A common question here is, "Why not just use MIDI Volume (CC #7) instead of Expression (CC #11)?" It's true that these two controllers essentially do the same things. However, we recommend setting each track to a global Volume level (this is controlled with your Mixer faders in Finale or Sibelius) for consistency's sake, and then any micro dynamics can be handled with Expression values. When in doubt, keep the Expression control set at its maximum value (127), and then reset it to 127 before and after any of these dynamic events.
 

Since VDL is primarily a library of percussion sounds, dynamics are most often controlled using Velocity (or "key velocity"). This is another standardized MIDI term that basically means the harder a note is struck, the higher value it receives (again from 0-127). Since the vast majority of sounds in VDL are "struck," the dynamics of each sampled sound are programmed with Velocity control. Furthermore, Velocity is the default dynamic control in Finale or Sibelius. For example, if you write a series of timpani notes in Finale at mezzo-forte, those notes might play back at an approximate Velocity value of 80. Add an accent to one of those notes, and its Velocity would increase to 95 or so.

Since VDL was programmed with a variety of samples at each Velocity, you'll not only hear louder/softer volumes as these variations occur, you'll actually hear louder/softer sampled sounds, creating real timbral changes. It's one of the reasons VDL sounds so lifelike! That's Velocity - and it's the most widely used form of dynamic control in VDL.

But what if you wanted a sustained concert snare roll to diminuendo over the course of two bars? This is different than the previous example because the sustained roll would be activated, and then held down for its 8-count duration. Other than the initial attack, Velocity isn't going to help much here because there's just one held note. That's where Expression comes in handy!

All VDL instruments are set to a default Expression value of 127 - the highest setting. So if you want this snare roll to mimic that diminuendo, simply enter a gradual Expression decrease from 127 to, say, 25 (depending on how soft you want it to get). In Sibelius, this can be done using the Cresc./Dim. Playback Plugin. In Finale, this can be accomplished via the MIDI Tool. In a DAW (digital audio workstation such as Logic Pro, Cubase, Digital Performer, etc.), you can simply draw an Expression line using its built-in MIDI automation control for CC #11.

Keep in mind, Velocity is still at play here. Here's why this is important to remember:  Let's take that same two-bar sustained snare roll, but instead of doing a diminuendo, let's create a big crescendo. By the end of the two bars, we'll want that roll to be pretty darn loud and aggressive. The Expression controller will increase the instrument's volume for the duration of the crescendo, however if the lower Velocity of the initial note strikes a piano dynamic sample of the snare roll, by the end of the crescendo you'll simply be hearing a louder rendition of that sustained piano dynamic sample. This isn't what we want. Instead, manually change the velocity of the roll's attack to the desired arrival level (velocity value of 120, say), then start an Expression event slightly before the note is struck (CC #11 value of 25) and proceed with the gradual Expression increase. So even though you'll be activating the start of the crescendo (piano dynamic) using a fortissimo velocity, it'll be restrained dynamically by the reduced Expression setting.
 

Hardware Expression Control

 

Express Yourself!