I too have wondered this? Although it is because if I had to fire in close quarters, hallway, car, etc. It should not be as loud right? I will want to be able to hear myself telling the dead guy not to move! JK
no, sub sonic means slower than the speed of sound, not less sound..
if you ever work the target pits of a really long range, you can uncover your ears and hear the sonic boom of the bullet long before the boom of the gunpowder..