A Flexible, Ultra-Thin Loudspeaker Developed By MIT Researchers – A Sneak-Peak At The Process

Researchers from the Massachusetts Institute of Technology (MIT) has in a new breakthrough developed an ultrathin loudspeaker that has the capability to turn any rigid surface into a high-quality, active audio source.

The research work funded, in part, by the research grant from the Ford Motor Company and a gift from Lendlease, Inc and published today (April 26, 2022) in IEEE Transactions of Industrial Electronics, had the thin film loudspeaker producing a sound with minimal or infinitesimal distortion as it uses just a fragment of the energy a traditional loudspeaker requires.

The team further demonstrated a hand-sized loudspeaker that weighs about as much as a dime, while generating high-quality prime sound irrespective of the surface the film is bonded to.

The engineers with the mandate to achieve these remarkable properties developed a simple fabrication technology involving three basic steps that can be scaled up to produce ultrathin loudspeakers big enough to accommodate the inside of an automobile.

The thin-film loudspeaker fashioned this way would be able to make provision for active noise cancellation in noisy environments (example an airplane cockpit), with the generation of sounds having the same amplitude but opposite phase, effectively making the sounds cancelling each other out.

The flexible is also useful in riveting entertainment as it can provide three-dimensional audio in a theatre or theme park ride, and due to its lightweight characteristic, it will need a little amount of power to operate. The device is hence well-suited for applications on smart devices where battery life is limited.

Vladimir Bulovic, the Fariborz Maseeh Chair in Emerging Technology, leader of the Organic and Nanostructured Electronics Laboratory (ONE Lab), director of MIT.nano, and senior author of the paper analyzing tidbits of the project said:

“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. It can be used anywhere. One just needs a smidgeon of electrical power to run it”.

A known impediment in works like this is that as thin-film loudspeakers have the designation to be freestanding to allow the film bend freely to produce sound, having these loudspeakers mounted on a surface has the tendency to impede the vibration, while hampering their ability to generate sound and it is on the aegis of this that the  MIT team rethought the design of a thin-film loudspeaker to overcome these issue. The team had their design relying on tiny domes on a thin layer of piezoelectric material that can individually vibrate instead of having the entire material vibrate.

Each of the domes having a few hair-widths across is encircled by spacer layers on the top and bottom of the film protecting them from the mounting surface, and this still allows them vibrate freely. The domes are protected from abrasion and impact due to day to day handling by the same spacer layers, in the process enhancing the durability of the loudspeakers.

The team made use of a laser to drill tiny holes in a thin sheet of PET, a lightweight plastic to build the  speaker, while they laminated the underside of that perforated PET layer with a very thin film (as thin as 8 microns) of piezoelectric material, (PVDF). They went further to apply vacuum above the bonded sheets and a heat source, at 80 degrees Celsius, underneath them.

Due to the thin layer of the PVDF, the pressure difference caused by the vacuum and the heat source will make  it bulge and as the PVDF cannot force through the PET layer, tiny domes will be protrude in areas where PET hasn’t blocked, causing the protrusions to self-align with the holes in the  PET later. The researchers not oblivious of this, had to laminate the other side of the PVDF with another layer of PET to act as a spacer between the domes and the bonding surface.

The lead author of the project, Jinchi Han, a ONE Lab postdoc highlighted the process when he said:

“This is a very simple, straightforward process. It would allow us to produce these loudspeakers in a high-throughput fashion if we integrate it with a roll-to-roll process in the future. That means it could be fabricated in large amounts, like wallpaper to cover walls, cars, or aircraft interiors”.

The domes 15 microns tall and about one -sixth the thickness of a human hair, can only move up and down about half a micron when they vibrate, and  which each dome being a single sound-generation unit, it takes thousands of these tiny domes vibrating together to produce audible sound.

The researchers made their work clearer by infusing tunabilty, a fabrication process where the size of the holes in the PET get changed as they seek to control the size of the domes. With the perfection of the fabrication technique, the team tested several different dome sizes and piezoelectric layer thicknesses to arrive at an optimal combination.

 “We have the ability to precisely generate mechanical motion of air by activating a physical surface that is scalable. The options of how to use this technology are limitless,” Bulovic says.

“I think this is a very creative approach to making this class of ultra-thin speakers,” says Ioannis (John) Kymissis, Kenneth Brayer Professor of Electrical Engineering and Chair of the Department of Electrical Engineering at Columbia University, who was not involved with this research. “The strategy of doming the film stack using photolithographically patterned templates is quite unique and likely to lead to a range of new applications in speakers and microphones.”

Reference: “An Ultra-Thin Flexible Loudspeaker Based on a Piezoelectric Micro-Dome Array” by Jinchi Han, Jeffrey Lang and Vladimir Bulovic, 26 April 2022, IEEE Transactions of Industrial Electronics.
DOI: 10.1109/TIE.2022.3150082