Drum roll
A drum roll (or roll for short) is a technique the percussionist employs to produce, on a percussion instrument, a sustained sound, "over the value of the written note." Rolls are used by composers to sustain the sound and create other effects, the most common of which is using a roll to build anticipation.
Types
Snare drum roll
A common snare drum roll is the closed (or "buzz" or "concert") roll. The concert roll is performed by creating 3 equal sounding bounces on each hand alternating right to left. The 3 bounces are made even more equivalent in sound by using the acoustic properties of the snare drum. Strokes closer to the rim will have a lower volume than strokes on or near the sweet spot of the drum. Therefore by placing the first of the 3 bounces closer to the rim and moving the second bounce towards the sweet spot and finally the third bounce in the sweet spot will produce three equal sounding bounces.
The open roll (or "double-stroke roll") is played with double strokes alternating between the left and right hands. Using a forearm stroke for the first and the fingers for the second stroke, the 2 strokes can be made to sound identical. This produces a near-continuous sound when the technique is mastered.