Active Noise Reduction

Written by Robert Mac
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Active noise reduction differs from passive noise reduction, but they both help quiet things down in noisy environments. While active noise reduction does use some passive techniques, its advantage is in destructive interference, which cancels low-frequency sounds. The goal of the passive technique is to block sound, not cancel it.

Understanding Active Noise Reduction

There are a couple of ways to reduce the noise level in an enclosed space, such as the cabin of a helicopter or exhaust pipes: active or passive. As the name implies, active noise reduction relies on actively monitoring sound waves and creating an opposite wave. The waves cancel each other out, so the net result is zero, or a significant reduction in background sound.

How does one create an opposite wave? Microphones pick up incoming sound waves and run them through an amplifier that quickly creates an opposite wave, nullifying both of them. Noise-reducing headphones that pilots use are examples of this active technology; they block low-frequency sounds but allow pilots to hear voices and sirens in the middle and high ranges.

On the other hand, passive reduction relies not on creating waves, but by blocking incoming sound waves. Foam ear plugs are a very simple example of this, but more complex headphones that block the ear as much as possible and filter out loud sounds are more effective. These types of headphones are more likely to be on shooting ranges or construction sights, because they filter loud, impulsive sounds.

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