Source: Academic Headlines
Recently, engineers at the Massachusetts Institute of Technology (MIT) have developed a new type of ultra-thin speaker, a flexible thin-film device that has the potential to turn any surface into a low-power, high-quality audio source.
This membrane speaker requires only a fraction of the energy required by conventional speakers, but is capable of producing high-quality sound with minimal distortion. According to the research team's demonstration, the palm-sized speaker weighs only a dime, and no matter what surface the film is glued to, it can produce high-quality sound.
To achieve these features, the researchers also pioneered a seemingly simple manufacturing technique that can be scaled up to produce ultra-thin speakers large enough to cover the wallpaper inside a car or room. In this way, membrane speakers can provide active noise reduction (i.e., having the two sounds cancel each other out) by producing sound of the same amplitude but opposite phase in noisy environments, such as the cockpit of an airplane.
In addition, the new device can also be used for immersive entertainment, such as providing 3D audio in theaters or theme park rides. And since it is lightweight and requires very little power to run, this device is ideal for smart devices with limited battery life.
"It looks like a thin piece of paper, put two clips on it, plug it into the headphone port of your computer, and start hearing the sound it makes, and it feels amazing." It can be used anywhere, and all it takes is one power to run it." Said Vladimir Bolovi, corresponding author of the research paper, director of MIT.nano, and head of the Organic and Nanostructured Electronics Laboratory (ONE Lab).
Bulovi co-authored the paper with lead author Jinchi Han, one Lab postdoc, and Jeffrey Lang, professor of electrical engineering. The study was published in IEEE Transactions of Industrial Electronics.
New slim speakers
We all know that traditional speakers in headphones or sound systems use current inputs when the ever-changing current input passes through a coil capable of generating a magnetic field and pushes the vibration of the speaker membrane, which in turn makes the air above it vibrate, thus producing the sound we hear.
In contrast, this new speaker device simplifies speaker design by using a piezoelectric material film 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 free-standing, the membrane material must be freely bent to produce sound. But mounting these speakers on the surface of an object hinders vibration and hinders their ability to produce sound.
To overcome this problem, the MIT team rethinked the design of the membrane speaker. Instead of vibrating the entire material, 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 the width of a few hairs, are surrounded by spacers at the top and bottom of the film, protecting them from mounting surfaces while still allowing them to vibrate freely. The same spacer layer protects the dome from wear and shock during daily operation, increasing the durability of the speakers.
The production process also seems very simple. First, the researchers used lasers to cut small holes in a sheet of PET (a lightweight plastic) that laminated a very thin layer of piezoelectric material (as thin as 8 microns) on the underside of the perforated PET, called PVDF; then they applied a vacuum above the bonded sheet and a heat source of 80 degrees Celsius below.
Since the PVDF layer is very thin, the pressure difference between the vacuum and the heat source causes it to expand. However, PVDF cannot force its way through the PET layer, so tiny domes protrude in areas that are not blocked by PET. These protrusions are aligned with the holes in the PET layer. The researchers then laminated the other side of the PVDF with another PET layer to act as a barrier between the small dome and the bonded surface.
"It's a very simple, straightforward process. If we integrate it with the roll-to-roll process in the future, it will allow us to produce these speakers in a high-throughput fashion. This means it can be manufactured in large quantities, just as wallpaper can cover the interior of a wall, a car or an airplane." Han said.
High quality, low power consumption, unlimited application potential
Each dome is a separate sound unit, and since the dome is 15 microns high, about one-sixth the thickness of a human hair, it can only move about half a micron up and down when vibrating, so thousands of such small domes need to vibrate together to produce an audible sound.
In addition, another benefit of the manufacture of this ultra-thin sounding device is its adjustability, because researchers can change the size of the holes in pet to control the size of the dome. Domes with larger radii push more air and produce a louder sound, but larger domes also have lower resonance frequency. The resonant frequency is the frequency at which the device operates most efficiently, and lower resonant frequencies cause audio distortion.
(Source: MIT)
After several tests, the researchers found the best combination of different dome sizes and piezoelectric layer thicknesses. They then tested their membrane speaker by mounting it to a wall 30 cm from the microphone.
When 25 volts of current passes through the device at 1 kHz (1,000 cycles per second), the speaker produces 66 decibels of high-quality, session-level sound. At 10 kHz, the sound pressure level increases to 86 dB, which is about the same volume level as city traffic.
This energy-efficient loudspeaker device requires only approximately 100 milliwatts of power per square meter of space. In contrast, an average home speaker may consume more than 1 watt of power to produce similar sound pressure at comparable distances.
The researchers explain that since only tiny domes of the device are vibrating, rather than the entire membrane, the speakers have high enough resonant frequencies to be effectively used in ultrasound applications such as ultrasound imaging. Ultrasound imaging uses very high-frequency sound waves to produce images, while higher frequencies produce better image resolution.
For example, the device could use ultrasound to detect where a person was standing in a room and track the position, just as bats use echolocation. If the vibrating domes of the films are covered with reflective surfaces, they can be used to create light patterns for future imaging techniques. If immersed in liquids, diaphragms can also provide a new way to stir chemicals, allowing chemical treatment techniques to use less energy than high-volume treatments.
"We have the ability to precisely generate mechanical movement of air by activating scalable physical surfaces. The choices for how to use this technology are limitless," Bulovi said.
Resources:
https://news.mit.edu/2022/low-power-thin-loudspeaker-0426
https://ieeexplore.ieee.org/document/9714188