MEMS Microphones A breakthrough innovation in sound sensing from STMicroelectronics

What are the advantages of MEMS microphones over traditional solutions? This article focuses on how MEMS Microphones have enabled breakthrough innovations in consumer, medical, security and automotive applications.

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STMicroelectronics Design and Aplication GmbH

A MEMS Microphone is a solid state integrated IC which can sense voice in the same way as ECM Microphone (Electret Condenser Microphone). They are getting increasing popular in modern devices such as Mobile Phone, Tablet, Laptop, Smart TV, Automotive voice recognition, gaming and Remote controller etc.

According to IHS iSuppli, the market for MEMS microphones for consumer electronics and mobile handsets is forecast to grow revenue at a CAGR of 23% between 2010 and 2014. The increased popularity of MEMS Microphone is attributed to its reliable monolithic structure, high tolerance of mechanical vibration, small footprint and height and optional digital output.

In addition, MEMS microphones enable dramatic advancements in sound quality in multiple-microphone applications. Such microphone arrays, facilitated by the small form factor, superior sensitivity matching and frequency response of ST’s microphones, enable the implementation of active noise and echo cancelling, as well as beam-forming, a sound-processing technology that helps isolate a sound and its location. These features are invaluable with the increasing use of cell phones and other devices in noisy and uncontrollable environments.

MEMS Microphone Construction

There are mainly two types of MEMS microphones – Analog which convert sound into corresponding voltage output and Digital which gives a digital output typically pulse density modulation [PDM].

MEMS microphone basically is an acoustic transducer.

• Transduction principle is the coupled capacity change between a fixed plate (back-plate) and a movable plate (membrane)

• The capacitive change is caused by the sound, passing through the acoustic holes, that moves the membrane modulating the air gap comprised between the two conductive plates

• The back-chamber is the acoustic resonator

• The Ventilation hole allows the air compressed in the back chamber to flow out and consequently allowing the membrane to move back

Key Parameters of MEMS Microphone

Sensitivity

• The sensitivity is the electrical signal at the microphone output to a given acoustic pressure as input. The reference of acoustic pressure is 1Pa or even 94dBSPL @ 1kHz**

• Sensitivity is typically measured:

• for Analog microphones in mV/Pa or even dBV = 20 * Log (mV/Pa / 1V/Pa)

• for Digital microphones in %FS or even dBFS = 20 * Log (%FS / 1FS)

**dBSPL= 20* Log(P/P0) where P0 = 20 μPa is the threshold of hearing. 20* Log(1Pa/20µPa)=94dBSPL

Directionality

• The directionality indicates the variation of the sensitivity response with respect to direction of arrival of the sound

• The STMicroelectronics MEMS microphones are OMNI-Directional which means that there is no sensitivity change at every sound source position in the space

• The directionality can be indicated in a Cartesian axis as sensitivity drift vs. angle or in a polar diagram showing the sensitivity pattern response in the space

Signal to Noise Ratio [SNR]

• The signal-to-noise ratio specifies the ratio between a given reference signal to the amount of residual noise at the microphone output

• The reference signal is the standard signal at the microphone output when the sound pressure is 1Pa @ 1kHz. In other words the microphone sensitivity

• The noise signal (residual noise) is the microphone electrical output at the silence. This quantity includes both the noise of the MEMS element and the ASIC

• Typically the noise level is measured in an anechoic environment and weighting-A the acquisition. The A-weighted filter corresponds to the human ear frequency response

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