A departure from conventional wisdom in loudspeaker design allowed MIT researchers to build paper-thin ones that can be mounted on walls, car interiors and IoT devices.
A research team from MIT has come up with a thin-film loudspeaker that can be mounted on any reasonably flat surface and turn it into an audio source. Consisting of a piezoelectric material sandwiched in between plastic and embossed with an array of microdomes, the device emits high-quality sound while using a fraction of the energy that a traditional sound system would use, the researchers claim. As a bonus, the ‘wallpaper loudspeaker’ can be manufactured with a roll-to-roll process.
“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 Bulović, leader of the Organic and Nanostructured Electronics Laboratory and senior author of the paper detailing the loudspeaker, published in IEEE Transactions on Industrial Electronics.
The researchers envision using their invention to provide an immersive sound experience, for example in a movie theater or a theme park ride. Another intriguing option would be active noise cancellation in planes, cars or even homes. Given its lightweight and low-cost nature and minimal power requirements, the thin-film loudspeaker could also add sound-producing capabilities to IoT devices. The domes could even generate ultrasound waves for imaging purposes.
The basic design of the loudspeakers found in headphones and audio systems hasn’t changed in 150 years: current is passed through a coil, generating a magnetic field that sets a membrane in motion, moving the air above it. This principle allows for thin designs, but these speakers invariably need to be freestanding, because the membrane needs room to vibrate.
MIT circumvented this issue by having the microdomes do the vibrating, rather than the entire material. The domes, 15 microns in height, are created by laminating a perforated sheet of PET plastic with an 8-micron layer of piezoelectric polyvinylidene fluoride (PVDF) and subjecting the stack to a vacuum and heat. This causes the PVDF to protrude through the holes in the PET. A couple of extra PET layers are added on each side – on the bottom to separate the domes from the bonding surface and on the top to protect them from abrasion damage.
“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,” says Jinchi Han, a postdoc in Bulović’s lab and first author of the paper.
The researchers tested their thin-film loudspeaker by mounting it to a wall 30 centimeters from a microphone to measure the sound pressure level. Passing 25 volts at 1 kilohertz through the device produced a 66-decibel sound, which is about as loud as a normal conversation. At 10 kilohertz, the sound pressure increased to 86 decibels, about the same volume level as city traffic. The loudspeaker requires 100 milliwatts of power per square meter, while an average home speaker requires over 1 watt to produce similar sound pressure.
Main picture credit: Felice Frankel