| Observations of an ANC Industry Insider 1 of 3 | | | | original sound wave. When identical sound waves |
| Active Noise Cancellation vs. Passive Noise | | | | are combined, they add their energy. Let's say the |
| Cancellation | | | | first wave at its peak has an energy of 1, and the |
| Any time I am asked what I do in a social setting I | | | | second wave at its valley has an energy of |
| have to brace my self for a lot of blank looks. In the | | | | -1. Adding the two together causes the combined |
| beginning I would answer with a very forthright; "We | | | | energy to equal zero! |
| produce active noise cancellation systems for heavy | | | | Why use Active Instead of Passive |
| industry." This was always followed by a lengthy | | | | If that explanation sounded very complicated, don't |
| silence until some brave soul would say, "So you make | | | | worry, it is. Implementing active noise cancellation is |
| insulation?" or something to that effect. To the lay | | | | even more complicated than describing it. So why |
| individual, when confronted with the words active noise | | | | bother with it all? Because there are certain waves |
| cancellation, all they really hear is noise cancellation, and | | | | that passive treatments are not able to stop. A |
| they fall back on their previous experience to help | | | | co-worker of mine is fond of saying "With enough |
| them categorize what I do, that generally means | | | | concrete and re-bar, anything can be made |
| insulation. | | | | quiet." Unfortunately there are lots of applications |
| In the last few years, active noise cancellation has | | | | where concrete and re-bar are not an option. |
| become more common place and understood | | | | Take your home for example. Usually, with the doors |
| because of the remarkable efforts of BOSE and | | | | and windows shut, you don't hear birds chirping, or the |
| others in producing active noise cancellation | | | | typical car drive by. But even with everything closed |
| headphones. There is still a significant amount of | | | | tight, you can still hear that car with the bass thumping |
| confusion though about what exactly active noise | | | | as it drives by. The reason for this is that low |
| cancellation (ANC) is and how it is different from | | | | frequency noises are able to penetrate physical |
| passive noise cancellation products such as | | | | barriers more effectively than high frequency |
| insulation. The purpose of this document is to try and | | | | noises. It's the same principle that allows you to get an |
| describe those differences in a manner that is both | | | | AM station while driving in the hills when you can't get |
| accessible and understandable to all. | | | | an FM station at all. AM waves are much lower in |
| The Basics | | | | frequency and not affected as much by physical |
| Simply put, passive noise cancellation treatments are all | | | | barriers. |
| about deflection and absorption. They are in essence | | | | That being said, passive treatments have a practical |
| a barrier between the observer and the noise | | | | lower threshold at which point they offer limited or no |
| source. Passive treatments take the form of foam | | | | reduction. The lower limit is debatable, but for the |
| plugs in the ear, foam lining between the engine of a | | | | most part, passive treatments start suffering reduced |
| car and the interior and even the 20 foot concrete | | | | effectiveness at about 1000 Hz, and their performance |
| barrier walls that line busy streets. Their sole purpose | | | | degrades as the frequencies get lower. Active |
| is to either absorb the sound, and turn it into something | | | | noise cancellation works best on noise below 300 |
| else (commonly heat), or reflect it in another direction. | | | | Hz. It is possible to use ANC on noise up to 500 Hz |
| Active noise cancellation on the other hand relies on | | | | but the reductions are limited and have to be |
| the principle of destructive interference; the idea of | | | | considered on a case by case basis. (For more |
| using one wave to cancel out another wave. The | | | | information on why ANC is only useful up to 500 Hz, |
| easiest way to visualize this principle is to use waves | | | | please read part 2 of this series.) As you can |
| in water. If I drop a pebble in a pond, it creates ripples | | | | imagine, with such distinct areas of effectiveness, the |
| (waves). By dropping a second pebble in the pond | | | | two methods for reducing noise are actually |
| shortly after the first, as the ripples from each rock | | | | complimentary to one another. Active noise |
| intermingle, small areas are created where there are | | | | cancellation should not be considered as a |
| no waves visible. In fact at those points the surface is | | | | replacement for passive treatment. In fact, we often |
| perfectly smooth. This phenomenon is a product of | | | | recommend to our customers that they first do |
| destructive interference. | | | | everything they can with passive noise cancellation, |
| Another way of visualizing it is by imagining a wave. | | | | and if the noise levels are still too much of an issue, |
| Every wave has two core properties: how fast it | | | | then consider active noise cancellation. |
| goes up and down (frequency) and how far it goes up | | | | Conclusion |
| and down (amplitude). Also, remember that a | | | | While active noise cancellation is a high tech solution to |
| complete wave has peaks and valleys. Now, imagine | | | | unwanted noise. It's not a substitute for a good piece |
| adding a second wave to the first. This second wave | | | | of high density foam. Because of the costs |
| is identical in frequency and amplitude, but the second | | | | associated with each system, we recommend doing |
| wave is out of synch with the first. So much so, that | | | | everything possible with passive forms of treatment |
| the peaks and valleys of the second wave line up | | | | and then re-evaluate your environment before deciding |
| exactly with the valleys and peaks respectively of the | | | | to implement active noise cancellation. |