11.4 MEMS microphones and speakers

MEMS microphones and speakers are revolutionizing audio tech in consumer electronics. These tiny devices use capacitive, piezoelectric, or electret mechanisms to convert sound to electrical signals and vice versa, enabling high-quality audio in compact gadgets.

MEMS audio components face challenges in miniaturization and performance. Researchers are developing new designs and materials to improve sound quality, efficiency, and directionality, paving the way for innovative audio experiences in smartphones, wearables, and smart home devices.

MEMS Microphone Types

Capacitive Microphones

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• Capacitive microphones operate based on the principle of capacitance change caused by the movement of a diaphragm in response to sound waves
• Consist of a fixed backplate and a movable diaphragm that forms a variable capacitor
• Sound waves cause the diaphragm to vibrate, resulting in a change in capacitance between the diaphragm and the backplate
• Capacitance change is converted into an electrical signal using a preamplifier circuit
• Offer high sensitivity, wide frequency response, and low noise levels compared to other types of microphones (electret and piezoelectric)
• Commonly used in smartphones, laptops, and smart home devices for voice recognition and audio recording applications

Piezoelectric Microphones

• Piezoelectric microphones utilize the piezoelectric effect to convert sound waves into electrical signals
• Contain a piezoelectric material (quartz, PZT) that generates an electric charge when subjected to mechanical stress caused by sound waves
• Sound waves cause the piezoelectric material to deform, producing a voltage proportional to the sound pressure
• Offer a wide dynamic range and can withstand high sound pressure levels without distortion
• Suitable for applications requiring high acoustic overload points such as in industrial environments or near loud sound sources (jet engines, explosions)

Electret Microphones

• Electret microphones are a type of condenser microphone that uses an electret material to maintain a permanent electric charge
• Consist of a fixed backplate and a movable diaphragm coated with an electret material (permanently charged dielectric)
• Sound waves cause the diaphragm to vibrate, changing the capacitance between the diaphragm and the backplate
• Capacitance change is converted into an electrical signal using a built-in FET (Field Effect Transistor) amplifier
• Offer good sensitivity, low cost, and simple construction compared to other microphone types
• Commonly used in consumer electronics such as headsets, voice recorders, and video cameras

MEMS Speakers

Miniaturization and Integration

• MEMS speakers aim to miniaturize traditional speaker technology to enable integration into compact electronic devices
• Utilize micromachining techniques to fabricate speaker components on a silicon substrate
• Miniaturization allows for the development of small-scale acoustic transducers suitable for portable devices (earbuds, smartwatches)
• Integration of MEMS speakers with other electronic components on a single chip reduces the overall size and cost of the device
• Enable the development of novel audio applications such as directional sound and personalized audio experiences

Acoustic Transducer Mechanisms

• MEMS speakers convert electrical signals into sound waves using various acoustic transducer mechanisms
• Piezoelectric MEMS speakers utilize the inverse piezoelectric effect to generate sound
  • Applying an electric field to a piezoelectric material causes it to deform and generate sound waves
• Electrostatic MEMS speakers operate based on the principle of electrostatic attraction
  • Consist of a fixed perforated backplate and a movable membrane
  • Applying a voltage between the backplate and the membrane creates an electrostatic force, causing the membrane to vibrate and produce sound
• Thermoacoustic MEMS speakers generate sound through the thermal expansion of air
  • Rapidly heating and cooling a thin film creates pressure variations in the surrounding air, resulting in sound waves

Challenges and Advancements

• MEMS speakers face challenges in achieving high sound pressure levels (SPL) and wide frequency response compared to conventional speakers
• Researchers are developing advanced MEMS speaker designs to improve performance characteristics
  • Optimizing the geometry and materials of the speaker components to enhance sound quality and efficiency
  • Investigating novel actuation mechanisms such as piezoelectric and thermoacoustic transducers to overcome the limitations of electrostatic speakers
• Advancements in MEMS fabrication techniques enable the integration of multiple speaker elements on a single chip for improved sound output and directional control
• Ongoing research focuses on enhancing the power efficiency and reliability of MEMS speakers for extended battery life in portable devices

Performance Characteristics

Frequency Response and Sensitivity

• Frequency response refers to the range of frequencies a MEMS microphone or speaker can accurately reproduce
• MEMS microphones typically have a flat frequency response over a wide range (20 Hz to 20 kHz) to capture various sound sources accurately
• MEMS speakers aim to achieve a wide frequency response to reproduce audio content faithfully
• Sensitivity represents the microphone's or speaker's ability to convert sound pressure or electrical signals into an output signal
• Higher sensitivity in MEMS microphones results in a stronger output signal for a given sound pressure level
• MEMS speakers with higher sensitivity require less power to produce a given sound pressure level

Signal-to-Noise Ratio and Directionality

• Signal-to-noise ratio (SNR) is the ratio of the desired signal to the background noise in a MEMS microphone or speaker
• Higher SNR indicates better sound quality and clearer audio capture or reproduction
• MEMS microphones with high SNR can capture clean audio even in noisy environments (crowded places, outdoor settings)
• MEMS speakers with high SNR can reproduce audio content with minimal distortion and background noise
• Directionality refers to the microphone's or speaker's ability to capture or reproduce sound from specific directions
• MEMS microphones can be designed with directional patterns (omnidirectional, unidirectional) to focus on sound sources from particular directions
  • Omnidirectional microphones capture sound equally from all directions
  • Unidirectional microphones (cardioid, hypercardioid) are more sensitive to sound coming from the front and less sensitive to sound from the sides and rear
• MEMS speakers can be arranged in arrays to achieve directional sound reproduction
  • Beamforming techniques can be applied to control the direction of sound propagation
  • Enables applications such as virtual surround sound and personalized audio zones in consumer devices (smartphones, laptops)