From MIT News
By Adam Zewe
Machine Heart Compilation
Machine Heart Editorial Department
This flexible thin-film device has the potential to turn any surface into a low-power, high-quality sound source.
Engineers at MIT have developed a speaker as thin as paper that can turn any surface into a sound source.
It weighs the equivalent of a 10-cent coin and produces a high-quality sound no matter what surface it is glued to.
This membrane speaker produces minimal sound distortion and uses much less energy than conventional speakers.
To achieve these properties, the researchers pioneered a seemingly simple manufacturing technique that requires only three basic steps. Using this technology, they can make ultra-thin speakers large enough to cover the interior of the car or the entire room.
In addition, such membrane speakers can actively reduce noise in noisy environments such as the cockpit of an airplane by producing sound of the same amplitude but opposite phase. This flexible device can also be used for immersive entertainment, such as providing 3D audio in theaters or theme parks. Because of its light weight and little power required to run, it is ideal for smart device applications with limited battery life.
The results of this research were recently published in the journal IEEE Transactions of Industrial Electronics.
Thesis link: https://ieeexplore.ieee.org/document/9714188
"It's a great feeling to pick up a piece of paper that looks thin, hold it in two clips, plug it into your computer's headphone jack, and start hearing it. It can be used anywhere and only needs a little electricity to run," said Vladimir Bulovi, director of MIT.nano and author of the paper.
How is this membrane speaker made?
Typical speakers common in headphones or audio systems use a current input that creates a magnetic field through the coil, which moves the speaker film, driving the air above the film, creating the sound we hear. In contrast, the new loudspeaker designed by MIT engineers simplifies the traditional design, using a molded piezoelectric material film. When a voltage is applied to it, the membrane moves, driving the air above it and producing sound.
Most membrane loudspeakers are designed to be free-standing (without relying on supports) because the membrane must bend freely to sound. Mounting these speakers on a surface can hinder vibrations and hinder their ability to produce sound.
To overcome this problem, the team at MIT rethought the design of membrane speakers. The solution they gave was not to let the entire material vibrate, but to rely on the vibrations of tiny domes on a thin layer of piezoelectric material, each of which vibrated individually. Each of these domes, only a few hairs wide, are surrounded by spacers at the top and bottom of the membrane, protecting them from mounting surfaces while still allowing them to vibrate freely. In daily operation, the same spacer layer protects the dome from wear and shock, improving the durability of the speakers.
To make the speakers, the researchers used lasers to cut tiny holes in a thin sheet of PET, a lightweight plastic. They attached a very thin (8 micron) film of piezoelectric material, called PVDF, underneath the perforated PET layer. They then vacuumed the bonded flakes above them and applied a heat source of 80 degrees Celsius below the flakes.
Due to the thin layer of PVDF, the pressure difference between the vacuum and the heat source causes it to expand. PVDF cannot force through the PET layer, so there will be tiny dome protrusions where they are not blocked by PET. These protrusions are self-aligned with the holes in the PET layer. The researchers then laminated the other side of the PVDF with another layer of PET to act as a barrier between the dome and the bonded surface.
"It's a very simple and straightforward process. If we combine that with the roll-to-roll process, we have the energy to produce these speakers and then cover them inside a wall, a car, or an airplane in a wallpaper-like way." Jinchi Han said that the first paper was written.
High quality, low power consumption
The small domes in membrane speakers are 15 microns high, about one-sixth the thickness of a human hair, and they can only move up and down about half a micron when they vibrate. Each dome is a separate sounding unit, so thousands of such small domes need to vibrate together to produce an audible sound.
Another benefit of the simple manufacturing process is the highly adjustable nature — researchers can change the size of the holes on the PET to control the size of the dome. Domes with larger radii can drive more air vibrations and produce louder sounds, but larger domes also have lower resonance frequencies, which can lead to audio distortion.
After refining the manufacturing techniques, the researchers tested several different dome sizes and piezoelectric layer thicknesses to achieve the best combination.
They mounted the membrane speaker on a wall 30 cm from the microphone and tested its sound pressure level in decibels. When a 25-volt voltage passes through the unit at a frequency of 1 kHz, the speaker produces a high-quality sound of 66 dB. At 10 kHz, the sound pressure level increases to 86 dB, which is roughly equivalent to the volume of urban traffic.
This energy-efficient device requires only about 100 milliwatts of power per square meter of speaker area, in contrast to an average household speaker that can consume more than 1 watt of electricity if it generates similar sound pressure at similar distances.
Han explains that because the miniature vaults are vibrating, rather than vibrating throughout the membrane, the speakers have high enough resonance frequencies to be effectively used in ultrasound applications, such as imaging. Ultrasound imaging uses very high-frequency sound waves to produce images, and higher frequencies produce higher resolution images.
Bulovi said the device could also use ultrasound to detect where humans are standing in a room, just as bats use echo localization and then follow the human movement to form sound waves. If a reflective surface is covered with a vibrating dome of a thin film, they can be used to provide ideas for the luminous patterns of future display technologies. If immersed in liquids, diaphragms can provide a new way to stir chemicals, allowing chemical treatment techniques to use less energy than high-volume treatment methods.
"We have the ability to precisely generate mechanical movements of air by activating retractable physical surfaces. The imagination space that this technology brings to people is infinite." Bulovi said.