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主动降噪技术原理

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发表于 2014-8-27 00:00:12 | 显示全部楼层 |阅读模式


Active Noise Cancellation
by Ron Kurtus (revised 18 March 2012)

Noise cancellation is a method to reduce or completely cancel out undesirable sound, such that you can't hear it. It is often called Active Noise Cancellation because the electronics involved actively cause the noise cancellation in real time.
In the case of a single sound frequency, you can add the same frequency 180° out of phase to cancel the sound. It is much more difficult with complex sounds, such as the spoken word. The most common electronic noise cancellation device consists of special earphones. Other types of devices that do not require earphones are being developed. Note that this is an emerging technology and is often not available for common noise problems.
Questions you may have about such a device are:
  • How can you cancel out simple waveforms?
  • How is complex sound cancelled out?
  • What are some noise cancellation applications?


Simple wave cancellationAlthough sound is a compression wave, each tone or frequency can be represented by a sine wave of a given wavelength.
Simple sine wave for single sound frequency
Slightly out of phaseSuppose another pure sound of the same frequency was emitted, but just a fraction of a second later. That means it is slightly out of phase from the first sound or sine wave. The visual representation of the two waves would be as in the next illustration.
One pure sound a fraction of a second after the next
Graphing the resultYou can graphically see what the resulting wave would look like by adding values above and below the centerline (zero). This is illustrated by the green dots in the following drawing. The green line is approximately a sine wave of the same frequency.
Sum of two waves slightly out of phase
You would hear the same frequency, except that the sound would be twice as loud as the original. Note that since the frequencies are the same, you would not hear beat sounds.
(See Beat Frequencies in Sound for more information on that subject.)
Sound cancellation Finally, if the sound waves were 180° or one-half a wavelength out of phase, the sum of the waveforms would be zero. They would cancel out each other and there would be no sound.
Sum of waves equals zero sound
How is it possible in air? Although it is difficult to accept that adding two sounds can result in zero sound, you need to remember that sound is a compression wave in air or other media. That means the wave first compresses to an amount greater than normal air pressure. This is the positive part of the sine wave graphic. Then the air expands to a pressure less than normal air pressure. This is the negative part of the sine wave—the part below the zero centerline. Adding the positive pressure and negative pressure will give you the normal air pressure.
Canceling complex waves Electronically, it would be simple to detect a simple tone, adjust it 180° out of phase and add it to the original, thus canceling out the sound. Unfortunately, most sound is much more complex than a simple sine wave.
Complex waveform of a spoken word
Spectrum of frequenciesA spoken word consists of a spectrum of frequencies of different amplitudes. This means that to cancel out each waveform, the electronics would need to filter each frequency separately, determine its frequency, create the same frequency and amplitude at 180° out of phase, and then add it to the original.
Not perfect Since there are so many frequencies and fractions of frequencies in some sounds, it is impossible to cancel them all out with this method. Instead, the electronics selects a narrow band of frequencies and averages out the result. This is a fairly good job at noise cancellation, but it is not 100%.
ApplicationsThere are a number of great applications for active noise cancellation devices.
Noise cancellation almost requires the sound to be cancelled at a source, such as from a loud speaker. That is why the effect works well with headsets, since you can contain the original sound and the canceling sound in an area near your ear. In applications where the sound comes from many areas, such as in a room, it is difficult to cancel the sound from each area. But, scientists and engineers are working on solutions.
HeadsetsOne obvious application is that people working near aircraft or in noisy factories can now wear these electronic noise cancellation headsets to protect their hearing.
Students can wear these headsets to cancel out unwanted noise, while being able to listen to their own music while they study. These headsets are now fairly inexpensive, costing around $50.



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发表于 2014-8-28 10:24:17 | 显示全部楼层
看的好吃力,楼主辛苦了。。。
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发表于 2014-8-28 10:25:19 | 显示全部楼层
我觉得原理性的东西可以用通熟易懂的中文说的
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 楼主| 发表于 2014-8-28 10:47:32 | 显示全部楼层
向着北极星 发表于 2014-8-28 10:25
我觉得原理性的东西可以用通熟易懂的中文说的

有兴趣参与中文版的普及公益工作么?
有光电传感器的,磁性传感器的和降噪的。
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发表于 2014-8-28 15:09:34 | 显示全部楼层
Excelpoint_Tayl 发表于 2014-8-28 10:47
有兴趣参与中文版的普及公益工作么?
有光电传感器的,磁性传感器的和降噪的。 ...

这个可以有噢顺便学习专业英语
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