DEP CTLR DSNDED ANOTHER ACFT TOO CLOSE TO RPTR'S WHICH CAUSED A TCASII RA. ACFT'S ALERT SYS SOUNDS OVER LOUDSPEAKERS ONLY; WHICH RPTR CONSIDERS A HAZARD BECAUSE THE PF DIDN'T HEAR IT. ALERT SHOULD BE HEARD THROUGH HEADSETS.

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JUST AFTER LEVELING AT 12000 FT AND ESTABLISHING CRUISE HOU DEP GAVE US A TURN SW TO INTERCEPT THE IAH 207 DEG RADIAL. ABOUT A MIN LATER DEP CALLED TFC AT 2 O'CLOCK 12500 FT NEBOUND. MY FO WHO WAS FLYING THE LEG ON THE AUTOPLT BEGAN LOOKING FOR THE TFC. ABOUT 10-15 SECONDS LATER WE GOT A TA FOR TFC AT 1 O'CLOCK; LESS THAN 2 MI AT 12400 FT AND DSNDING. I CALLED THE TA AND ITS INDICATED LOCATION AND CONTINUED MY VISUAL SEARCH FOR THE TFC. ABOUT 8-10 SECONDS LATER WE HAD THE RA TO DSND AT 1500 FPM. I ALMOST DID NOT HEAR THE RA AND MY FO DID NOT HEAR IT. BY THE TIME I REALIZED HE HAD NOT HEARD IT AND WAS NOT RESPONDING WE WERE GETTING A SECOND AURAL WARNING. I TOOK OVER THE ACFT AND FLEW THE REQUIRED DSCNT UNTIL THE 'CLR OF CONFLICT' CALL. I THEN STARTED A CLB BACK TO 12000 FT AND HAD THE FO INFORM ATC OF THE RA. WE LEVELED AT 12000 FT AND CONTINUED WITH NO FURTHER PROBS. THE DEP CTLR SAID THERE WAS NO CONFLICT AS THE TFC WAS PAST US BEFORE HE WAS GIVEN HIS DSCNT CLRNC. OUR TCASII CLRLY SHOWED HE WAS FROM 1-2 MI IN FRONT OF US. WE NEVER SAW THE ACFT SO I CANNOT CONFIRM ITS LOCATION. OUR TCASII INSTALLATION GIVES ITS AURAL WARNING ONLY OVER THE COCKPIT SPEAKERS NOT THROUGH HEADSETS AS ALL OTHER WARNINGS COME. IN THIS CASE THIS WAS A PROB. AS MANY PLTS DO; BOTH THE FO AND MYSELF HAVE SOME HEARING LOSS. IN THE HIGH NOISE ENVIRONMENT OF A TURBOPROP WITH HEADSETS ON I ALMOST DID NOT HEAR THE WARNINGS AND MY FO; WITH HIS BACK TURNED; LOOKING OUT HIS SIDE WINDOW FOR THE TFC DID NOT HEAR IT. THIS CAUSED A CONSIDERABLE DELAY; WELL PAST THE REQUIRED DSCNT PARAMETERS; IN THE INITIATION OF THE DSCNT. I FEEL THIS NEEDS TO BE LOOKED INTO AS A MAJOR INSTALLATION FLAW. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT IDENTED AS EMB-120; BRASILIA. RPTR CAPT SAYS; IN THE TKOF AND CLB ENVIRONMENT WITH ALL THE NOISE FROM THE HIGH RPM ENGS; PLUS RADIO CLUTTER FROM ATC/FLCS; IT IS VERY DIFFICULT TO HEAR TCASII ALERTS BECAUSE THEY COME OVER THE COCKPIT LOUDSPEAKERS ONLY. RPTR CONSIDERED THEIR SIT HAZARDOUS BECAUSE THE FO WAS FLYING AND DIDN'T HEAR THE INITIAL RA OR THE DSND COMMAND. CONSEQUENTLY; THE CAPT TOOK OVER THE CTLS AND FOLLOWED THE TCASII COMMAND. RPTR BELIEVES THE ALERTS SHOULD ALSO BE HEARD THROUGH THE HEADSETS.