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