Almost every mobile device has ditched its old-fashioned electret microphone invented way back in 1962 at Bell Labs. Most microelectromechanical (MEMS) microphones just copy the basic principle driving electrets, electrically biased diaphragms whose capacitance changes as sound wave vibrate it. On the frontier, however, are piezoelectric MEMS microphones, which promise unheard of signal-to-noise ratios (SNR) of up to 80 dB (versus 65 dB in the best current capacitive microphones) in 2015.

Either way, MEMS microphones are growing like gangbusters, because engineers have found a whole bunch of applications that can use them as a substitute for more specialized sensors starting in 2015 -- getting more bang for the buck (or, if you want to be all official about it, a lower bill of materials).

The total MEMS market is predicted to top US$14 billion in 2015 and US$20 billion by 2018, Jean-Christophe Eloy, CEO and president of Yole Development, told the MEMS Executive Congress, the annual meeting of the nonprofit MEMS Industry Group. JeremieBouchaud, principal analyst at IHS, said more than US$1.4 billion of that will be for MEMS microphones.

The biggest reason for the fast growth curve is that smartphone and tablet makers are putting as many as eight MEMS microphones into their devices as they discover new ways to use them to eliminate the need for sensors that are often more expensive, such as proximity sensing -- beam forming to zero in on the speaker's location. But one of the most novel new applications is ultrasonic sensing.

Everyone who has a smartphone or tablet is sick to death of having to clean greasy fingerprints over and over and over again. Wouldn't it be cool if you could engage the icons, buttons, and whatnot while hovering just a few millimeters above the screen? Believe it or not, prototypes are already demonstrating that it is possible by using the MEMS microphones that increasingly surround the screen for noise canceling anyway. If those microphones are tuned to the ultrasonic range, then ultrasonic emitters will be able to sense the finger just above the screen, thus enabling touchless interaction (and no more cleaning over and over and over). There are dozens of other research projects under way to harness MEMS microphones -- which use very little power -- to do tasks that currently require power-hungry dedicated devices, like MEMS gyroscopes.

MEMS microphones are already providing increasingly high-definition audio quality for video recordings by canceling out the noise and following the voice of the speaker being filmed (beam forming). HD MEMS microphones are also drastically improving the accuracy of voice command functions. In 2015, they should allow devices to mix touch and voice for some functions. There are also burgeoning markets for MEMS microphones for Internet of Things devices, medical devices, and wearable electronics of all kinds. Every smart watch has a MEMS mic in it. So do smart glasses like Google Glass, even though the top buyers of MEMS microphones remain smartphone and tablet vendors, principally Apple and Samsung.