The world of microphones is changing at least on handheld gadgets. MEMS Mics Taking Over article is telling that 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 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.
MEMS microphones target all audio applications where small size, high sound quality, reliability and affordability are key requirements. MEMS microphones market is growing fast. Global MEMS microphone shipments expanded to 2.6 billion units in 2013, up 37% from 1.9 billion in 2012. By 2017, shipments will reach 5.4 billion units.
The biggest reason for the fast growth curve is that smartphone and tablet makers are putting as many as eight MEMS mics into their devices.In addition to mobile devices MEMS microphones are finding their way to connected automobiles and hearing aids. In addition to audio applications there are other uses for MEMS microphones: engineers have found a whole bunch of applications that can use them as a substitute for more specialized sensors starting in 2015.
A MEMS microphone package is unique among semiconductor devices, in that there is a hole in the package for the acoustic energy to reach the transducer element. In electrically biased diaphragm MEMS microphone the transducer is essentially a variable capacitor with an extremely high output impedance in the gigaohm range, and the transducer’s signal is sent to a preamplifier before it can be used. An analog MEMS microphone’s output impedance is typically a few hundred ohms. There are also digital interfaces. PDM is the most common digital microphone interface; this format allows two microphones to share a common clock and data line. With PDM interface decimation happens in the codec or DSP (microphone supplies just pulse modulated signal at around 3 MHz clock). Some Digital microphones move the analog-to-digital conversion function from the codec into the microphone (outputs for example I2S format serial data), enabling an all-digital audio capture path from the microphone to the processor.
MEMS micrphones are no longer special devices that you can buy from just few specialized source. Now major distributors have a good selection of MEMS microphones available. For example Mouser is an authorized distributor for many MEMS microphone manufacturers including Knowles Acoustics, STMicroelectronics, & Wolfson Microelectronics, and they seem to have a good selection of MEMS microphones. Also Digikey has a selection of MEMS micrphones.
If you plan to use MEMS microphone in your design, check out the following application notes and articles: