An excerpt from a favorite book of mine:
"LOUDNESS [as a term] is used specifically and precisely for the listener's perception. Loudness is much more difficult to represent in a metering system, in fact it's best presented as a series of numbers rather than as one overall figure of "loudness." Two pieces of music that measure the same on a flat level meter can have drastically different loudness[es]. A true loudness meter makes a complex calculation using SPL, frequency content, and duration. Exposure time also affects our perception; after five minute rest, the music seems much louder, but then we get used to it again--another reason why it is wise to have an SPL meter around to keep us from damaging our ears.
Contrary to popular belief, the levels on a digital peak meter have (almost) nothing to do with loudness. Here is an illustration. Suppose you are doing a direct two-track recording and you've found the perfect mix. Leaving the faders alone, you let the musicians do a couple of takes. During take one, the performance reached -4dB on the meter, and in take two, it reached 0dB for a brief moment during a snare drum hit. Does that mean that take two is louder? No: because in general, the ear responds [most] to average levels, not peak levels when judging loudness. If you raise the master gain of take one by 4dB so that it too reaches 0.0dBFS peak, it will sound 4dB louder than take two, even through they both now measure the same on the peak meter.
An analog tape and digital recording of the same source peaked to full scale sound very different in terms of loudness. If we make an analog tape recording and a digital recording of the same music, and then dub the analog recording to digital, peaking at the same level as the digital recording, the analog dub will have about 6dB more intrinsic loudness than the all digital recording. Quite a difference! This is because peak to average ratio of an analog recording can be as much as 12-14dB compared with as much as 20dB for an uncompressed digital recording. analog tape's built in compressor is a means of getting recordings to sound louder. That's why pop producers who record digitally may have to compress or limit to compete with the loudness of their analog counterparts."
-Bob Katz, Mastering Audio: The Art and the Science