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Microelectromechanical systems (MEMS) microphones have many of the typical advantages of MEMS devices, including ultra-small size, low power consumption, and stable performance (not changing with time and temperature). However, the audio characteristics of these microphones are not sufficient to meet certain design requirements, such as the need to capture sound from a distance or when multiple microphones are required. Today, high-performance MEMS microphones are realizing new possibilities, and many acoustic experts will point out that self-noise is the first feature to consider.
Self-noise - things you need to know
Any microphone produces a level of noise, including its electronic circuitry, sensor components, and housing. This inherent noise is called "self-noise." Anyone who has used a mobile phone is familiar with this sound. For example, when the phone is connected but no one is talking, the buzz you hear is self-noise.
For electronic designers, microphone self-noise is a permanent constraint. Our goal is simply to have the microphone provide as much signal as possible to the rest of the signal chain. The inherent self-noise of the microphone is also called the noise floor, and some of the signals captured from the audio source will be lower than the noise floor.
The higher the noise of the microphone, the less signal you get. The lower the noise of the microphone, the more margin there is to isolate the desired sound from the unwanted noise. This way, the processor (DSP or codec) has more signals to process. Therefore, a quieter microphone can make the output of the signal chain clearer.
The higher the signal-to-noise ratio of the microphone, the quieter the microphone, and vice versa.
While high SNR is useful in any situation, this feature is not that important if the audio source is very close to the microphone. For such near field applications, there will usually be enough signals. In applications where the microphone is far from the sound source, if a low-SNR, high-noise microphone is used, the resulting signal will be very poor or inaudible.
Meaning of the feature
The self-noise or noise floor of the microphone largely determines the quality of the audio that can be captured and transmitted to the signal chain. Signal-to-noise ratio (SNR) and equivalent input noise (EIN) are two characteristics that describe where the noise floor is located.
Compared to previous technologies, the self-noise of MEMS microphones has been greatly reduced (see Figure 1).
Figure 1 MEMS microphone is greatly reduced from noise
1 Signal to noise ratio (SNR)
SNR is the ratio of the reference signal to the noise floor of the microphone. The SNR of the microphone is the difference between its inherent self-noise and the standard 1 kHz, 94 dB SPL (1 Pa) reference pressure. This characteristic is usually expressed as an A-weighted value (dBA) at a bandwidth of 20 kHz. A-weighted means that the provided SNR includes a correction factor that corresponds to the sensitivity of the human ear to different frequency sounds. When comparing the SNRs of different microphones, make sure they are based on the same weight and bandwidth. If the measurements do not use the same weight and bandwidth, the comparison will be inaccurate.
2 equivalent input noise (EIN)
The equivalent input noise is to represent the output noise level of the microphone as a theoretical source of acoustic noise applied to the microphone input. The unit of measure is the sound pressure level, expressed in decibels (dB SPL). The SPL less than the EIN level is lower than the noise floor of the microphone.
EIN can be determined directly from the SNR characteristics of the microphone:
EIN =94dB-SNR
MIMO performance doubles SNR performance
Early microphones in the MEMS industry provided an SNR of approximately 58-60 dB, which was less acoustic than an electret condenser microphone (ECM). Now, this situation is changing, and the performance of MEMS microphones made by leading manufacturers has been greatly improved.
Ultra-low noise MEMS microphones The ADMP504 and ADMP521 have a noise floor that is more than 2 times lower than earlier MEMS microphones. The ADMP504 and ADMP521 are the first MEMS microphones to achieve 65dBA SNR (29dBA EIN).
The SNR of 65dBA is quite good even for electret microphones, but in the case of equivalent SNR, the size of the ECM is usually much larger than that of the MEMS microphone. When the ECM is downsized, its SNR drops rapidly (see Figure 2). In addition, ECM does not have the other advantages of MEMS microphones, such as consistent response to sound at all operating temperatures.
Figure 2 MEMS microphone VS ECMs
Capture distant sounds
What are the benefits of high performance MEMS microphones? While almost any application can benefit, you can now consider applying these microphones to areas that were previously unusable.
In applications such as video conferencing, professional audio, and industrial systems, sound sources are often not near the microphone. Far-field applications like this are where low-noise MEMS microphones come into play.
A specific example is a video call (such as Skype) using a webcam and tablet. MEMS microphones are now able to support high-definition audio capture of these products, and MEMS microphones are packaged very compact enough to fit into the smallest consumer electronics devices.
Another possibility is to use a MEMS microphone as an acoustic sensor. In industrial equipment design, it is not always feasible to place the microphone inside the machine casing. However, when the microphone picks up the sound transmitted through the hard barrier, there is a large amount of signal loss.
A low-noise microphone can collect enough signals. For example, in a flow control application, a microphone can detect production problems by listening to materials flowing through the pipeline.
Multi-microphone application
For any multi-microphone beamforming algorithm, low noise floor is also critical. Beamforming algorithms tend to result in higher system noise levels than single microphones in the array. Therefore, it is necessary to ensure that each microphone in the array has a high SNR.
Since beamforming can improve the directionality of the microphone array, such arrays are very popular in video conferencing systems. The video conferencing system includes both fixed video conferencing equipment in the company's conference room and a television set-top box where users can make video calls from their living rooms.
Multi-microphone beamforming is also used for security applications. Security and surveillance equipment is typically installed at a fixed location, but not all suspicious activity occurs within the field of view of the camera. With low-noise MEMS microphones, security cameras for residential and commercial buildings can use audio to detect which direction the sound comes from, pointing the lens at the target.
Consider the lowest noise of the microphone
For challenging audio capture applications, although more than one aspect of microphone performance will eventually be considered, a low noise floor is a determining success or failure. In the past, electret microphones had to be chosen if high SNR was required. Today, the choice is no longer limited to traditional microphone technology.
Today's MEMS microphones are more than twice the SNR of previous products, making them suitable for many new high-performance applications. With the maturity of MEMS microphone technology, modern MEMS microphones have achieved ultra-low noise, small size and reflow-resistant packages and all the other features that everyone expects.
Author: ADI company Jerad Lewis, Paul Schrei
——This article is selected from the electronic enthusiast network November, “Special Edition of Video and Audio Technologyâ€, “Perspective New Design Columnâ€, please indicate the source, please do not investigate it!
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