How come modern rock recordings seem to be so relentlessly loud? Essentially, the answer comes down to the misuse of compression, a technique by which the peaks and valleys of volume in a recording are electronically leveled out.
There's a popular YouTube video circulating that gives a pretty simple explanation. After a recording is finished but before it's mastered to CD, engineers use electronic tools to increase the volume of the entire recording, then squash out any peaks over a certain level. The result is a really loud CD that is supposed to stand out to radio programmers and other types who judge music based on the first 10 seconds of each song, but that has very little dynamic range, which makes it hard for actual music listeners to enjoy it for more than a few minutes.
Many fans of Canadian rock band Rush criticized their 2002 album Vapor Trails for this problem. Bob Dylan has noticed it. I really noticed it on Beck's 2002 album Sea Change: here's an album of gorgeous acoustic tracks about heartbreak, best meant for late nights alone, and it's one of the most relentlessly loud CDs I've ever heard.
But just because compression has been misapplied, that's no reason to throw the baby out with the bathwater. In live settings, compression has helped many a weak singer shine through--everybody wants to hear the lyrics, so you have to crank the vocal track, and compression prevents the occasional peak from distorting. It's also useful for bass, where one big thumb-hit on the low E string can overload the sound system. And it's almost always used in recording. Sometimes an engineer will apply a compressor during recording to make sure a particular instrument--kick drum, bass--doesn't spike too high, creating distortion or digital clipping. More often, it's applied during mixing and mastering to change the character of a particular instrument, or of the whole recording.
Which brings me to Joe Meek, a producer of instrumental records, mostly surf, from the early and mid-1960s. (He committed suicide in 1967.) Meek essentially invented electronic compression, and a line of compressors still bears his name. Many of his recordings sound like music from an interstellar circus, transmitted across millions of light years. It's a musical fetish, but a pleasurable one if you're bored with the same-old-same-old.