BA32 DSNDS OUTSIDE OF T75 CLASS B AIRSPACE WHEN CLRED FROM ILS TO VISUAL APCH. PLT CONCERNED WITH ATC RECOGNITION OF SIT.

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ON ARR INTO THE STL AREA; WE WERE TOLD TO EXPECT RWY 30R. SOON AFTER; THEY ASSIGNED RWY 24; WHICH WOULD ALLOW FOR AN EARLIER ARR. WE WERE ASSIGNED 5000 FT AND GIVEN A HDG TO JOIN THE RWY 24 LOC. WE CALLED THE FIELD IN SIGHT ABOUT 25 MI OUT. AT THAT POINT I NOTIFIED MY FO THAT I WOULD BE OFF AT COM #1 TO NOTIFY OUR COMPANY THAT WE WERE IN RANGE ON COM #2. IN THAT PERIOD; WE WERE CLRED TO 3500 FT. SHORTLY THEREAFTER; WE WERE CLRED FOR THE VISUAL APCH TO RWY 24. WHEN CLRED TO 3500 FT; MY FO STARTED HIS DSCNT RIGHT AWAY. DURING THE DSCNT; WE WERE BELOW CLASS B AIRSPACE FOR A FEW MOMENTS; WHICH I HAD NOT REALIZED UNTIL AFTER THE FACT. HENCE; WE MAY HAVE EXCEEDED THE 200 KT RESTR BELOW CLASS B AIRSPACE. HOWEVER; ATC DID NOT MENTION ANYTHING PERTAINING TO THE MATTER. CONTRIBUTING FACTORS WERE: ALLOWING MYSELF TO BE DISTR WITH AN UNNECESSARY (AT THE TIME) RADIO CALL TO COMPANY; LETTING MY GUARD DOWN DURING A CLR VFR DAY VISUAL APCH; AND MY FO'S LOW EXPERIENCE (IT WAS HIS FIRST WK OFF OF IOE). ALSO; I AM CONFUSED ON WHY ATC WOULD GIVE A CLRNC TO AN ALT WHICH WOULD PUT YOU BELOW CLASS B AIRSPACE AT ALL! IF SO; THEY SHOULD GIVE A WARNING OR ADVISORY.