Remember, drum corps perform on football fields outside. In a concert hall, the sound from the instruments bounces everywhere, but the walls are carefully shaped to direct the sound to the audience. Outside the sound still bounces everywhere, but the sound that is not pointing directly forward flies away and is wasted. Because of that, marching instruments are shaped differently; the big "bell" points towards the audience, so it is as loud as possible for them.  The four most common brass instruments in corps are the trumpet, mellophone (like a French horn), baritone, and contrabass (like a tuba). For a long time, DCI rules have banned all other brass instruments besides the ones listed above. Now, DCI have changed the rules and have allowed more brass instruments, like the trombone and euphonium. These other instruments are used in a manner that only one or a couple people play them in the corps. As a result, the instruments need microphones and speakers, or else the audience would not hear them over the dozens of marching instruments on the field.