How does a smartphone microphone work?

A microphone is a device that converts sound waves into electrical signals. In smartphones, microphones serve the essential purpose of capturing audio for phone calls, videos, voice assistants, and other features that rely on speech input. At a basic level, a microphone contains a thin membrane that vibrates when hit by sound waves. These vibrations then get converted into an electrical signal via electromagnetic induction. The microphone sends this audio signal to the smartphone’s processor, which can interpret the electrical signal as sound and enable many useful functionalities. Without a high-quality microphone to capture our voices and surrounding sounds accurately, smartphones would lack many of the convenient and practical features we use daily.

This article provides an in-depth look at how different types of microphones work in smartphones, examining the components and engineering that enable our mobile devices to hear. We will explore the clever noise-cancellation technologies that allow microphones to capture our voices clearly, even in loud environments. Additionally, we will consider key specifications like frequency response, sensitivity, and directionality that determine microphone performance. With an understanding of both microphone hardware and the audio processing software behind the scenes, we can better appreciate the impressive acoustic abilities of our ubiquitous smartphones.

Microphone Design

A MEMS (microelectromechanical system) microphone has a very different design from traditional microphones. Rather than using a movable coil or diaphragm suspended in a magnetic field, a MEMS microphone uses a very small diaphragm etched into silicon with a stationary backplate behind it. As sound hits the diaphragm, it vibrates and changes the capacitance between the diaphragm and backplate, converting the acoustic sound waves into an electrical signal.

The key components of a MEMS microphone are:

  • Diaphragm – A very thin membrane, usually made of silicon or plastic, that moves in response to sound waves.
  • Backplate – A rigid, perforated plate that forms a capacitor with the diaphragm. The changing distance between the diaphragm and backplate creates a variable capacitance.
  • ASIC – An application-specific integrated circuit that converts the capacitance changes into a digital output signal.

Compared to traditional microphones, MEMS microphones are much smaller, more sensitive, and more resilient. They can be batch manufactured using semiconductor fabrication techniques, enabling miniaturization and integration. However, they have a higher noise floor and nonlinearities compared to high-end condenser microphones.

Overall, MEMS microphones have revolutionized microphone technology by enabling tiny microphones to be embedded into consumer electronics like smartphones, hearing aids, laptops and more. Their small size and digital interface have enabled many new microphone applications and audio processing capabilities.

Microphone Types

There are three main types of microphones used in most smartphones today:

MEMS (Microelectromechanical systems) microphones – These microphones contain a miniature mechanical diaphragm and electrical components on a silicon chip. They are tiny, durable, and low cost. Most modern smartphones use MEMS microphones in their primary microphone assemblies (https://hearingreview.com/practice-building/practice-management/continuing-education/back-basics-smartphones-microphones).

ECM (Electret condenser microphones) – An electret material that holds a permanent electric charge replaces the mechanical diaphragm and capacitor plate found in traditional condenser microphones. ECM microphones offer better performance than MEMS mics and are found in some higher-end smartphone models (https://mynewmicrophone.com/what-kind-of-microphones-are-used-in-cell-phones/).

Studio-quality microphones – Some accessory microphones that can connect to smartphones via the charging port or wirelessly provide studio-quality audio capture. These include shotgun mics for video, lavalier mics for interviews, and small diaphragm condenser mics for music recording (https://www.engadget.com/best-mobile-microphones-for-recording-with-a-phone-154536629.html).

Sound to Electrical Signal

The key component that allows a microphone to convert sound into an electrical signal is the diaphragm. The diaphragm is a thin membrane that vibrates when it is impacted by sound waves. As sound waves reach the microphone, they cause the diaphragm to vibrate back and forth. Most smartphone microphones use either an electromagnetic or electrostatic technique to convert this vibration into an electrical signal.

In electromagnetic microphones, the diaphragm is attached to a coil of wire surrounded by a magnet. As the diaphragm vibrates, it causes the coil to move back and forth within the magnetic field, inducing an electrical current. The current generated by the coil matches the original sound wave that caused the vibration.

In electrostatic or condenser microphones, the diaphragm forms one plate of a capacitor, while a fixed backplate forms the second plate. As the diaphragm vibrates and its distance from the backplate changes, it leads to a change in capacitance between the two plates. This change in capacitance generates the electrical signal. Thus, the diaphragm’s mechanical vibrations are converted into an equivalent electrical signal through capacitance change.

In summary, the microphone’s diaphragm transforms pressure variations in sound waves into mechanical vibrations. Then through electromagnetic induction or capacitance change, these vibrations are converted into an analogous electrical signal for audio recording and processing.

Noise Cancellation

Noise cancellation in smartphone microphones utilizes technologies like dual-microphone systems to reduce background noise and improve audio quality. Most smartphones have a primary microphone for capturing the user’s voice, as well as a secondary microphone facing away from the user to pick up ambient noise in the environment. The smartphone’s digital signal processor (DSP) analyzes the ambient noise and generates an inverse audio waveform to cancel it out. This process, known as destructive interference, combines the ambient noise waveform with the inverse version to effectively cancel out the background sounds.

In addition to dual-microphone noise cancellation, smartphones apply filters and audio processing algorithms to further suppress noise. High-pass filters remove lower frequency noises like humming or traffic, while sound enhancers boost the frequencies of human speech. More advanced noise reduction can be achieved using machine learning models that are trained to identify speech versus ambient noise. Overall, the combination of multiple microphones, DSP filtering, and audio processing optimizations enable the clear capture of voice on a smartphone, even in noisy environments.

Microphone Specifications

The key specifications for a smartphone microphone include frequency response, sensitivity, and signal-to-noise ratio (SNR).

Frequency response refers to the range of frequencies the microphone can capture, typically 20Hz to 20kHz for a smartphone mic. Wider frequency responses allow the mic to pick up more nuances in sound. According to the paper “FlowSense: Monitoring Airflow in Building Ventilation Systems Using Commodity Smartphones” (https://dl.acm.org/doi/pdf/10.1145/3517258), smartphone mics have varying frequency responses, with some as wide as 100Hz to 7kHz or 20Hz to 16kHz.

Sensitivity measures how well the microphone converts acoustic sound pressure into an electrical signal. Higher sensitivity mics need less sound pressure to produce output signals. Smartphone mics generally have sensitivities around -40 to -42 dBV/Pa.

SNR compares the level of the desired signal to the level of background noise. Higher SNR indicates less noise and distortion. For smartphone mics, typical SNR is around 60 to 65 dB, though top models may reach 70 dB for crisp, clear audio.

Other key specifications include maximum SPL, the loudest sounds the mic can capture before distorting, and omnidirectionality, its ability to pick up sounds equally from all directions.

Audio Processing

Once the microphone converts the acoustic sound waves into an analog electrical signal, additional processing is required to prepare the audio for digital storage and transmission. Modern smartphone microphones utilize analog-to-digital converters to transform the analog signal into a digital format.

The audio is then compressed using algorithms such as MP3, AAC, or Opus to reduce the size of the digital audio file. This compression removes some fidelity but enables efficient storage and streaming. The digital audio is also encoded into a standard format like PCM or ADPCM in preparation for storage or transmission.

Microphone Placement

The placement of microphones on a smartphone is very important for capturing high quality audio. Most smartphones have multiple microphones located at key points on the device.

Many smartphones have one microphone on the bottom or lower rear of the device. This allows the microphone to be closer to the user’s mouth when holding the phone for a phone call 1. Rear microphones may also be placed near the camera module for capturing audio during video recording 2.

In addition to a bottom/rear microphone, most smartphones also have a microphone at the top or front of the device. Having multiple microphones allows for noise cancellation and beamforming techniques to isolate the user’s voice over ambient noise 3. This creates microphone arrays that focus on the user’s voice.

Emerging Technologies

Microphone technology in smartphones continues to advance with new innovations that improve audio quality and expand functionality.

Waterproofing

Making microphones fully waterproof allows users to record audio even in wet conditions. Water-resistant coatings and membranes allow air and sound to pass through while blocking moisture. This enables underwater audio recording.

Miniaturization

Microphones keep getting smaller to fit into ever-slimmer smartphone designs. MEMS (micro-electro-mechanical system) microphones use silicon chips with embedded microphone membranes, taking up less space.

Multicapsule Microphones

Having multiple microphones allows directional audio pickup and advanced noise cancellation. Smartphones now pack in 3-4 microphones. Multiple mic capsules can also enable features like beamforming to isolate audio sources.

Conclusion

Smartphone microphones have come a long way from the early crude designs used in the first mobile phones. Advances in miniaturization, noise cancellation, and audio processing have enabled smartphone mics to capture high-quality audio for calls, videos, and other applications. Today’s microphones utilize MEMS technology to pack multiple mic elements into a compact package to enable advanced capabilities like beamforming and directional pickup. Further improvements on the horizon include the use of ultrasonic frequencies to capture spatial and directional sound information. Companies like Sound of Things are pushing the envelope with technologies like binaural recording and 3D audio capture.

As smartphones continue to displace traditional cameras and video equipment, their microphones will need to keep pace in terms of audio quality and features. More advanced noise cancellation will likely become standard across devices. It’s likely future smartphone mics will be able to pinpoint sound sources and filter out unwanted noise even better than today. Immersive audio applications like augmented reality will further drive new microphone designs and placements. There are still plenty of opportunities for improvement, but it’s clear that the humble microphone will continue to be an essential component enabling smartphones to capture the world around us.

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