What is the Difference Between the Flexboom/Midi/and ST mics on the 91

The ST mic is a sound tube- which is pretty much the exact same thing as a voice tube - but Plantronics had probably already copywritten voice tube, hence sound tube, or ST. So no noise canceling on the ST.

The midi boom is slightly worse than the sound tube as far as noise-cancelling but not much - the benefit to this is that it doesn’t extend as far out in front of your mouth for a more discreet, modern look.

The Flex-boom is a longer, FLEXible BOOM mic, that IS noise cancelling.

Left - the MIDI Boom mic 9120, far left, the ST 9120