I generally think of the various volume 'controls' in MIDI to be like this:

CC7 (volume) is like your amp volume. Set it at the beginning of the track and leave it.

CC11 (expression) this is like the volume pedal on the organ or guitar volume pedal. Use it for crescendos, diminuendos, and other track volume changes throughout the song.

The beauty of having two is this: You can do all your changes in the track with CC11. However if you find when you are all done and compare the volume of your track to the others, say it's too loud or too soft, you can use CC7 to adjust that without having to change anything else in the track.

Velocity is the third one. It's called velocity because in the early days of MIDI they measured the interval of time between depressing a 'piano' style key and hitting the bottom. The quicker it went from top to bottom, the harder you hit the key, and the louder the note should be.

So the relative volume between notes for accents and other expressive differences between individual note volumes is expressed with velocity.

These are the basics.

In addition there are Aftertouch, Channel Pressure, and Breath Controller which can influence volume, but that depends on your MIDI controller and MIDI sound source. I advise new MIDI people to get the first 3 down pat before going on to these controls.

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