May 17, 2020 by Biljana Ognenova
Today’s devices feature hundreds of electroacoustic transducers the average user interacts with daily, knowingly or unknowingly. From smartphone assistants to voice control in autonomous vehicles and voice-activated robots, the field is open for design to EEs.
On account of discoveries in communication technology, electroacoustics, magnetic field design methods, and materials sciences; such as fibre optics, silicon transducers, piezoelectric polymer devices, and polymer-electret transducers, as well as improved material-processing capabilities—electroacoustic transduction has been on the rise in the last several decades.
Microphones and speakers are so widespread today that would be hard to imagine a world where we don’t rely on sound input and output devices. The audio and sound equipment market shows a growing trend of 40% a year, mostly due to selling headphones and earphones, true wireless stereo (TWS) headsets, and smart speakers.
While it was difficult to predict the direction development of electroacoustic transducers will take ten years ago, the growing significance of voice-user interfaces and MEMS (micro-electromechanical systems) indicates the area of widespread application of electroacoustic sensors and sound transmitters we may be seeing in the near future.
MEMS microphones minimise the disturbance to the sound field, come with low mass diaphragm results in low vibration sensitivity and low manufacturing costs, as well as advantages in IC fabrication, rapid device modification, and finer specification tolerance and reproducibility.
Some of the historical disadvantages related to MEMS microphones are the low signal-to-noise ratio (S/N or SNR) although what is a good SNR can be argued, considering factors such as the strength of the source signal, environmental noise, and the noise of the microphone itself.
There are ways to improve MEMS microphone noise, but it is not an easy nut to crack as it requires the usage of complex geometry and non-linear coupled physics, which includes handling the challenges of disproportionate changes in input and output. A common solution is to design an equivalent circuit model or build a hybrid condenser microphone model.
A cross-section drawing of a condenser (electret) microphone. Image Credit: Ibersensor.
New generation MEMS microphones include an array of benefits, the most vital being their noise-cancelling properties, the joint matching of the amplitude and phase of the audio signals of two microphones operating in parallel, and the high ambient temperature robustness, which makes them suitable for automatic assembly of printed circuit boards.
These benefits are the reason why we enjoy using them in popular consumer electronics applications — smartphone video sharing, voice calls over wearables, smart home device control, and voice-controlled digital assistants.
Another promising application of electroacoustic transduction with MEMS microphones is virtual reality. VR applications require immersive user experiences with small-scale devices, such as VR headsets and they can benefit from ambisonic microphones. Ambisonic microphones provide a stable, undisturbed sphere or sound in a rotating sound field, spreading sound information proportionally in all directions.
The simplified version of an ambisonic microphone is built of four closely packed microphone capsules designed as cardioid polar patterns that are recording signals in the “ambisonics A-format” and later converting mono, stereo, and surround signals into a B-format.
Video conferencing for multiple remote work teams depends on solid sound systems, as well as sophisticated speech control in home environments where we will need to see more reliable, fine-tuned, high-performing microphones.
In the field of biomedical application of ultrasound, we see acoustically driven microbubbles that improve image contrast and use of ultrasound to transport therapeutic agents across the blood-brain barrier.
A microphone attached to a speaker.
Undoubtedly, optimizing microphones is essential for MEMS becoming more and more ubiquitous, but loudspeakers are no less important for micromachining and microfabrication of connected electronic devices. Connected devices are rarely one-directional in energy conversion. Many devices need to both “listen” and “speak” at least, if not learn, analyze and interpret data to take action in the IoT network.
As electroacoustic transducers that convert electromagnetic waves into sound waves, loudspeakers play a major role in the IoT devices, whose proliferation and successful implementation will depend much on improvements in directional sound amplification.
Even today, when we are just scratching the surface of embedded electroacoustic transducers as far as IoT devices are concerned, when designing and redesigning sensors and actuators we must think of controlling ambient sound levels.
A widely researched and developed area for audio sensors is intelligent home systems, where microphones and speakers encompass a variety of data transmission and activation methods for smart TVs, HVAC systems, and CCTV and building security.
Audio sensors replicate human biological systems for hearing and speech in robots, providing them with the means to communicate with the environment. Commercially speaking, loudspeakers have always been a lucrative resource for industries that try to please users hungry for the most perfect ambient sound. With new 3D-printed speakers produced with laser sintering, that may be possible if you are willing to pay a price of $30,000 per pair.
Regardless of the race for marketing the ideal portable speaker, it is not only aesthetics that will drive electrical engineers when designing future electroacoustic transducers.
We have mentioned the importance of advancements in materials science, such as this example of a graphene microphone or this thin and flexible SATURN self-powered microphone that leverages a triboelectric nanogenerator. But there is more to come, with our humble predictions aiming mostly towards improvements in MEMS and sound directionality, and, as it is the case with most devices, to minimising power loss.
