The amplitude response seems simple: Faster attack with higher amplitudes. Attack stage gets shorter as the velocity increases.
I'm implementing this in SuperCollider as
attack = (1 / velocity) - 0.99. (This implementation gives very long attack times at low velocity, but I need to know the attack ranges before scaling it -unless someone can suggest a better implementation, which I'm sure there is-)
The problem is that I don't know the attack time ranges of a typical brass instrument. What's a common/typical fast attack time in brass instruments? What's a common/typical slow attack time in brass instruments?
All these assuming no crescendo, or other modulations, are being applied. Just the amplitude envelope trying to emulate a note being played on a brass instrument without crescendo.