Mic Check 1, 2, 3
Sound is the most natural way for human beings to communicate, and in the near future microphone technology will enable us to use it to control the world around us.
As some of the world’s largest tech companies race to take advantage of ecosystems based on voice recognition, the billion-dollar mic industry is going to have to keep pace with advancements in technology.
In this article, I examine the current $1 billion global market for microelectromechanical systems (MEMS) microphones and consider potential frontiers for improvement, as well as some of the startups and research ideas disrupting this industry.
The microphone market today
The use of microphones is increasing rapidly. YoY growth is about 18% and IHS predicts that the use of MEMS microphones will rise to about 6 billion units in 2019.
The biggest reason for this fast growth curve is that smartphone, tablet, and even smart speaker makers are putting multiple microphones into their devices. For instance, the Amazon Echo has as many as seven MEMS mics; even in some of the latest smartphones, you’ll find up to four microphones.
Traditionally, the market has focused on using microphones in mobile applications. In contrast, we envision growth in areas such as IoT, automotive, industrial, and smart homes and buildings, driven largely by voice input and contextual awareness.
Existing microphone technology
A MEMS microphone is typically composed of a fixed back plate and a moveable diaphragm. When sound comes along, the moveable diaphragm is distorted by sound pressure, changing the gap (and capacitance) between its surface and the backplate. The change in capacitance is then measured and converted to an electrical signal by a digital ASIC. This electrical signal represents sound.
Since the gap between the diaphragm and the backplate is essential for the capacitive MEMS microphone to function, it can also cause many failure modes. The graphic below shows some of those failure modes: 1) normal, 2) dust / particle damage, 3) water entry, and 4) stiction failure, which may be caused by a sudden acceleration or loud sound (acoustic overload) that throws the diaphragm toward the back plate.
As you can see, the capacitive MEMS microphones that have dominated the market for the past 10–15 years are easily damaged by common environmental contaminants such as water, dust, and particulate matter.
The poor reliability of capacitive MEMS microphones is a direct result of their architecture. The industry is keenly aware of these problems, and product designers are actively seeking a better option.
Piezoelectric microphones
Building on research conducted at the University of Michigan, engineers from Vesper Technologies have built a microphone without a gap, called the VM1000 — the first commercially available piezoelectric MEMS microphone.
The VM1000 consists of a single layer of flexible plates for both the backplate and the diaphragm. Changes in sound pressure cause these plates to bend and experience stress. As the plates are built from a sandwich of piezoelectric materials, the stress generates an electrical charge, which allows for direct measurement of sound. This creates a microphone that does not need a backplate.
In a nutshell, piezoelectricity is the appearance of an electrical charge across the sides of certain solid materials when you subject them to mechanical stress. This material is not new: smartphones contain dozens of piezoelectric radio-frequency (RF) filters.
The RF filter industry is worth billions of dollars. So there is no shortage of reliable and replicable manufacturing tools to build piezoelectric MEMS. Vesper is capitalizing on this infrastructure to mass-produce the first piezoelectric MEMS microphone on the market.
Piezoelectricity also enables Vesper’s systems to draw incredibly low power and wake upon sound. Without requiring the push of a button to activate Siri with the AirPods or Alexa with the Echo Tap, Vesper’s technology could make truly always-listening interfaces possible even on the smallest of battery-powered devices.
Optical microphones
Historically, microphones have always been based on mechanically moving parts, whether it’s a capacitive membrane or a piezo. Because these devices are mechanical, they are prone to interference, mechanical disturbance, and the inert mass common in conventional microphones.
Israeli startup VocalZoom has released a voice biometrics solution based on its optical sensor that measures voices using facial vibrations, eliminating the need for microphones and the noise reduction software of traditional acoustic solutions.
The laser is directed at the face of the person talking, and measures vibrations “in the order of tens and hundreds of nanometers — so small that nothing else can pick them up.” These micro-measurements of the skin are converted into audio.
Because of their precision, aim, and ability to sidestep sound waves, no other surrounding noise interferes with the clarity of the voice. In addition, by detecting the direction of arrival, the sensor can verify that the person of focus is in the right direction and at the right distance so the sensor only listens to a particular user.
The company is working with most voice-recognition software systems and headset manufacturers, and is also working on a car mirror integration approach and with MEMS manufacturers who are interested in combining VocalZoom’s technology with classic acoustic audio.
Graphene microphones
Most commercially available microphones today use nickel to make microphone membranes. However, following the development of graphene as a promising material for various electronics devices, it has also been studied for MEMS microphones.
In November 2015, researchers at the University of Belgrade in Serbia built the world’s first graphene-based microphone, leveraging graphene’s ability to detect faint and high-frequency sound waves.
The team grew a multi-layer graphene membrane on a nickel foil substrate using a chemical vapor deposition process, then etched the nickel foil away. The resulting membrane can act as the vibrating membrane that converts sound to an electrical current in a microphone.
The team’s prototype graphene mic boasts 10 decibels higher sensitivity than commercial microphones, at frequencies of up to 11 kHz. And model simulations indicate that even greater sensitivity is theoretically possible.
With 300 layers rather than the prototype’s 60, a graphene vibrating membrane may be able to detect frequencies of up to 1MHz — approximately fifty times higher than the upper limit of human hearing.
Recently, several companies have expressed interest in using graphene membranes for speakers or microphones. In May 2017, Apple was a granted a new patent (filed in 2015) that details an audio device that uses a diaphragm made from a graphene-enhanced composite material. Apple’s graphene membrane can be used in a speaker, microphone, or headphone device.
As devices become smaller and lighter, it will be more and more challenging to provide high-quality audio using conventional materials and researchers are looking at innovative ways to improve the mechanical response of these audio devices — graphene being one.
Frontiers for improvement.
While development areas such as piezoelectric and optical are improving the performance of existing microphones today, I’m interested in learning more about “new” microphones — companies or research that are radically influencing the way audio capture is done.
For instance, Abe Davis’s research from MIT uses a regular camera and a potato chip bag, the ultimate in a low-tech/dirty environment/open back chamber.
Requiring no additional sensors or detection modules, Davis’s cost-effective method of speech recognition is capable of producing accurate and efficient representations of surround-sound acoustics.
Gierad Laput’s research from CMU uses a single small sensor board to detect dozens of phenomena of interest in a room, including sounds, vibration, light, heat, electromagnetic noise, and temperature.
One small device can function as an all-purpose super sensor, which can be plugged in and deployed for any sensing application. You can then program next-level applications, for example, turning on a warning light when the faucet is being turned on or calculating wear and tear on a forklift.
The ability to communicate over a wide range of modalities could drive many new applications, potentially leading to drastic improvements over the directional microphones included in the Amazon Echo or Google Home.
I’ll be watching advances in adjacent industries (like piezos, lasers, and cameras) and if you’re working on innovative ideas within this space, I would love to hear from you!
