Noise-cancelling technology

噪声抑制技术

Opting for the quiet life

选择安静的生活

Tiny microphones provide a new way to eliminate background noise

微型麦克风使背景噪声的成为可能

Feb 11th 2010 | From The Economist print edition

MANY of those who talk loudly into their mobile phones are just inconsiderate show-offs for whom no punishment is too evil. Sometimes, however, there is an excuse. Noise in the background can make it hard for your interlocutor to hear what you are saying. Raised voices are an inevitable consequence

有许多对着手提电话高声谈论的人仅仅是为了炫耀自己，这些毫不顾及旁人感受的人得不到任何惩罚是件令人厌憎的事情。虽然如此，但有时也确实有一些原因。（比如在）背景噪声使对方难以听清你到底在说什么时，提高话音也就难以避免了。

Soon, though, this excuse will vanish. Thanks to advances in manufacturing techniques, which allow miniature mechanical components to be built into electronic chips, it is now possible to add better noise-cancelling features to phones, and also to other products, such as the small “earbuds” used to listen to music players.

虽然如此，但是很快，这个借口就将消失了。得益于制造技术的进步，使得在芯片上集成微型机械组件得以实现。如今在手提电话上可以增加改善了的噪声抑制功能，同样包括其他产品，比如用来输出音乐播放器的耳塞机。

The idea behind active noise-cancellation goes back to the 1950s. Sound is a pressure wave in the air. By making an identical but inverted version of that wave and playing it through a loudspeaker, peak should fall on trough, and trough on peak. The two waves should thus, in theory, cancel each other out—leaving silence.

主动噪声抑制技术可以追溯到上个世纪50年代。声音是在空气中传播的压力波。通过扬声器发送和噪声频率相同，相位项反的声波，使声波的波峰和噪声的波谷叠加，波谷和噪声的波峰叠加，这两种声波叠加的结果在理论上将使它们相互消失，而只有寂静留下。

But the practice is hard. Unless the sources of the noise and the antinoise coincide, the wave patterns will not overlap properly and the effect will be lost. A good approximation to perfect overlap can, however, be made by playing the anti-noise as close to the ear as possible, so that the part of the noise which is actually doing the irritating, namely that going into the listener’s head, gets cancelled, even if the rest does not. The problem with this approach is that you have to pick up the incoming soundwave with a microphone, work out in an instant what the antisoundwave needs to look like, and then get ahead of the original soundwave by sending a signal to a loudspeaker that will broadcast the antisoundwave at precisely the moment the soundwave arrives.

但是实现却很困难。如果噪声和反噪声不同步，它们的波形将无法准确地叠加，从而失去抑制效应。一个较好的xx叠加的方法可以通过将反噪声源的位置尽可能的靠近耳朵，这样一部分确实让人感到不快的噪声，也就是可能传播到听者头部的噪声可以被xx，即便是有部分噪声依然存在。这种方法的关键在于，需要一个麦克风截获噪声，并立即计算出反噪声频率以及发射时机，在一个xx的时刻（在噪声波抵达时）发出驱动扬声器的反噪声叠加信号。

Bose on

In the 1980s Amar Bose, an electrical engineer at the Massachusetts Institute of Technology and founder of the audio company of that name, pulled off this trick for the first time by designing pilots’ headphones that reduce the noise of jet engines. Now, retail versions of such noise-cancelling headsets are a favourite with passengers, too, and the technology is spreading.

上个世纪80年代，Amar Bose，麻省理工学院的电子工程师，也是Bose博士音响公司的创始人。首次将这个理论实现在为飞行员设计的减少航空发动机噪音影响的头戴式受话器上。现如今，各种版本的抑制噪声的头戴式耳机也很受飞机乘客的欢迎。这种技术还在扩散。

The closer to the ear the microphone is located, the simpler the process becomes. For that, the smaller the microphone is, the better. Small mikes are easier to fit into earpieces, and recently they have been getting very small indeed. Jérémie Bouchaud and his colleagues at the German office of iSuppli, a market-research company that has been analysing the sector, are increasingly finding a new sort of tiny microphone in the products which they tear apart in order to identify manufacturing trends.

麦克风的位置距离耳朵越近，处理过程将变得越简单。因此，麦克风的尺寸越小越好。微型麦克风很容易安装在耳塞中，最近它们确实变的非常小了。iSuppli，一家市场分析公司的德国办事处的Jérémie Bouchaud和他的同事分解了一种新型微型麦克风，以找出制造业的发展趋势，在对这个领域的分析过程中，他们有了更多的发现。

These mikes use a technology known as MEMS (microelectromechanical systems). A silicon membrane is fabricated over a tiny cavity. This allows it to vibrate in sympathy with sound waves, like the diaphragm in a conventional microphone. The vibrations change the electrical properties of the device and this electrical signal is converted into an audio signal. One of the smallest MEMS microphones found by iSuppli is a device just over 1mm thick. It is fitted into the latest version of Apple’s ultra-slim iPod Nano to record sound for its video camera.

这些话筒的使用MEMS (microelectromechanical systems微机电系统)技术制造。在微xxxx内嵌入硅薄膜。这使得它就象常见的麦克风振动膜一样可以和声波产生共振。共振改变了这种装置的电特性，电信号转变为音频信号。iSuppli 在一种装置中发现的MEMS麦克风厚度刚好超过1 mm。它被安装在Apple{zx1}款的超薄型iPod Nano上，为其中的摄影机记录声音信号。

One mike, however, is often not enough. Mr Bouchaud is finding more than one MEMS microphone inside ever more of the products he disassembles, and these extra mikes are often being used to add noise-cancelling features. Two of the newest mobile phones, Google’s Nexus One and Motorola’s Droid, both use MEMS microphones in this way. Mr Bouchaud expects this feature will be incorporated into more products. Indeed, last year, Knowles Acoustics of Itasca, Illinois, which has been making MEMS microphones since 2003, became the first manufacturer to have shipped a billion of the devices.

然而，一个麦克风是远远不够的。Bouchaud先生在他分解的产品中总是发现远远多于一个的MEMS麦克风。这些多出来的麦克风被用来增强噪声抑制特性。两种{zx1}型的手提电话，Google的Nexus One以及Motorola的 Droid 也如此使用MEMS麦克风。Bouchaud先生预期这种特性将被应用到更多的产品中。事实上，在去年，位于伊利诺斯州的伊塔斯卡的Knowles Acoustics（楼氏声学公司）成为{zd0}的MEMS麦克风制造商，发运了10亿个这样的装置，该公司从2003年开始生产MEMS麦克风。

Nokia has been working with Wolfson Microelectronics of Edinburgh to produce a wireless headset for mobile phones and music players that uses ten MEMS microphones: two to record speech and eight to cancel noise. Sony has new noise-cancelling earbud-type headphones that use MEMS technology. It claims these can eliminate more than 90% of background noise. The earbuds have three separate settings: one for planes, one for trains and buses, and one for general office hubbub.

Nokia和爱丁堡的Wolfson Microelectronics（欧胜微电子有限公司）合作制造的移动电话以及音乐播放器使用的无线耳塞拥有十个MEMS麦克风：两个用于话音收录，八个用于噪声抑制。Sony也有新的使用MEMS技术的有噪声抑制功能的耳塞式受话器，它称其产品可以抑制90%的背景噪声。这种耳塞式受话器有三种分别的设置：一种是在飞机上，一种是在列车或公共汽车上，另外一种用于嘈杂的办公室环境。

Other uses are emerging. Akustica, based in Pittsburgh, Pennsylvania, puts a MEMS microphone complete with the circuitry needed for digital output into a package less than 5mm square. Digital output helps protect against radio and electromagnetic interference, which can cause audio problems with microphones mounted on computer displays. It also allows for elaborate signal processing.

其他应用也浮现出来。位于宾夕法尼亚州的匹兹堡的Akustica在小于5平方毫米的面积上集成了一个MEMS麦克风以及满足数字输出需要的电路元件。数字输出信号帮助抑制可以导致安装在计算机显示器上的话筒出现干扰音频的无线电波以及电磁干扰信号。这种装置同样也可以用来处理复杂的信号。

Using more than one microphone improves sound quality. Microsoft, for example, has been developing software that takes advantage of the different times an acoustic wave arrives at each microphone in an array around the screen of a computer. By analysing these signals the software is able to make the array work like a directional microphone. In effect it turns an electronic ear towards someone who is speaking—when communicating with Skype, say—and thus suppresses background noises. That should mean that there is no longer any reason for people to shout at their computers, either.

使用远多于一个的麦克风可以提高声音品质。比如，Microsoft,已经开发了一种软件利用声波到达安装在计算机显示器上的麦克风阵列的不同时间，通过分析这些信号，软件可以使这个阵列在定向传声器状态下工作。实际上，它开启了一个面向说话者的电子耳-当利用Skype通讯时-同时抑制背景噪声。也就是说，对那些对着他们的显示器高声叫嚷的人们再也没有任何理由这么做了。