Noise reduction headphones actively produce their own sound waves in order to cancel out external sound waves.

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.