This is all to do with reference points. First off, you have to realise that dB aren't an absolute measure of anything - a dB is only a step relative to the one next to it. Your reference to 40dB being 'somewhat quiet' is actually meaningless on its own, because it doesn't tell you what this 'quietness' is in comparison to - and that's rather important.
So, to get around this, there are several different reference points used, depending on what you are trying to give an indication of.. In acoustics, for air we tend to use a definition which relates to pressure variations of 20uP (microPascals), which is the level of the quietest sound a human can percieve at 1kHz. If you compare this to the standard pressure level for sound in water (1uP), you'll note that there's quite a difference - by the time it's converted from linear pressure (Pascals) to logarithmic dB, that's a difference of 26dB in the reference points.
But when it comes to electrical signals and the recording of them, it's slightly different. Here we are most concerned with the highest levels that we can record, not the quietest ones. So the absolute ceiling, beyond which you cannot go, is the reference point, and that's expressed as 0dB - because it's the only absolute reference point that is common to all digital signals. But even that's not a 'real' absolute reference, because it's only the point where distortion starts, not an absolute electrical reference - and there are several of those as well, just to add to the complication. Anyway, all signal levels in digital audio are referenced to 0dB, and therefore every signal quieter than that is represented by a minus number.
But, you don't want your audio anywhere near the 0dB level, especially when you are recording it. So in recordings we leave what is known as 'headroom' - usually between 12 and 15dB - just to allow for unexpected peaks. But if you do that, then the relative levels of all of your signals are lower by this amount. And it's normal to compensate for this later in the processing you do, to bring the peak level much nearer 0dB. But this time, it's under your control. So at that point, the relative level differences between quiet and loud remain the same, because we are using a logarithmic reporting device, but the absolute levels will shift, according to the amount of headroom you've left.
There are other factors you have to take into account as well, which can complicate things further. The bottom line is that you can't just quote dB numbers at people without stating an absolute reference, unless it's only the difference between two levels. It's best simply not to do it, because relatively few people understand what they actually don't mean... and this is just the simplified explanation; there's a heck of a lot more to it as well, that you can find if you search for it.
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