The versatile, thin-film gadget has the potential to make any floor right into a low-power, high-quality audio supply.
MIT engineers have developed a paper-thin loudspeaker that can turn any surface into an active audio source.
This thin-film loudspeaker produces sound with minimal distortion while using just a fraction of the energy required by a traditional loudspeaker. The hand-sized loudspeaker the team demonstrated, which weighs about as much as a dime, can generate high-quality sound no matter what surface the film is bonded to.
To achieve these remarkable properties, the researchers pioneered a deceptively simple fabrication technology that involves only three basic steps and can be scaled up to manufacture ultrathin loudspeakers large enough to cover the inside of an automobile or wallpaper a room.
Used this way, the thin-film loudspeaker could provide active noise cancellation in clamorous environments, such as an airplane cockpit, by generating sound of the same amplitude but opposite phase; the two sounds cancel each other out. The flexible device could also be used for immersive entertainment, perhaps by providing three-dimensional audio in a theater or theme park ride. And because it is lightweight and requires such a small amount of power to operate, the device is well-suited for applications on smart devices where battery life is limited.
“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,” says 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.
Bulovic wrote the paper with lead author Jinchi Han, a ONE Lab postdoc, and co-senior author Jeffrey Lang, the Vitesse Professor of Electrical Engineering. The research is published today (April 26, 2022) in IEEE Transactions of Industrial Electronics.
A new approach
A typical loudspeaker found in headphones or an audio system uses electric current inputs that pass through a coil of wire that generates a magnetic field, which moves a speaker membrane, that moves the air above it, that makes the sound we hear. By contrast, the new loudspeaker simplifies the speaker design by using a thin film of a shaped piezoelectric material that moves when voltage is applied over it, which moves the air above it and generates sound.
Most thin-film loudspeakers are designed to be freestanding because the film must bend freely to produce sound. Mounting these loudspeakers onto a surface would impede the vibration and hamper their ability to generate sound.
To overcome this problem, the MIT team rethought the design of a thin-film loudspeaker. Rather than having the entire material vibrate, their design relies on tiny domes on a thin layer of piezoelectric material which each vibrate individually. These domes, each only a few hair-widths across, are surrounded by spacer layers on the top and bottom of the film that protect them from the mounting surface while still enabling them to vibrate freely. The same spacer layers protect the domes from abrasion and impact during day-to-day handling, enhancing the loudspeaker’s durability.
To build the loudspeaker, the researchers used a laser to cut tiny holes into a thin sheet of PET, which is a type of lightweight plastic. They laminated the underside of that perforated PET layer with a very thin film (as thin as 8 microns) of piezoelectric material, called PVDF. Then they applied vacuum above the bonded sheets and a heat source, at 80 degrees Celsius, underneath them.
Because the PVDF layer is so thin, the pressure difference created by the vacuum and heat source caused it to bulge. The PVDF can’t force its way through the PET layer, so tiny domes protrude in areas where they aren’t blocked by PET. These protrusions self-align with the holes in the PET layer. The researchers then laminate the other side of the PVDF with another PET layer to act as a spacer between the domes and the bonding surface.
“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,” Han says.
Top quality, low energy
The domes are 15 microns in top, about one-sixth the thickness of a human hair, they usually solely transfer up and down about half a micron after they vibrate. Every dome is a single sound-generation unit, so it takes 1000’s of those tiny domes vibrating collectively to supply audible sound.
An added good thing about the crew’s easy fabrication course of is its tunability — the researchers can change the scale of the holes within the PET to regulate the scale of the domes. Domes with a bigger radius displace extra air and produce extra sound, however bigger domes even have decrease resonance frequency. Resonance frequency is the frequency at which the gadget operates most effectively, and decrease resonance frequency results in audio distortion.
As soon as the researchers perfected the fabrication approach, they examined a number of completely different dome sizes and piezoelectric layer thicknesses to reach at an optimum mixture.
They examined their thin-film loudspeaker by mounting it to a wall 30 centimeters from a microphone to measure the sound stress stage, recorded in decibels. When 25 volts of electrical energy had been handed via the gadget at 1 kilohertz (a price of 1,000 cycles per second), the speaker produced high-quality sound at conversational ranges of 66 decibels. At 10 kilohertz, the sound stress stage elevated to 86 decibels, about the identical quantity stage as metropolis visitors.
The energy-efficient gadget solely requires about 100 milliwatts of energy per sq. meter of speaker space. In contrast, a mean house speaker would possibly devour greater than 1 watt of energy to generate related sound stress at a comparable distance.
As a result of the tiny domes are vibrating, reasonably than the complete movie, the loudspeaker has a excessive sufficient resonance frequency that it may be used successfully for ultrasound functions, like imaging, Han explains. Ultrasound imaging makes use of very excessive frequency sound waves to supply photos, and better frequencies yield higher picture decision.
The gadget may additionally use ultrasound to detect the place a human is standing in a room, identical to bats do utilizing echolocation, after which form the sound waves to observe the particular person as they transfer, Bulovic says. If the vibrating domes of the skinny movie are coated with a reflective floor, they may very well be used to create patterns of sunshine for future show applied sciences. If immersed in a liquid, the vibrating membranes may present a novel technique of stirring chemical substances, enabling chemical processing methods that would use much less power than massive batch processing strategies.
“We have now the flexibility to exactly generate mechanical movement of air by activating a bodily floor that’s scalable. The choices of how you can use this know-how are limitless,” Bulovic says.
“I believe it is a very inventive strategy to creating this class of ultra-thin audio system,” says Ioannis (John) Kymissis, Kenneth Brayer Professor of Electrical Engineering and Chair of the Division 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.
This work is funded, in part, by the research grant from the Ford Motor Company and a gift from Lendlease, Inc.