Noise reduction headphones actively produce their own sound waves in order to cancel out external sound waves. A microphone in the headphones receives waves of one frequency. A loudspeaker in the headphones then produces a wave of that frequency but of a different phase.

What is the phase difference between the external sound wave and the wave produced by the loudspeaker in the headphones?
A. 90
B.180
C 270
D 360

Question

Noise reduction headphones actively produce their own sound waves in order to cancel out external sound waves.  A microphone in the headphones receives waves of one frequency. A loudspeaker in the headphones then produces a wave of that frequency but of a different phase. 

What is the phase difference between the external sound wave and the wave produced by the loudspeaker in the headphones?
A. 90
B.180
C 270
D 360