NPR: One of the more dramatic changes we’ve heard in music over the last 10 years is that it seems to have gotten louder.The reason is compression, the dynamic compression that’s used a lot in popular music. The idea is to make a song jump out of your iPod compared with the song that preceded it.However, the technique can make listening to music fatiguing as well as distort the sound of the instruments.This issue came to a head last year with the release of Metallica’s album Death Magnetic.That record is so loud that the International Telecommunication Union has set standardization measurements for long-term loudness, says Bob Ludwig, a record mastering engineer. “And that Metallica record is one of the loudest records ever produced.”