Paper-thin and just as light, a new thin-film loudspeaker developed at MIT can generate high-quality sound no matter what rigid surface it is bonded to, turning pretty much anything into an active audio source. The process used to achieve these properties is both quite simple and easily scalable, requiring only three basic steps to potentially wallpaper a room, a car interior, or an airplane cockpit, offering either high-quality sound or noise-cancellation in loud environments.
Where most existing thin-film loudspeakers need to stand on their own and bend freely — mounting them would impede vibration and dampen or distort sound — this new approach works on any surface because the entire loudspeaker doesn’t need to vibrate. Instead, the design features tiny domes distributed on a thin layer of piezoelectric material, each of which vibrates individually. Space on the top and bottom of the domes allows them to vibrate without being damaged.
The thin speakers are fabricated by using a laser to cut tiny holes into a thin sheet of PET, the underside of which is then laminated with a film of the piezoelectric material PVDF as thin as 8 microns. A vacuum is then applied above the bonded sheets while a heat source is applied underneath them, causing tiny domes of the PVDF to protrude through the holes in the PET layer. Finally, the other side of the PVDF is laminated with a second PET layer to create the necessary space between the domes and the bonding surface.
The device is also low-power, needing only about 100 milliwatts of power per square meter of speaker area. A hand-sized speaker made of the film was used to demonstrate the design, which can produce 66 decibels at 30 cm away at 1kHz with 25V of electricity passing through. At 10kHz, the output ramped up to 86dB. While this shows that in the current model, the small diaphragms may struggle to create much volume at lower frequencies, a bonus of the design is its tunability. The size of the holes can be altered to create domes of varying radii, which the researchers have taken advantage of to test for optimal parameters.
Since the speakers can be bonded to any surface, they could be an excellent sound-proofing solution. By generating the sound of the same amplitude but an opposite phase, they could be used to cancel out clamorous environments. Of course, it could also be useful for creating immersive environments for entertainment, perhaps for 3D audio in theaters and theme park rides. The team is optimistic about the large range of options for using the novel technology, which could lead to a range of new applications in speakers and microphones.