Opting for the quiet life
Tiny microphones provide a new way to eliminate background noise
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.