It's called the "Cocktail Party Effect." You may be immersed in the middle of a crowd of audiophiles all talking at once, but when someone says something that catches your attention, such as your name, you can focus on the sound of that person's voice and exclude the babble. The noise suppression can be 9–15dB; ie
, the sound being concentrated on seems to be three to four times louder than the ambient noise, according to Wikipedia
. The exact mechanism of the Cocktail Party Effect is not known, but it is conjectured that it has something to do with the binaural nature of human hearing: the fact that we have two ears allows us to apply spatial discrimination to what would otherwise be a jumble of sound.