Have you ever asked yourself what is micro microphones in electronic world?
Now is time to reveal that information.
New sensors detect sound using light and heat The "micro" in microphone refers to the tiny sound the instrument must pick up, not to the devices themselves, which have hardly shrunk since Alexander Graham Bell`s first telephone receiver in 1876. Despite years of attempts, engineers have had little success adapting the methods of the microprocessor industry to make microphones that are as cheap and diminutive as computer chips. That is now changing. This past March, designs for several truly microscopic acoustic sensors were unveiled at a conference in Berlin. These micromicrophones may soon find use in nearly invisible hearing aids, experimental aircraft wings, and ultrasonic cameras that let divers see through dark and murky water. Some of the devices work just as an ordinary microphone does, only on a smaller scale. Engineers at Microtronic, a Danish firm, and Robert Bosch GmbH in Stuttgart, Germany, have created whole silicon wafers full of condenser microphones, in which the vibration of a charged membrane just 400 nanometers thick converts sound into an electrical signal. Although the device is only two millimeters square, it performs as well as conventional microphones 10 times its size. Others presented more radical designs. Jörg Sennheiser, chairman of one of Europe`s biggest microphone companies, demonstrated a "microflown" that uses no moving parts. Instead of sensing changes in pressure, the microflown detects the minuscule wind that accompanies each passing wave front. Two narrow bridges of platinum and silicon nitride, each lane only 10 microns wide, cross channels etched into a silicon wafer. Electrical current passed through the spans heats them to 300 degrees Celsius or more. As sound waves push air particles across the parallel lanes, the breeze transfers heat from the first bridge to the second. The effect is small, but it changes the resistance of the wires enough that the electrical signal rises well above the noise level. "The device is only sensitive up to about five kilohertz, but this is high enough for receiving speech clearly," says Hans-Elias de Bree, who invented the microflown as a graduate student and has since founded a company, Microflown Technologies, to develop the sensor for market. "The performance is now close to the condenser mikes used in telephone receivers," he says, "but [the microflown] is much more directional, so it eliminates background noise." Other applications may take advantage of the sensor`s ability to work where heat, dirt or vibration would damage conventional microphones. The need to operate in extreme environments spurred other researchers to invent micromachines that translate sound reflections into light patterns. In a prototype built by Young C. Cho and his colleagues at the National Aeronautics and Space Administration Ames Research Center, red laser light passes through an optical fiber, bounces off a gold-coated silicon nitride membrane and heads back into the fiber. As the membrane vibrates--by distances less than the width of an atom--it creates variations in the intensity of the light that are easily translated into electrical signals. Cho reports that the new device is 1,000 times more sensitive than any previous fiber-optic pressure sensor and bests even commercial reference microphones. Its small size also makes it much easier to mount on airplane and spacecraft surfaces for measuring air turbulence in wind-tunnel tests. Engineers at Boston University are developing a similar design in the hope of etching an array of 10,000 optical micromicrophones on a single silicon wafer. The U.S. Navy wants such a chip so that it can make acoustic cameras that SEALs can use to spot underwater mines at night and in turbid water. Robin Cleveland remarks that he already has single sensors working and predicts that his team will have an array ready within two years. --W. Wayt Gibbs in San Francisco