The difference between Volume and Gain, and ”loudness” overall (Audio Engineering)
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You can imagine most audio amplification as occurring not in one leap but in a series of stair steps.
Microphones and turntables produce a very low voltage signal. They require boosting to a medium voltage signal called “line level” that is still too low to drive a speaker but is equivalent to what electronic sources like CD players, synthesizers, audio players, etc. produce. That initial step from a very low input to line level is often called “gain.” On recording gear it is typically variable and has its own knob, since different signals require different amounts of boost to get to line level. Confusingly, this “gain” stage is also sometimes called “mic preamp” or “phono preamp” which can be confusing since there is another preamp stage we will hear about next.
Line level signal is not enough to make any sound come out of a speaker. To get there you need to further amplify it to a much more powerful “speaker level.” This second stair step typically has two parts that happen in sequence. First, it goes through something called a preamp (not to be confused with the mic or phono preamps described above), which applies volume control to the line level signal. Second it goes through something called a power amplifier stage, which typically takes whatever line level signal it is given and boosts it by a fixed amount. To be clear the amount varies per power amplifier but for a given power amplifier that amount always remains the same. When you see a knob labeled “volume” it is adjusting the amount that the preamp is attenuating or boosting the line level signal before it is fed to the power amp.
Then, the power amp sends this now-much-higher speaker level signal to the speakers, which makes them move and produce sound waves that travel through the air to your ears. How loud this sounds is measured in decibels and will vary based on a combination. of all of these things:
1. The initial low level signal (if the sound is coming from a low level source like a mic or turntable)
2. The amount of gain applied to that by the mic or phono preamp (ditto)
3. The position of the volume control on the preamp
4. The fixed amount of boost provided by the power amp
5. The sensitivity of the speakers (they can produce more or less sound with the same signal depending on their design)
6. The distance of your ears from the speaker
7. How much reverberance happens in the room (more reverberance will mean more sound hits your ears)
Now the terms “gain” and “volume” are sometimes used in other contexts but in general the overall rule holds that in a given system the chain is input -> gain control -> volume control -> output. Not all systems have a gain control because not all systems are designed to handle low level or highly variable input, so some just have input -> volume control -> output.
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