The following statements belong to the control of noise sources ( ) The measures to block the spread of noise are ( ) The measures to prevent noise from entering the human ear

There are three ways to control noise: one is at the sound source; the other is during the propagation process; and the third is at the human ear.

Among the following statements, the ones that control noise sources are (3. Move the airport away from residential areas), which is at the sound source; the measures that block the spread of noise are (2. After lighting the firecrackers, evacuate while Cover your ears), this is during the transmission process; measures to prevent noise from entering human ears are (1. Pilots flying fighter jets wear helmets), this is at the human ear.