Table of Contents



PRODUCT SPOTLIGHT:
MEMS MICROPHONES

Introduction

MEMS microphones cover image

Welcome to the CUI Devices Product Spotlight on MEMS microphones. This resource will take a look at MEMS microphone technology, including a brief introduction to the two most common microphone technologies, an overview of key MEMS microphone specifications, and a discussion on microphone arrays and applications. CUI Devices' MEMS microphone product offering and features will also be presented.

Objectives

  • Introduce the two most common microphone technologies
  • Outline key specifications for MEMS microphones
  • Discuss MEMS microphone arrays and other potential applications
  • Highlight CUI Devices' range of MEMS microphone products

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What is a Microphone?
MEMS Microphone Example ECM Example

MEMS Microphones

Electret Condenser Microphones (ECM)

Microphones are electromechanical products used to convert sounds into electrical signals. The two most common microphone technologies today are MEMS and electret condenser. MEMS microphones are the newest of the two technologies and continue to grow in use thanks to their compact package sizes, improved audio quality, and more stable performance. On the other hand, electret condenser microphones have been a staple of audio recording applications for many years. Their flexible mounting configurations, wide operating voltages, and additional unidirectional and noise canceling directivity options, make them a viable microphone for many applications.

Applications

Microphones are used in a variety of applications across a range of industries, including consumer, industrial, and medical. While commonly used for audio recording or voice capture in products like cell phones or hearing aids, microphones can additionally be used to monitor voices and sounds. Examples of this include sound detection of glass breaking in security systems, sound recognition in baby monitors, or even microphones used as a sensor to detect unusual sounds in industrial machinery. Pairs of microphones are often used to record sounds in stereo, whereas arrays of MEMS microphones allow selective monitoring of sound sources while attenuating background noises. This can be useful when operating speakerphones in noisy environments.

Use Cases

  • Audio recording and voice capture
  • Detection sensors
  • Activity monitoring
  • Machinery failures

Industry Applications

  • Consumer
  • Scientific
  • Industrial
  • Medical
  • And More

MEMS Microphone Overview
Typical MEMS microphone construction

A MEMS device (Micro-Electro-Mechanical System) is a mechanical structure fabricated on a semiconductor die. In the case of a MEMS microphone a diaphragm is fabricated on a silicon die. A top or bottom sound port is then fabricated in the package to allow sound to enter. Once sound enters it causes movement of the diaphragm and the movement produces an electrical signal that is amplified and buffered by an electronic circuit in the microphone package. In some MEMS microphones an analog waveform is provided as the output signal. However, the inclusion of electronic circuits in the microphone enables an analog-to-digital converter (ADC) to convert the analog output signal to a digital signal inside the microphone. This digital signal is less susceptible to electronic noise than an analog output signal and is a convenient format if the output is being sent to a digital circuit.

Analog MEMS Microphones

MEMS microphones are constructed with multiple semiconductor die in a single package. Analog MEMS microphones will include an audio MEMS transducer connected to an audio preamplifier. The output of the audio preamplifier is made available for the user, meaning that analog MEMS microphones do not require any digital circuits in the application system. This makes the application circuits for MEMS microphones easy to design, but the analog output signal can be sensitive to external electrical noise. Analog MEMS microphones are available in 2-pin configurations for compatibility with electret microphone applications. However, the most common applications for MEMS microphones with analog outputs use 3-pin configurations with separate circuits for power and the output signal.

Analog, 2-pin configuration

Analog, 2-pin configuration

Analog, 3-pin configuration

Analog, 3-pin configuration

Digital MEMS Microphones

Digital MEMS microphones will include an audio MEMS transducer connected to an audio preamplifier and an analog-to-digital converter (ADC). The output of the ADC is a single bit digital signal and is made available for the user. These microphones require digital circuits in the application system to receive and process the output signal, but this digital output signal is not sensitive to external electrical noise. Digital MEMS microphones typically have 5 pins with provisions for sharing clock and output data pins between two microphones in a stereo application. The most common format used in digital microphones is pulse density modulation (PDM), which allows for communication with only a clock and single data line. I²S outputs are another option that include an internal decimation filter, which allows for processing to be completed in the microphone itself. This means the microphone can connect directly to a digital signal processor (DSP) or microcontroller, eliminating the need for an ADC or codec in many applications.

Digital Output Construction

Applications image

Key Specs: Top & Bottom Ports
Top and Bottom port configurations

In addition to analog or digital outputs, there are a few other specifications for MEMS microphones to consider when making the proper selection. This slide and the following slides will discuss those parameters in greater detail.

First, comes the decision of whether to select a top port or bottom port microphone. Top port MEMS microphones have a sound hole fabricated in the top of the metal case that houses the internal circuitry, while the bottom port has a hole drilled on the bottom next to the solder pads. With a bottom port, a hole will also need to be drilled in the PCB to allow sound to enter. In general, the decision to select a top or bottom port MEMS microphone comes down to how the microphone fits in with the overall design.

Key Specs: Sensitivity & Sensitivity Tolerance

A MEMS microphone’s sensitivity rating is a measure in dB of how much output a microphone produces for a given sound level. A larger sensitivity number is better, but the sensitivity number will typically be negative. Therefore, a sensitivity rating of -5 dB is better than a sensitivity rating of -25 dB. Analog outputs usually measure sensitivity in dBV which is decibels relative to 1 volt RMS, while digital is measured in decibels relative to full scale output or dBFS.

Sensitivity tolerance is another important specification especially when using MEMS microphones for noise cancelation or array applications. Typical MEMS microphones carry sensitivity tolerances from ±3 dB down to ±1 dB, which allows for closer matching of sensitivities from microphone to microphone. This is one specification where MEMS microphones hold a distinct advantage over traditional electret condenser microphones.

Sensitivity Equations

Ratio of microphone output signal with 1 Pa sine wave at 1 kHz to reference output signal:

  • Analog - Sensitivity (dBV) = 20 x log10(SmV/Pa ÷ Ref); Ref = 1000 mv/Pa
  • Digital - Sensitivity (dBFS) = 20 x log10(S%FS ÷ Ref); Ref = 1.0

Key Specs: Signal to Noise Ratio

Top Port Configuration

Signal-to-noise ratio (SNR) is an indicator in dB of how much background electrical noise the microphone’s MEMS element and ASIC will introduce into the system. This ratio looks at the reference signal (measured when sound pressure is 1 Pa at 1 kHz) and the residual noise at the microphone output. A larger SNR is desired meaning an SNR of 59 dB is better than 36 dB.

  • Ratio of the desired signal to the undesired noise measured in dB
  • SNR = 20 log(PS/PN)
    • PS – Output signal power level
      • Measured at 1 Pa (94 dB SPL) at 1 kHz
    • PN – Noise signal power level
      • Measured at 20 kHz bandwidth, A-weighted in quiet anechoic chamber

Key Specs: Dynamic Range & Acoustic Overload Point (AOP)

The dynamic range is a measure in dB of the loudness range over which a microphone is useful. The minimum signal represents what the microphone can distinguish from residual noise, while the maximum signal is how high the microphone can perform without distortion. A larger dynamic range number is better, so a dynamic range of 95 dB is better than 63 dB. The maximum signal of the dynamic range can also be referred to as the acoustic overload point (AOP). This is the sound level where distortion rises rapidly and is typically capped at a distortion level of 10%.

MEMS Microphone Arrays
Example of MEMS array applications

MEMS microphones can be placed in physical arrays to enhance the signal quality of the output from the microphones - also known as beamforming. Signals can be extracted from noisy environments by adding the input signals of the desired sound and subtracting the input signals of the undesired sounds. This is useful in environments where ambient noise is high, such as conference rooms or motor vehicles. Another practical application of microphone arrays is to locate the direction a particular sound is coming from. Implementing these microphone arrays requires microphones with well matched characteristics to generate the signals, which is where the tight sensitivity tolerances of MEMS microphones come into play. The host system will incorporate digital signal processing (DSP) capabilities to perform the necessary calculations on the microphone signals.

Broadside Microphone Arrays

Example of a Broadside Array

Two of the most common MEMS microphone arrays are broadside and endfire. Well suited for a computer monitor or TV screen, broadside microphone arrays are a one or two-dimensional array of microphones placed perpendicular to the source of the desired sound. The signals from each of the microphones are then summed to produce the desired electrical signal. Sounds perpendicular to the array, either in front or behind, will arrive at the microphones at the same time and sum constructively, while sounds not perpendicular will arrive at differing time delays, resulting in a lower level electronic signal.

Endfire Microphone Arrays

Example of an Endfire Array

Endfire microphone arrays are required to be pointed at the sound of interest and will only capture sound that is directly in front of the array, making handheld microphones a good application for this topology. They are constructed by arranging a line of microphones in the direction of the desired sound source, where the desired sound arrives at each microphone with a different time delay. Each microphone’s processing circuit may employ an electronic time delay to compensate for the audio time delay of the microphones.

Sound Location Detection Arrays

Example of a Location Detection Array

Sound location detection microphone arrays are another common array topology, which can typically be implemented by placing microphones on the perimeter of a circle or a sphere. Electronic signal processing is then used to determine the desired signal from each microphone, while the relative time delay of the desired signal between each microphone is used to pinpoint the source of the sound relative to the microphone array. This is often used for gunshot detection by the police and military.

Available Products
Available MEMS options

CUI Devices offers a wide selection of MEMS microphones in compact, low profile packages as small as 2.75 x 1.85 x 0.95 mm. Available in analog, digital pulse density modulation (PDM), or digital I2S output types, the models feature sensitivity ratings from -44 up to -26 dB, signal-to-noise ratios from 57 up to 65 dBA, and tight sensitivity tolerances down to ±1 dB. CUI Devices' MEMS microphones offer top or bottom port locations, with low current draw down to 80 microamps (μA) and operating temperature ranges from -40 up to 105°C. The models are also reflow solder compatible, adding to their flexibility during the manufacturing process.

Features

  • Analog, digital PDM, or digital I2S outputs
  • Compact, low profile footprints
  • Top or bottom sound port models
  • Low current consumption down to 80 μA
  • -44 up to -26 dB sensitivity ratings
  • 57 up to 65 dBA signal-to-noise ratios
  • Tight sensitivity tolerances for array applications
  • Reflow solder compatible
  • Rectangular or round form factors
  • -40 up to 105°C operating temperature ranges

Summary

MEMS microphones are a growing technology in the audio market, thanks to a number of advantages over traditional electret condenser microphones. Their improved audio performance, reduced vibration sensitivity, and tight sensitivity tolerances make them an ideal solution for a range of applications and industries. CUI Devices offers a range of MEMS microphone models with a variety of key performance specifications to match the unique audio needs of an engineer’s design.