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MEMS Technologies: Microphone

  Date: 20/06/2013

MEMS Microphone – a breakthrough innovation in sound sensing

Vishal Goyal – Technical Marketing Manager, ST Microelectronics India

This article focuses on MEMS Microphone, its application and advantages. The article highlights the advantages of MEMS microphone over traditional solution. The article also introduces readers to MEMS Microphone construction and its key parameters. The article also focused on how MEMS Microphone has enable breakthrough innovations in consumer, medical, security and Automotive application. Finally article also highlights the leading position which STMicroelectronics has acquired through its innovative technology and supply chain management.

Introduction

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.

mic

ECM Microphone

mems microphone

MEMS Microphone

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


mems microphone



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)

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

Dynamic Range and AOP:
• The dynamic range is the difference between the minimum and maximum detectable sound by the microphone without distortion:
• The maximum signal that the microphone can “listen” without distortion is also called acoustic overload point (AOP). For both analog and digital ST microphones the AOP is 120dBSPL as sound pressure
• The minimum signal that a microphone can “listen” depends on its SNR. In other words, the minimum signal is equivalent to the residual noise in terms of dBSPL

Frequency response:
The frequency response of a microphone in terms of magnitude:
• Indicates the sensitivity variation across the audio band. Or even, this parameter describes the deviation of the output signal from the reference 0dB
• Typically the reference for this measurement is the exactly the sensitivity of the microphone à 0dB = 94dBSPL @ 1kHz
• The typical frequency response of a microphone shows a roll-off at low frequency due to ventilation hole and an rise up at high frequency due to Helmholtz effect
• The frequency response of a microphone in terms of phase:
• Indicates the phase distortion introduced by the microphone. In other words the delay between the sound wave moving the microphone membrane and the electrical signal at the microphone output
• This parameter includes both the distortion due to the membrane and the ASIC


ST’s Microphone offers Best acoustical performances - Best frequency response, Omni-directional polar pattern, Best Sensitivity , Reduced phase rotation, Optimized audio quality (SNR > 60dB), Precise unit-to-unit-matching. It also offers Highest reliability and robustness, Superior stability of humidity, temperature and dust parameters.

Directional acoustic patterns using MEMS Microphone

An omnidirectional microphone response is generally considered to be a perfect sphere in three dimensions. The smallest diameter microphone gives the best Omni-directional characteristics at high frequencies. This is the reason that makes the MEMS microphone the best Omni-directional microphone
But MEMS Microphone can also be used in array to modify the response according to desired acoustic patterns

mems microphone


As an application example

The signals of a couple of microphones are processed** to shape the response along the x direction

mems microphone


The physical and acoustic parameters of ST’s MEMS microphones perfectly fit the challenging requirements of distant-speech interaction systems. The small form factor allows the researchers to easily embed entire arrays of microphones in the walls, desks, or speech-enabled appliances of the automated home, while the microphones’ excellent acoustic characteristics, coupled with sophisticated signal-processing technologies, will make it possible to identify and capture an individual speaker from several meters away, in a crowded room with music playing.

The distant-speech interaction capability will not only dramatically change the way people interact with technology, but can make a real difference for those who can’t easily move around, such as the elderly or the motor-impaired. In addition to the home scenarios, the distant-speech interaction systems can find use in robotics, tele-presense, surveillance and industry automation.

STMicroelectronics advantages:
ST MEMS microphones are available in plastic packages. The patented technology breakthrough saves space and increases durability in consumer and professional voice-input applications, from mobile phones and tablets to noise-level meters and noise-cancelling headphones.

While other MEMS microphone manufacturers still produce devices with metallic lids, ST leads the way with industry-unique, innovative plastic packages. ST’s MEMS microphones are suitable for assembly on flat-cable printed-circuit boards that simplify design in today’s space-constrained consumer devices. The patented package technology allows equipment manufacturers to place the ‘sound hole’ either on the top or the bottom of the package to ensure the slimmest possible design and shortest acoustic path from the environment to the microphone. While the microphones with the sound hole on the top (top-port) suit the size and sound-inlet position requirements of laptops and tablets, the bottom-port microphones are mostly used in mobile phones.

ST has recently introduced - MP34DT01- first MEMS microphone in the market that couples the advantage of a top-port sound-inlet position with unparalleled signal-to-noise ratio (SNR) of 63 dB and flat frequency response in the full audio band of 20–20,000 Hz. The device’s best-in-class SNR makes it also suitable for applications beyond typical consumer applications, such as phonometers – sound-level meters that require high dynamic range. ST’s MEMS Microphone can keep frequency response even after the reflow soldering.

ST ‘s industry-unique capability to manage the whole supply chain and leading-edge MEMS production capacity enables short product-development cycles and time-to-volume for high-performance, cost-competitive silicon acoustic devices.


Conclusion:
MEMS microphone are entering new application areas such as voice-enabled gaming, automotive voice systems, acoustic sensors for industry and security applications, and medical telemetry. Its unique construction, performance and form factor has made possible what was unthinkable earlier. The readers are encouraged to use MEMS Microphone to take a lead over their competition and ST is committed to support its customers.


 

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