The SPD-30 contains a parameter called "Muffling" which adjusts the decay of an instrument. The "decay" is how long the sound of the instrument will sustain after it has been triggered. When using higher values of the Muffling parameter, more muffling is applied to the instrument - hence making the decay(sustain) shorter. With lower Muffling values, the instrument's decay is longer.
Here's how to adjust the Muffling for an instrument in the SPD-30:
1. Choose a Kit to edit.
2. Press MENU.
3. Press the function 3 button (ENTER).
4. Press function 2 (EDIT).
5. Turn the function one knob to highlight "Muffling."
6. Hit the pad that you'd like to adjust the Muffling for.
7. Turn the function 2 or function 3 knobs to adjust the Muffling for the A or B instruments - (0-50).
8. Repeat Steps 6 and 7 for any remaining pads.
9. Press the BACK button when you're finished. The new settings are saved automatically.