Airplane passengers hate the loud noise produced by airplane engines. Like all sound waves, these noises travel at about 340 meters per second. When they reach your ear, the sound waves make your ear drum vibrate.

Some airlines have started experimenting with anti-sound headphones. These headphones contain speakers that give off “anti-sound” waves. The anti sound waves are designed to cancel out the sound waves coming from the engines. As a result, the passenger hears softer engine noise. Early testing shows that these anti-sound headphones work fairly well.

(i) Anti sound headphones reduce the noise level heard by the passenger primarily by taking advantage of –