Maybe I'm just excessively geeky, but I found this fascinating. Someone measured the dynamcic range (difference between the loudest and quietest notes) on one band's entire recorded catalogue, and asked "why are your newer albums so much louder:
(Would have been clearer if they had sorted by release year, but you can still see the trend.)
Without knowing the technicalities, I've always thought modern rock music is "louder" than it used to be. Or, as I put it in my musical ignorance, "more dense", i.e. with less quiet bits to give you a breather. And I don't think it's such a good thing.
Anyway, through the miracle of the Internet, the band's bass player, Al Barrow, gave us the answer he got from his sound engineer, and again it's fascinating if you're of a geeky persuasion and wonder about how these things work:
OK Here is a answer for how Magnum do it and Sheena Sear Magnums studio engineer sees it. Hope this helps.
Ok so - digital audio always has the same absolute maximum volume which is 0dB. You can't push it louder than that. Things were a bit different back in analog days where with tape you could push it over the 'maximum' and it would saturate (this is where it rounds the top of the waveforms). That's actually a kind of distortion but a sort of pleasing one. It's the same thing with valves, but as you know if you drive them so much they add warmth and texture but if you drive them a lot what they output sounds completely different from the input was, which is obviously not desirable on a whole mix. So similarly tape saturation gets to the point where it no longer sounds pleasing, but everyone had a different opinion on how much did sound good. And with vinyl there was a physical limit to how deep you could cut the groove, and as I understand it if you made the master too loud the needle would physically jump out of the groove.
When we create a raw mix as it comes out of the control room it's got a really high dynamic range, that is the difference between the loudest and quietest sounds is very large which would result in a very big number in that table. But because in the digital world the peak can only be 0 to the end user this basically means the mix would sound quiet. Ok so they can turn it up, but imagine if you're listening to the radio or your iTunes and one rock track finishes and another one starts and it's noticeably louder. Those who know nothing about sound and mastering might go - 'wow I love this band they're so heavy and LOUD'. So in order to sound louder than the next band people started to compress their mixes - making the quiet bits louder and the loud bits quieter, which is reducing the dynamic range, and making smaller numbers in that table. Because the peak is always the same the overall effect of this is that the mix as a whole sounds louder. The louder you go it becomes hard to do well as you're constantly fighting distortion and most engineers would argue it is not a good thing as by removing the dynamic range you are losing a lot of the depth and subtlety of the music.
To give you a photography analogy it's like producing a deliberately high contrast colour-saturated photo in order to make it stand out on a magazine page. It might draw the eye but a lot of detail will have been sacrificed.
This went on for years with people pushing their mixes louder and louder all the time. Up until around the turn of the millennium when it got so OTT that even the general public started to notice it (google RHCP Californication mastering and you'll get the point). Since then the overall volume of masters has remained roughly the same.
I do master loud - i.e. near the top end of what has become normal. I make no apologies for it, Tony likes it and it's kind of the case of we didn't start the war but we can't undo it either. If everyone in the world was producing high contrast over saturated photos would you want to be the one trying to publish natural ones which would look wishy-washy by comparison? If I was a top mastering engineer in a top studio I might be brave enough to fight the cause a bit but I'm not and at the end of the day most people still like their rock bands to sound loud. It would certainly make my life a lot easier if overnight everyone went 3dB quieter but it's not really going to happen.
More to the point if you went to that resource and looked up any band with a back catalogue spanning 30-40 years you would almost certainly see the same thing.
So there it is. The loudness wars and mastering in a nutshell.
Hope that helps!