2.5. How can adding sound make a system quieter?
Description
This article is from the Active Noise Control FAQ, by Dr. Chris Ruckman
2.5. How can adding sound make a system quieter?
It may seem counter-intuitive to say that adding more sound to a system can reduce noise levels, but the method can and does work. Active noise control occurs by one, or sometimes both, of two physical mechanisms: "destructive interference" and "impedance coupling". Here is how they work:
On one hand, you can say that the control system creates an inverse or "anti-noise" field that "cancels" the disturbance sound field. This works by the principle of destructive interference. A sound wave is a moving series of compressions (high pressure) and rarefactions (low pressure). If the high-pressure part of one wave lines up with the low-pressure of another wave, the two waves interfere destructively and there is no more pressure fluctuation (no more sound). Note that the matching must occur in both space *and* time -- a tricky problem indeed.
On the other hand, you can say that the control system changes the way the system "looks" to the disturbance, i.e., changes its input impedance. Consider the following analogy:
Picture a spring-loaded door, one that opens a few centimeters when you push on it but swings shut when you stop pushing. A person on the other side is repeatedly pushing on the door so that it repeatedly opens and closes at a low frequency, say, twice per second. Now suppose that whenever the other person pushes on the door, you push back just as hard. Your muscles are heating up from the exertion of pushing on the door, but end result is that the door moves less. You *could* say that the door opens and that you "anti- open" it to "cancel" the opening. But that wouldn't be very realistic; at least, you would not actually see the door opening and anti-opening. You would be more accurate to say that you change the "input impedance" seen on the other side of the door: when the other person pushes, the door just doesn't open.
(The spring-loaded door is supposed to represent the spring effect of compressing air in a sound wave. This is not a perfect analogy, but it helps illustrate impedance coupling.)
In some cases, destructive interference and impedance coupling can be two sides of the same coin; in other cases destructive interference occurs without impedance coupling. The difference is related to whether the acoustic waves decay with distance traveled:
Sound from a speaker hanging in the middle of a stadium decays (is less loud) at a distance because of "spherical spreading." The sound energy is spread out over an increasingly large area as you get farther away. Go far enough away and, for all intents and purposes, the sound decays completely down to nothing. On the other hand, sound in a "waveguide" such as a duct can travel long distances without significant decay.
If a control system actuator is close to the disturbance source, destructive interference and impedance coupling can both occur. But what about when the actuator is far away from the disturbance, so far away that any wave it creates decays completely down to nothing before reaching the disturbance? There can still be destructive interference near the actuator, even though the actuator cannot possibly affect the impedance seen by the disturbance. Example: the tiny speaker in an active control headphone will not affect the impedance seen by a cannon firing a mile away, but it can create destructive interference within the headphone.
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