Akai XR 20 Reference Manual page 7

What's a Voice?
Each of the 12 large pads triggers a voice. A voice is a sound-generating element with
several variable parameters: sound assignment, tuning, volume, output assignment
(the voice's audio output can go to either one of two sets of stereo outputs, and
furthermore, to anywhere within the stereo field of the chosen set of outputs), MIDI
note number, etc.
Each pad is velocity-sensitive: the harder you hit the pad, the louder the sound
assigned to the pad will play. Thanks to the dynamic articulation techniques mentioned
earlier, the timbre will often change as well, just like the way that "real" instruments
produce sound when played with different amounts of force.
There are eight levels of pad volume resolution, from soft to loud. However, when
using the XR20 as a drum sound expander and triggering sounds via MIDI, they
respond to all 127 MIDI volume levels.
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