Gain is well, how much the wave will be amplified.
Amplitude is the level of decibels the wave arrives at each cycle without being influenced by gain.
velocity is mostly used for MIDI.
Volume is the final mixdown amplitude of all the waves.
Is that correct?
Gain just means an increase. That increase will be measured in amplitude and volume.
Velocity is a midi term that describes how forceful a virtual note is played.
Amplitude is a measurement of how 'large' a sound wave is, but there is no linear scale between amplitude and volume. Volume (perceived loudness) is based on a lot of things one of which is frequency. A high amplitude of a high frequency won't seem as loud as a medium amplitude of a mid-range frequency (unless you're a dog).
Volume is most often measured with decibels, but there are a lot of different kinds of decibels. The one we use most often is dBFS of full scale. With this 0dB is the point of distortion for a sine wave. You'll use this scale to measure 2 main things with regards to volume, peak and RMS. Peak is your momentary maximum, while RMS (Root Mean Squared) is basically your average level. You'll find you can have a peak of -1 but your RMS will be at -20. The meters on your board or DAW are most likely peak meters to make sure your recording isn't clipping while a master RMS meter will tell you the 'volume' or how loud something appears to be.
Those are just the bullet points, this could easily be a 50page explanation of how those terms work 
Thanks that was helpful. I guess I need an engineering degree in acoustics to understand volume. Hehehe
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You're so wise CCH! haha. The terms can get muddled up a lot from different sources I have found, which can be rather annoying at first when you're learning.
You're so wise CCH! haha. The terms can get muddled up a lot from different sources I have found, which can be rather annoying at first when you're learning.
Why thank you (c: It took me a while when I was still a studio apprentice to wrap my brain around how 'loudness' works.
