Active Noise Control – the technological challenge

Introduction

The basic idea behind Active Noise Control (ANC) has been known for more then 70 years. Producing an opposing signal (anti-noise) with the same amplitude as the noise you want to reduce (unwanted noise) but with the opposite phase, yields a significant reduction in the noise level. According to the superposition of waves, two waves with equal amplitude and identical phase have an additive effect, resulting in a doubling of the overall amplitude. On the other hand, two waves with equal amplitude but opposite phase, have a subtractive effect, resulting in a decrease in the overall amplitude. The phase describes the relative position of the wave in its rising and falling cycle. If two waves are in phase, they rise and fall together but if they are exactly out of phase, one rises as the other falls and so they cancel out one another. ANC tries to eliminate sound components by adding the exact opposite sound. The level of attenuation is highly dependent on the accuracy of the system for producing the amplitude and the phase of the reductive signal (anti-noise).