Reading Comprehension

Read the passage and answer the question.

Noise-cancelling headphones work by fighting sound with sound. Tiny microphones on the headphones detect the steady hum of surrounding noise, such as the drone of an airplane engine. The device then generates a new sound wave that is the mirror image of the unwanted one. When these two waves meet, their peaks and troughs cancel each other out, leaving the listener with relative quiet. The technology works best against constant, low-pitched sounds; sudden or high-pitched noises, like a nearby voice, are much harder to predict and therefore much harder to cancel.