2.1. What is active control of noise/vibration?
Description
This article is from the Active Noise Control FAQ, by Dr. Chris Ruckman
2.1. What is active control of noise/vibration?
The question is usually posed like this: "I heard about a new noise control technology called Active Something-Or-Other ... can I use it to make my house quiet when the kid next-door plays 'Black Sabbath' on his electric guitar?"
The technology in question is "active noise control," a.k.a. "active noise cancellation," a.k.a. "anti-noise," and it is one of the hot research topics in acoustics these days. Here is the bottom line: yes, active noise control works in the proper circumstances, but no, you cannot use it to soundproof an entire house.
Active control is sound field modification, particularly sound field cancellation, by electro-acoustical means.
In its simplest form, a control system drives a speaker to produce a sound field that is an exact mirror-image the offending sound (the "disturbance"). The speaker thus "cancels" the disturbance, and the net result is no sound at all. In practice, of course, active control is somewhat more complicated; see below.
The name differentiates "active control" from traditional "passive" methods for controlling unwanted sound and vibration. Passive noise control treatments include "insulation", silencers, vibration mounts, damping treatments, absorptive treatments such as ceiling tiles, and conventional mufflers like the ones used on today\u2019s automobiles. Passive techniques work best at middle and high frequencies, and are important to nearly all products in today\u2019s increasingly noise- sensitive world. But passive treatments can be bulky and heavy when used for low frequencies. The size and mass of passive treatment usually depend on the acoustic wavelength, making them thicker and more massive for lower frequencies. The light weight and small size of active systems can be a critically important benefit; see later sections for other benefits.
In control systems parlance, the four major parts of an active control system are:
* The plant is the physical system to be controlled; typical examples are a headphone and the air inside it, or air traveling through an air-conditioning duct.
* Sensors are the microphones, accelerometers, or other devices that sense the disturbance and monitor how well the control system is performing.
* Actuators are the devices that physically do the work of altering the plant response; usually they are electromechanical devices such as speakers or vibration generators.
* The controller is a signal processor (usually digital) that tells the actuators what to do; the controller bases its commands on sensor signals and, usually, on some knowledge of how the plant responds to the actuators.
Analog controllers may also be used, although they are somewhat less flexible and thus more difficult to use.
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