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Minimising self noise for MEMS microphone apps

Posted: 13 Dec 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Microelectromechanical system? mics? signal-to-noise ratio?

The ultra-low noise Analog Devices ADMP504 and ADMP521 MEMS microphones have lowered the noise floor of earlier MEMS mics by more than 2x. The ADMP504 and ADMP521 are touted as the first MEMS microphones to reach the level of 65 dBA SNR (29 dBA EIN).

An SNR spec of 65 dBA is good even for an electret microphone, but ECMs tend to be much larger than MEMS mics with comparable SNRs. As the size of an ECM gets smaller, its SNR drops quickly (figure 2). ECMs also don't offer the other advantages that MEMS mics do, such as a consistent response to sound across all operating temperatures.

Figure 2: As the size of an electret condenser microphone gets smaller, its SNR drops quickly.

Capturing sound at a distance
Where can you take advantage of the SNR of a very high performance MEMS microphone? Though almost any application would benefit, you can now consider these microphones where you might not have been able to before.

In applications such as video conferencing, professional audio, and industrial systems, the source of the sound is often not next to the mic. Far-field applications like these are good examples of where a low-noise MEMS microphone will be beneficial.

One specific example is video calling (think Skype) with webcams and tablet computers. Now MEMS microphones can enable high-definition audio capture for these products, and MEMS mic packages are compact enough to place inside even the smallest consumer electronics devices.

Using MEMS microphones as acoustic sensors is another possibility. In industrial equipment designs, positioning a microphone inside the housing of a machine isn't always practical. However, you sacrifice a lot of signal when the microphone has to pick up sound transmitted through a solid barrier.

A microphone with low self noise will be more capable of acquiring an adequate signal. For instance, a microphone in a flow control application could identify production problems by listening for material flowing through a tube.

Multiple microphone applications
A low noise floor is also critical for any multi-microphone beamforming algorithm. Beamforming algorithms often result in a higher system noise level, as compared with a single microphone in the array. Therefore it becomes critical for each mic in the array to have a high SNR.

Because beamforming increases the directionality of a microphone array, these arrays are popular in videoconferencing systems. That includes stationary videoconference equipment for corporate conference rooms, as well as the TV set-top boxes people use to make video calls from their living rooms.

Multi-microphone beamforming is also used in security applications. Security and surveillance equipment is normally mounted in a fixed spot, and of course, not all suspicious activity will take place within the camera's field of view. With low-noise MEMS microphones, security cameras on homes and commercial buildings can use audio to detect which direction a sound is coming from and point the camera lens at that target.

Consider how low the microphone can go
Though you will ultimately consider more than one aspect of a microphone's performance, a low noise floor is a make-or-break spec for challenging audio capture applications. So if you needed high SNR in the past, you would probably have had to choose an electret mic. These days your options are no longer limited to traditional microphone technology.

MEMS microphones are prepared for a number of new high performance applications now that some mics offer more than double the SNR of their predecessors. The maturing of MEMS microphone technology has added ultra-low noise to the compact size, reflow-compatible packaging, and everything else you expect from modern MEMS microphones.

References
1. Microphone Specifications Explained by Jerad Lewis Analog Devices Application Note AN-1112).

About the authors
Jerad Lewis is a MEMS microphone applications engineer at Analog Devices. He joined the company in 2001 after getting his BSEE from Penn State University. Since then, he's supported different audio ICs, such as converters, SigmaDSPs, and MEMS microphones.Paul Schreier is product marketing manager for MEMS Microphones in Analog Devices' MEMS and Sensor Technology Group and holds degrees in Computer Engineering and Finance from the University of Nebraska and an MBA from Boston College. Prior to joining ADI, Mr. Schreier held various sales and marketing positions at National Semiconductor, Texas Instruments and NXP.

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