Active+and+Passive+Noise+Cancellation

__ Introduction __

 * Active ** and ** Passive ** Noise Cancellation, or noise control, means reducing sound emissions of ambient noise (noise pollution) to increase personal comfort or environmental considerations.


 * Active Noise Cancellation **: refers to sound reduction using a power source. Advantages include: more effective at low frequencies, less bulky, able to block noise effectively.
 * Passive Noise Cancellation : **refers to sound reduction by using insulation materials, rather than a power source. Advantages include: More effective above 2000Hz, does not require a power source, cheaper than active noise cancellation technology.

__ Applications of the Topic __
How it works: It reduces unwanted sounds by electro-acoustical means. First, adaptive devices analyze the waveform of the background noise. Second, an internal circuitry produces a sound that is the exact mirror-image of the disrupting sound. (It's important to note the sound waves are longitional waves, consisting of a compression phase and rarefraction phase.) The internal cirucuitry creates waves with the same amplitude of the background noise, with the compressions (crests) line up with the rarefractions (troughs), and the two waves will collide with eachother, and cancel each other out (destructive interference), and the result is no sound at all. This is especially useful for headphones, so a person can listen to their music and not background noise. How it works : It works by using insulating materials to dampen or blocking the sound waves, reducing the noise by 15-20 decibals. It accounts for 99% of headphones on the market because it's more accessible and cheaper to produce. __ Relation to Sound and Waves __ It relates to sound and waves because the main purpose of Active or Passive Noise Cancellation is to block out or cancel sound waves. Active Noise Cancellation uses the idea of total destructive interference by producing waves that will cancel the longitional waves of sound. Passive Noise Cancellation uses the idea that as waves travel through a medium, they get weaker, therefore lessening the sound levels.
 * Active Noise Cancellation Examples **: headphones, speakers, microphones, air-conditioning ducts, mufflers, submaries, aircrafts.
 * Passive Noise Cancellation Examples: **headphones, walls

[] [|http://en.wikipedia.org/wiki/Active_noise_control#Active_vs._passive_noise_control] [] []