACFT Y AND THE PLT RESPONDED. WHEN THE ACFT WERE LATERALLY SEPARATED I TURNED ACFT Y TO A 060 DEG HDG FOR ADDITIONAL TFC AND LATER CLRED HIM TO INW. I CLRED ACFT X DIRECT HEC. I FELT THIS WAS A PLTDEV ON ACFT X'S PART. THE PLT WAS INSTRUCTED TO CALL IN TO ZLA ON THE NEXT FREQ. IT TOOK ABOUT 20 MINS TO RELIEVE ME FROM THE SECTOR AND AFTER I WAS RELIEVED I TOOK A WALK AROUND THE FACILITY PARKING LOT TO WORK OUT SOME OF THE ADRENALINE THAT WAS IN MY SYS. WHEN I RETURNED I WAS TOLD BY THE OPS MGR THAT THEY HAD CONFIRMED THAT ACFT X WAS ASSIGNED AND ACKNOWLEDGED FOR FL340 AND THAT THEY WERE FILING A PLTDEV AGAINST THE PLT. I WOULD NEED TO WRITE A STATEMENT AND THE OM SET UP THE FALCON SYS FOR REPLAY WITH AUDIO. THE OM ASKED IF I WOULD LIKE A UNION REP AND I SAID AFFIRMATIVE AND WHILE WAITING FOR THE REP I PLAYED THE FALCON DISPLAY. I THOUGHT I HEARD SOME NOISE AFTER ACFT Z RESPONDED TO MY FL240 CLRNC AND I REPLAYED IT AGAIN AND TURNED UP THE VOLUME. YOU CAN HEAR ACFT Z RESPONDING TO THE CLRNC AND IF YOU LISTEN CAREFULLY THERE IS AN 'ACR X 240.' THIS WAS OBVIOUSLY ACFT X RESPONDING TO ACFT Z'S CLRNC. I ASKED THE OM TO LISTEN AND HE CONFIRMED WHAT I HEARD ALTHOUGH HE WAS SURPRISED SINCE BOTH HE AND MY SUPVR LISTENED TO THE FALCON AND NEITHER ONE HEARD ACFT X RESPONDING. SUBSEQUENTLY OTHER MGRS HAVE LISTENED TO AUDIO AND THEY ALSO MISSED THIS READBACK. THE OM ASKED ME NOT TO TAKE THIS PERSONALLY; BUT SINCE I HAD MISSED THE READBACK IF A LOSS OF SEPARATION HAD OCCURRED I WOULD BE RESPONSIBLE. THE PLTDEV WAS THEN CANCELED AND THE PLT WAS INFORMED. THE OM SAID HE HAD A LONG DISCUSSION WITH THE PLT ABOUT THE BAD READBACK SINCE ACFT Z AND ACFT X SOUND NOTHING ALIKE. I INFORMED THE OM THAT I AM NOT CERTAIN THAT ACFT X'S BAD READBACK EVER MADE IT THROUGH THE ELECTRONICS TO MY HEADSET. I NEVER LET NOISE; HETERODYNE; OR GENERALLY POOR QUALITY READBACKS GO BY WITHOUT QUESTIONING THE PLTS. AS A GENERAL OBSERVATION PLTS ARE BECOMING VERY LAID BACK LATELY WITH THEIR RADIOS. I AM CONSTANTLY NEEDING TO REPEAT MYSELF AND CORRECT READBACKS. THEY ARE VERY BAD WITH NOT USING THEIR CALL SIGNS AND WITH MAKING UP THEIR OWN POOR PHRASEOLOGY. THERE NEEDS TO BE SOME EMPHASIS PLACED ON GOOD RADIO COMS WITH THE PLT THROUGH THEIR OPERATOR/OWNERS AND WITH THE FAA IN GENERAL. IF ACFT X HAD BEEN PAYING ATTN THIS NMAC WOULD NEVER HAVE OCCURRED. SUPPLEMENTAL INFO FROM ACN 758378: I WAS PF OF ACFT X. WE WERE DSNDING FROM FL400 AND GIVEN INTERMEDIARY DSCNTS. TO THE BEST OF MY RECOLLECTION; THE LAST CLRNC ATC GAVE US WAS TO DSND TO FL240. I HEARD THE PNF CONFIRM FL240 WITH ATC. FL240 WAS ROLLED INTO OUR ALT SELECTOR TO WHICH I REITERATED; 240; ALT SELECT. WE WERE ROUGHLY AT FL360 AT THAT POINT. I SET UP THE VNAV IN ANTICIPATION OF DSNDING TO THE NEXT 12000 FT RESTR AND NOTICED IT WOULD TAKE A 2000 FPM DSCNT TO COMPLY. AT ABOUT 33700 FT; ATC INFORMED US THAT OUR TFC WOULD STOP AT FL330 AND THAT WE COULD EXPECT LOWER AFTER PASSING TFC. AT THAT MOMENT WE REALIZED SOMETHING WAS WRONG. WE WERE CLOSING THE DISTANCE BTWN OUR TFC AND DSNDING TO THEIR ALT. WE DISENGAGED THE AUTOPLT AND STARTED A R TURN. JUST THEN; ATC COMMANDED A R 90 DEG TURN. TFC WAS GIVEN THE SAME INSTRUCTION. WE GOT A TA ALERT IN THE TURN AND CONTINUED OUR DSCNT TO INCREASE SEPARATION. AFTER WE WERE CLR OF THE CONFLICT; THE CTLR GAVE US A CLRNC TO CLB BACK TO FL340 AND SHORTLY AFTER A DSCNT TO FL280 DIRECT HEC. WE WERE GIVEN A NUMBER TO CONTACT ZLA UPON LNDG. WE WERE TOLD THAT THE ZLA INVESTIGATION FOUND THAT WE REPLIED TO ANOTHER CLRNC TO FL240 AND THEY DID NOT CATCH THE MISTAKE. THEY ADVISED NO ACTION WOULD BE TAKEN.