Trombone Slide Difficulties
The unique construction of the trombones with their slides gives them a possibility to do certain things that other brass instruments can’t do (e.g. seamless glissandi) but also limits them in certain situations. Especially in the lower register where there are not several different possibilities available to play a specific note the agility of the trombone is considerably lower than on instruments with valves.
The reason lies in the pure physicalities of pushing a valve down with a small finger movement vs. moving the slide with an elbow movement and in some cases needing to move the slide from completely in to completely out, which can also happen on notes very close together.
The most problematic one for a tenor trombone is between Bb a major 9th below middle C and B a semitone higher. While on a real tenor trombone (usually played by the first trombone player which doesn’t come with a fourth and/or fifth valve as other trombones), the Bb can only be produced with the slide completely in while the B only with the slide completely out. So a nightmare passage for a player would be quick staccato passages between these two notes. The higher you get the more options to play the same note are available on the trombones so the slide movement can be reduced but down there, there’s no option.
So when you’re writing for brass, keep an eye on the speed factor for your (low) trombones.