MIT engineers have developed a paper-thin speaker that can turn any surface into an active audio source.
This thin-film speaker produces sound with minimal distortion while using a fraction of the energy required by a traditional speaker. The hand-sized speaker the team demonstrated, which weighs about a penny, can deliver high-quality sound no matter what surface the film is glued to.
To achieve these properties, the researchers developed a deceptively simple manufacturing technique that requires just three basic steps and can be scaled up to produce ultra-thin speakers large enough to cover the interior of an automobile or upholster a room.
Used in this way, the thin-film speaker could provide active noise cancellation in noisy 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 delivering three-dimensional sound in a theater or theme park. And because it’s 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’s remarkable to take what looks like a thin sheet of paper, attach two clips to it, plug it into your computer’s headphone port, and start hearing the sounds coming from it. It can be used no anywhere. It only takes a little electrical power to make it work,” says Vladimir Bulović, Fariborz Maseeh Chair in Emerging Technologies, Head of the Organic and Nanostructured Electronics Laboratory (ONE Lab), Director of MIT .nano and main author of the article.
Bulović authored the paper with lead author Jinchi Han, a ONE Lab postdoc, and co-lead author Jeffrey Lang, a professor of electrical engineering at Vitesse. The research is published today in IEEE Industrial Electronics Transactions.
A new approach
A typical loudspeaker found in headphones or an audio system uses electrical current inputs which pass through a coil of wire which generates a magnetic field, which moves a loudspeaker membrane, which moves the air above it, which produces the sound we hear. In contrast, the new speaker simplifies speaker design by using a thin film of shaped piezoelectric material that moves when voltage is applied to it, displacing air above it and generating his.
Most thin-film speakers are designed to be self-contained because the film must bend freely to produce sound. Mounting these speakers on a surface would prevent vibration and hinder their ability to generate sound.
To overcome this problem, the MIT team re-engineered the design of a thin-film speaker. Rather than vibrating the whole material, their design relies on tiny domes on a thin layer of piezoelectric material that each vibrate individually. These domes, each only a few hairs wide, are surrounded by spacer layers at the top and bottom of the film that protect them from the mounting surface while allowing them to vibrate freely. The same spacer layers protect the domes from abrasion and impact during daily handling, improving speaker durability.
To build the speaker, the researchers used a laser to cut tiny holes in a thin sheet of PET, which is a lightweight type of plastic. They laminated the underside of this perforated PET layer with a very thin film (as thin as 8 microns) of piezoelectric material, called PVDF. Then they applied a vacuum above the glued sheets and a heat source, at 80 degrees Celsius, below.
Because the PVDF layer is so thin, the pressure difference created by the vacuum and the heat source caused it to swell. The PVDF cannot force its way through the PET layer, so tiny domes protrude in areas where they are not blocked by the PET. These protrusions automatically line up with the holes in the PET layer. The researchers then laminate the other side of the PVDF with another layer of PET to act as a spacer between the domes and the bonding surface.
“It’s a very simple and straightforward process. It would allow us to produce these high-speed speakers if we put it into a roll-to-roll process in the future. That means it could be made in large quantities, like wallpaper to cover walls, cars or airplane interiors,” says Han.
High quality, low consumption
The domes are 15 microns high, about one-sixth the thickness of a human hair, and they only move up and down about half a micron when vibrating. Each dome is a single sound generating unit, so thousands of these tiny domes need to vibrate together to produce audible sound.
An added benefit of the team’s simple fabrication process is its adjustability – researchers can change the size of the holes in the PET to control the size of the domes. Domes with a larger radius move more air and produce more sound, but larger domes also have a lower resonant frequency. The resonant frequency is the frequency at which the device operates most efficiently, and a lower resonant frequency results in audio distortion.
Once the researchers perfected the fabrication technique, they tested several different dome sizes and piezoelectric layer thicknesses to arrive at an optimal combination.
They tested their thin-film speaker by mounting it to a wall 30 centimeters from a microphone to measure the sound pressure level, recorded in decibels. When 25 volts of electricity passed through the device at 1 kilohertz (a rate of 1,000 cycles per second), the speaker produced high-quality sound at conversational levels of 66 decibels. At 10 kilohertz, the sound pressure level rose to 86 decibels, roughly the same volume level as city traffic.
The energy-efficient device requires only about 100 milliwatts of power per square meter of speaker surface. In contrast, an average home speaker can draw over 1 watt of power to generate similar sound pressure at a comparable distance.
Because the tiny domes vibrate, rather than the entire film, the speaker has a high enough resonant frequency that it can be used effectively for ultrasound applications, such as imaging, Han says. Ultrasound imaging uses very high frequency sound waves to produce images, and higher frequencies give better image resolution.
The device could also use ultrasound to detect where a human is in a room, much like bats do using echolocation, and then shape the sound waves to track the person as they move, explains Bulovic. If the thin film’s vibrating domes are coated with a reflective surface, they could be used to create patterns of light for future display technologies. If immersed in liquid, vibrating membranes could provide a new method of agitating chemicals, enabling chemical processing techniques that could use less energy than large-batch methods.
“We have the ability to precisely generate mechanical air movement by activating a scalable physical surface. The options for using this technology are limitless,” says Bulović.
This work is supported, in part, by the Ford Motor Company Fellowship and a grant from Lendlease, Inc.