Where Sound Meets Flexible Electronics

Voice-securing your ATM card. Talking to your newspaper over coffee. Projecting your voice to a room full of people using only a thin, lightweight loudspeaker that fits in your pocket. With new research published last week in the journal Nature Communications, a team of scientists from Michigan State University and Georgia Institute of Technology has opened the door to these possibilities.

Last week, the team introduced the first ultrathin, flexible, scalable device that can convert mechanical energy to electrical energy and vice versa—acting like both a microphone and a loudspeaker. To showcase the audio capabilities of this device, the researchers created a flag that can blow in the wind while blaring music, a paper-thin sheet that can record a symphony, and a very sensitive voice recognition security key.

These applications are based on something called a ferroelectret nanogenerator, or FENG. Introduced by researcher Nelson Sepulveda’s group (Michigan) in late 2016, FENG is a flexible, paper-thin generator that was originally designed to harness human motion for generating electricity. When you push down on a FENG, the “pushing” energy is turned into electrical energy that researchers have shown can power items like keyboards, LED lights, and touch-screens. Now, the team has shown, its applications go beyond power generation.

Researcher Nelson Sepulveda holding up an array of ferroelectret nanogenerators
(FENGs), similar to that used in the flag.
Image Credit: Michigan State University.

At its core, a loudspeaker is a system that turns electrical signals into sound waves. Traditional loudspeakers do this with a cone-shaped diaphragm mounted on a rigid frame, a suspended coil that carries the electrical signal, and a permanent magnet that sits near the coil. Electrical signals traveling through the coil induce a magnetic field. This magnetic field interacts with the permanent magnet, and the changing forces between the two cause the diaphragm to move back and forth, generating sound waves. A FENG can accomplish the same task, turning electrical signals into sound waves, and it can do so with much less bulk.

Electrical signals from an audio input source change the current in the voice coil. This current creates a magnetic field through the loops of the coil, which is repelled by the stationary magnet behind it, creating pressure waves that correspond to the input signal and producing sound.
Image Credit: HowStuffWorks

The researchers start with a thin sheet of polypropylene, a low-density plastic, that contains tiny foreign particles sprinkled throughout. By stretching the film, the researchers put stress on the plastic that leads to the formation of small holes, called voids, at the particle sites. Through a series of processing steps, the researchers cause the voids to swell and then stabilize. The result is a foam-like material that contains voids similar in shape to M&M candies.

By applying a large electric field, the researchers cause a kind of electric arcing between the upper and lower surfaces of the voids. This turns the voids into large dipoles with positive charges lining one side and negative charges lining the other. Since plastics like polypropylene are good insulators , these charges tend to stay put. Finally, a thin layer of silver is deposited onto the top and bottom of the polypropylene and the FENG is ready for action.

Exactly how is this foam a loudspeaker? Remember that speakers turn electrical signals into sound waves. If you connect the silver layers of a FENG to a source of electrical signals (say, the fight song for the Michigan State University Spartans on your iPod), things start happening. As electrical signals travel through the silver, the electric field between the plates pulls and pushes on the charges within the voids—attracting opposite charges and repelling like charges. These forces cause the voids, and therefore the FENG, to physically change shape, vibrating in sync with the signal and mechanically producing audible sound waves.

The opposite of loudspeakers, microphones turn sound waves into electrical signals that are usually recorded or routed to loudspeakers. To create a FENG-based microphone, therefore, you simply do the reverse. Instead of feeding it an electrical signal, you feed it sound waves. By compressing the material, by speaking into it or putting it in front of a loudspeaker for example, you create changes in how the charges are distributed inside of the voids. These rearrangements exert forces on charges in the silver layers, resulting in an electrical current that can be captured and recorded.

In addition to exploring the physics behind the acoustic applications of FENG-based devices, the researchers constructed and tested prototype applications. You can see for yourself how the music-playing flag works in the video below. The flag looks just like any other, waving in the wind, but it has a matrix of FENG loudspeakers sandwiched between two layers of fabric.

A demonstration of the loudspeaker flag.
Credit: Wei Li, David Torres, Ramón Díaz, Zhengjun Wang, Changsheng Wu, Chuan Wang, Zhong Lin Wang & Nelson Sepúlveda (CC BY 4.0).

To explore its use as a microphone, the researchers played a complex aria through a traditional loudspeaker aimed at a FENG-based microphone. The microphone turned the sound waves into an electrical signal that was recorded. By comparing the spectrum of frequencies captured by the FENG-based microphone to that in the original music, the team verified that the new microphone design is very high quality.

The high-sensitivity of this application became even more clear in a prototype voice-protected security feature the researchers designed for a computer. Not only did a user have to say the correct password to unlock a computer, but the spectrum of frequencies produced during speech had to match that of the person who originally recorded the password—something not so easy to accomplish unless you are that person!

Wearable technologies like fitness sensors, digitally enhanced clothing, smart watches, and smart glasses are coming of age, but as the researchers point out, most of these technologies interact with us visually and through touch. FENG brings to light a whole other category of personal technologies that interact with us through sound. It will be interesting to see what other applications emerge. If history is any indication, sound and the devices on which it is played remain closely tied to our personal experiences and shared cultural memories.

Kendra Redmond

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