As thin as paper! MIT scientists develop new high-quality speakers with imaginative applications

Engineers at MIT have developed a new type of ultrathin speaker, a flexible, thin-film device that could potentially turn any surface into a low-power, high-quality audio source.

The film speaker requires only a fraction of the energy required by traditional speakers, but can produce high-quality sound with minimal distortion. According to the research team's demonstration, the palm-sized speaker weighs only a dime and can produce high-quality sound no matter what surface the film is bonded to.

To achieve these properties, the researchers also pioneered a deceptively simple manufacturing technique that can be scaled up to produce ultrathin speakers large enough to cover the interior of a car or wallpaper in a room. In this way, the thin-film speakers can provide active noise cancellation (i.e., making two sounds cancel each other out) in noisy environments such as an airplane cockpit by producing sounds of the same amplitude but opposite phase.

The new device could also be used for immersive entertainment, such as providing 3D audio in theaters or theme park rides. And because it is lightweight and requires very little power to operate, the device is ideal for use on smart devices with limited battery life.

"It's amazing that it looks like a thin piece of paper, you stick two clips on it, you plug it into the headphone port of your computer, and then you start hearing sounds from it. It can be used anywhere and you only need a single source of electricity to run it," says Vladimir Bolović, director of MIT.nano and head of the Laboratory for Organic and Nanostructured Electronics (ONE Lab), the corresponding author of the study.

Bulović co-authored the paper with first author Jinchi Han, a ONE Lab postdoc, and Jeffrey Lang, a professor of electrical engineering. The research was published in IEEE Transactions of Industrial Electronics.

New ultra-thin speakers

We all know that traditional speakers in headphones or audio systems use current input. When the constantly changing current input passes through a coil that can generate a magnetic field, it pushes the vibration of the speaker membrane, which in turn causes the air above it to vibrate, thereby producing the sound we hear.

The new device, by contrast, simplifies speaker design by using a thin film of piezoelectric material that moves when voltage is applied to it, causing the air above it to vibrate and produce sound.

Because membrane speakers are designed to be freestanding, the membrane material must be free to flex in order to produce sound. But mounting these speakers on a surface would impede vibrations and hamper their ability to produce sound.

To overcome this problem, the MIT team rethought the design of thin-film speakers. Rather than having the entire material vibrate, their design relies on tiny domes on a thin layer of piezoelectric material, allowing each dome to vibrate individually.

(Source: MIT)

These small domes, only a few hairs wide, are surrounded by spacer layers on the top and bottom of the membrane, protecting them from the mounting surface while still allowing them to vibrate freely. The same spacer layers protect the domes from wear and impact during daily operation, thereby increasing the durability of the speaker.

The manufacturing process looks very simple. First, the researchers used a laser to cut small holes in a thin sheet of PET (a lightweight plastic), and laminated a very thin layer of piezoelectric material (as thin as 8 microns) called PVDF on the underside of the perforated PET; then they applied a vacuum above the bonded sheets and a heat source of 80 degrees Celsius below.

Because the PVDF layer is so thin, the pressure difference created by the vacuum and heat source causes it to expand. But the PVDF cannot force its way through the PET layer, so the tiny domes protrude in areas not blocked by the PET. And these protrusions align with the holes in the PET layer. The researchers then laminated the other side of the PVDF with another PET layer, which acts as a spacer between the small domes and the bonding surface.

“It’s a very simple, straightforward process. If we integrate it with a roll-to-roll process in the future, it will allow us to produce these speakers in a high-throughput way. This means it can be manufactured in large quantities, just like wallpaper can cover a wall, or the interior of a car or an airplane,” Han said.

High quality, low power consumption, unlimited application potential

Each dome is a separate sound-generating unit, and because the dome is 15 microns high, about one-sixth the thickness of a human hair, and can only move up and down about half a micron when vibrating, it takes thousands of these small domes vibrating together to produce audible sound.

Another benefit of making this ultra-thin sound-generating device is its tunability, as researchers can change the size of the holes in the PET to control the size of the dome. Domes with larger radii push more air and produce louder sounds, but larger domes also have lower resonant frequencies. The resonant frequency is the frequency at which the device operates most efficiently, and lower resonant frequencies can cause audio distortion.

(Source: MIT)

After many tests, the researchers found the best combination of different dome sizes and piezoelectric layer thicknesses. They then tested their film speaker by mounting it on a wall 30 centimeters away from a microphone.

When 25 volts of electricity is passed through the device at 1 kilohertz (1,000 cycles per second), the speaker produces 66 decibels of conversational-level, high-quality sound. At 10 kilohertz, the sound pressure level increases to 86 decibels, about the same volume level as city traffic.

The energy-efficient speaker device requires only about 100 milliwatts of power per square meter of area. In comparison, a typical home speaker might consume more than 1 watt of power to produce similar sound pressure at a comparable distance.

The researchers explain that because only the tiny dome of the device is vibrating, rather than the entire membrane, the speaker has a high enough resonant frequency that it could also be used effectively in ultrasound applications, such as ultrasound imaging. Ultrasound imaging uses very high-frequency sound waves to produce images, and higher frequencies produce better image resolution.

For example, the device could use ultrasound to detect where a person is standing in a room and track the location, much like bats use echolocation. If the film's vibrating domes are covered with a reflective surface, they could be used to create light patterns for future imaging technologies. If immersed in a liquid, the vibrating membrane could also offer a new way to stir chemicals, enabling chemical processing techniques that use less energy than bulk processing methods.

“We have the ability to precisely generate mechanical movement of air by activating a physical, expandable surface. The options for how this technology can be used are endless,” Bulović said.

References:

https://news.mit.edu/2022/low-power-thin-loudspeaker-0426

https://ieeexplore.ieee.org/document/9714188

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