Question 33
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°
Reference: Past Exam Paper – June 2013 Paper 11 Q29
Solution:
Answer: B.
The noise can be cancel out by destructive interference (using the principle of superposition).
By producing a wave of the same frequency and amplitude but with a phase difference of 180° (this is the condition for destructive interference), the loudspeaker can cancel the noise.
A phase difference of 360° means that the waves are completely in phase (there is no phase difference between them.). Constructive interference occurs causing the amplitude of the noise to become greater.
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