According to NFPA 5.5.2.1 you do need a detector. If dust is present choose a heat detector, if not chose a smoke detector. I would also design for a "notification applicance". I would use a "strobe" only. A speaker in the pit would produce an extremly high db. (assuming its a small room and your speakers volume is not adjustable; i have not found an adjustable speaker yet). Hope it helps.
Please note you should also check your state fire codebook.