What we hear depends in large measure on what we can screen out. We have the mysterious ability to tune in on one conversation amid the babble of a crowded room - a phenomenon called the cocktail-party effect. But even when we have filtered out most distractions, certain sounds we care about will jump out at us. We can't help noticing the sound of our own name, for example, however softly it is spoken. In the same way, a sleeping mother will be wakened by a cry from her baby, and a single wrong note by one player in a huge symphony orchestra will catch the ear of the conductor.
A country dweller who visits the city is often appalled by the relentless clamor of vehicles and people, which an urban cousin no longer hears; and a teenage student can concentrate while loud rock music is playing, to the amazement of his parents. Also, depending on their attitudes toward the source of the noise, people may hear noises as louder or softer. In one study, for example, people living near a military airport found the sound of jets taking off less noisy when they believed that the airport was vital to the national defense.