COMMUTER ACFT DSNDS BELOW ASSIGNED ALT.

Narrative

DURING CRUISE I HEARD ZLA TELL ACR X (DSNDING) TO LGB 'PLAN ON 35 DME N OF SXC (SANTA CATALINA VOR) AT 11000 FT.' I MADE NOTE. AND SINCE WE WERE ON THE SAME ROUTING; I FIGURED THEY WOULD GIVE US THE SAME ALT RESTRICTION. 3 MINS LATER THEY SAID 'SMT PLAN ON 35 N OF SXC AT 11000 FT.' BOTH THE CAPT AND I HAD REMOVED OUR HEAD SETS FOR THE 1.5 HR FLT. WE ALSO TURNED OFF THE 'SIDE TONE' TO AVOID LOUD SQUEALS WHEN XMITTING. WE HAD APPROX 75 KTS TAILWIND DURING OUR DSCNT. REALIZING I'D BETTER START DOWN TO MEET THE RESTRICTION; WE ASKED CTR FOR LOWER. THE CTLR SAID 'SMT DSND AND MAINTAIN 11000 FT.' AT THIS POINT I BEGAN CALCULATING WHAT RATE OF DSCNT WOULD BE REQUIRED TO MEET THE 35 N AT 11000 FT RESTRICTION. (IN HINDSIGHT; THE CTLR NEVER GAVE US THE RESTRICTION; JUST PLAN ON IT.) SINCE THE CTLR SAID PLAN ON THE RESTRICTION; I WAS BUSY PLANNING THE DSCNT. AFTER WE HAD STARTED DOWN I FAILED TO HEAR A CALL FROM CTR AND FAILED TO HEAR THE CAPT READ BACK THE CLRNC 'SMT DSND AND MAINTAIN 13000 FT FOR TFC AT 12000 FT.' AGAIN WE HAD OUR SIDE TONE TURNED OFF. SINCE THE ALT ALERTER IS CLOSER TO THE FO SIDE OF THE COCKPIT; I USUALLY SET THE CLRNC ALT. BUT THE LAST CLRNC I HEARD WAS 11000 FT; THAT REMAINED IN THE ALERT BOX. AS WE CONTINUED THE DSCNT; CTR CALLED OUT TFC AS AN ACR WDB AT 12 O'CLOCK; 'DO WE HAVE HIM IN SIGHT?' WE RESPONDED 'AFFIRMATIVE; WDB IN SIGHT.' DSNDING THROUGH 11800 FT THE CTLR ASKED 'SMT; WHAT IS YOUR ALT?' WE RESPONDED 11800 FT FOR 11000 FT.' SHE RESPONDED 'NEGATIVE; YOU WERE ASSIGNED 13000 FT.' 'SMT YOU WERE INVOLVED IN A POTENTIAL PLT ALT VIOLATION. CONTACT CTR [WITH THE TELEPHONE NUMBER] ON THE GND.' (ON THE TAPE; THE CAPT READ BACK 13000 FT; BUT WE BOTH FAILED TO ADJUST THE ALT ALERTER.)