MIT's thin plastic speakers fall flat. And that's by design

The walls are alive with the sound of music

Video Engineers at MIT have created paper-thin speakers using a plastic film and a piezoelectric layer embossed with tiny domes.

These sheet speakers could potentially be applied to any surface for sound output or input: think surround sound or noise cancellation in aircraft. The technology also has potential for ultrasound imaging and echolocation, among other possibilities.

The work is described in a paper published recently in the journal IEEE Transactions on Industrial Electronics, "An Ultra-Thin Flexible Loudspeaker Based on a Piezoelectric Micro-Dome Array."

"It feels remarkable to take what looks like a slender sheet of paper, attach two clips to it, plug it into the headphone port of your computer, and start hearing sounds emanating from it," Vladimir Bulović, leader of the Organic and Nanostructured Electronics Laboratory (ONE Lab), director of MIT.nano, and senior author of the paper, told the MIT News Office.

The other two co-authors of the paper are Jinchi Han, a ONE Lab postdoc, and Jeffrey Lang, professor of electrical engineering at MIT.

The boffins have produced a short YouTube video that demonstrates the technology:

Youtube Video

Traditional speakers translate electrical signals into a magnetic field that moves a speaker membrane to create audible vibrations. The MIT academics got rid of the coiled wire that generates the membrane-moving magnetic field by using piezoelectric material – which converts electrical energy to mechanical energy (and vice versa) – to move a microarray of domes that generate sound.

Their design relies on a sheet of plastic known as PET (polyethylene terephthalate) that's perforated and layered with a thin film of piezoelectric material, called PVDF (polyvinylidene fluoride).

When these sheets are heat bonded to create a vacuum, the result is a flat array of tiny domes that move when voltage is applied. The design creates enough separation of materials to allow the domes to vibrate without interference when the plastic film has been mounted to a surface.

At a distance of 30cm (12 inches), it's claimed, these thin-film speakers can generate high-quality sound equivalent to the volume of a typical conversation (66 decibels) through the application of 25 volts of electricity at 1 kilohertz. At 10 kilohertz, sound output reached the level of city traffic (86 decibels).

The sheet-speakers supposedly require only 100 milliwatts of power per square meter of speaker area, significantly less than home speakers that use more than 1 watt of power for comparable sound pressure at the same distance.

Jinchi Han told The Register in an email that the speakers have been tested between 100 Hz to 100 kHz.

"They are able to produce sound at low frequencies, but sensitivities (acoustic pressure generated under unit voltage) are usually higher at high frequencies than those at low frequencies," said Han.

According to the MIT News Office, the technology is simple to fabricate and can be scaled up to cover the interior of a room or vehicle – the research was funded in part by Ford Motor Company. This would allow covered surfaces to emit sound for listening or for noise cancellation.

Ioannis (John) Kymissis, professor of electrical engineering and Chair of the department of electrical engineering at Columbia University, told the MIT News Office that the technology could also be used to record sound – the domes, acting as tiny microphones, would capture vibrations.

MIT professor Jeffrey Lang confirmed as much in an email to The Register.

"The same device can work as a microphone," said Lang. "It can be mounted on the surface of any object and used for sound recording. The device itself is passive and generates voltage signal under incident acoustic waves. But we apply a small transimpedance amplifier in order to obtain a large signal-to-noise ratio."

"We actually have an upcoming paper that reports the microphonic performance of the same device. The amplifier is the only part that consumes power. If a standalone design is needed, usually the signal storage/processing and wireless transmission consume much more power than the amplifier itself."

"But we can either use a battery or integrate energy harvesting components to make it standalone without wiring to external power. For instance, our group is also developing thin-film solar cells and it's possible to integrate that with the acoustic thin film to provide the energy."

Han said the technology has potential for ultrasound applications because it can generate the high resonance frequency necessary for ultrasound imaging. Echolocation is another possible application, according to Bulović, who also suggested the micro-speaker array combined with a reflective surface could be used as a display technology by creating patterns of light. ®

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