Mis à jour le 12 May 2022.
Engineers at the Massachusetts Institute of Technology, better known as MIT, have just invented the thinnest speaker driver in the world. As thin and light as a sheet of paper, this driver could revolutionize the way we view loudspeakers and listen to music. Its ultra thin design would allow it to be lined on walls and inside vehicles. The very notion of what a loudspeaker is could change.
A loudspeaker driver only 0.12 mm thick
A real technological feat, the loudspeaker invented by MIT researchers takes the form of a rectangular membrane just 0.12 millimeters thick and weighing 2 grams. Despite this extreme thinness, the speaker is able to produce powerful sound with minimal distortion, say the researchers. “It’s a remarkable feeling to take something that looks like a sheet of paper, attach two clips to it, plug them into your computer’s audio output, and start listening to music ,” says Vladimir Bulović, head of the study.
How does such a thin speaker work?
With a standard loudspeaker, the cone is attached to a surround and glued to a coil, generally made of copper. The latter is surrounded by a magnetic equipment to control its movement. When the voice coil receives the alternating current from the amplifier, it starts moving by being alternately attracted and repelled by the magnetic system. This coil then moves the membrane thus generating the sound waves. Many design parameters, including size, materials or even the magnetic system, influence the operation and performance of a loudspeaker driver. For headphones and earbus , the operating principle is the same, but we sometimes refer to it as a transducer.
The ultra-thin loudspeaker developed by the MIT simplifies the principle of operation of standard loudspeakers. To do this, the researchers rely on a sheet with an inverse piezoelectric effect. The latter is capable of chaging shape when an electric current is applied to it. It therefore acts as a cone thanks to its ability to adopt different shapes depending on the current sent by the amplifier. However, the drawback of this technology is that the driver has to be placed away from any surface so that it can move and create sound. To overcome this constraint, MIT engineers covered the piezoelectric material with thousands of microscopic domes. Six times thinner than a hair, they vibrate together to produce audible sound. This layer is then separated from the fixing sheet by an air gap. Thanks to this innovation, the MIT loudspeaker driver can be placed on any surface while maintaining the same freedom of movement.
The MIT researchers tested their ultra thin driver by mounting it to a wall 30 centimeters from a microphone to measure the sound pressure level. When a voltage of 25 volts at 1 kilohertz passes through the system, the driver delivers quality sound at a level of 66 decibels. At 10 kilohertz, the sound pressure level rose to 86 decibels, roughly the same volume level as city traffic.
What applications for the MIT loudspeaker driver?
The MIT’s new speaker driver offers countless advantages, including a very simple manufacturing process. Its design requires an assembly between an 8-micron-thick PET plate and a piezoelectric material, called PVDF. Everything is heated, then passed under a vacuum chamber through a mold to form the domes. “It’s a very simple, straightforward manufacturing process, ” says Jinchi Han, postdoc in the Vladimir Bulović lab. “ It allows for high throughput production, so it could be used to cover entire walls, car interiors, airplane cockpits, and so on. ”
In an airplane cockpit or the cabin of a car, scientists believe that these ultra-thin speaker drivers could be used to reproduce ambient sounds in reverse phase to cancel out unwanted noise. In other words, the principle of operation would be the same as noise-cancelling headphones , but on the scale of a car or an entire room. These wafer-thin drivers could also one day cover the walls of our living rooms for a fully immersive listening experience. In addition, this driver is very energy efficient since only 100 mW of power per square meter is needed to drive it, much less than traditional speaker drivers. The application on very large surfaces is therefore facilitated.
This new technology is full of promise and could be used in many areas far from the traditionnal uses of loudspeakers. Researchers are already studying the possibility to used these domes to create ultrasound for medical imaging. Another possibility would be to reverse the direction of operation of the loudspeakers to create an echolocation system. Finally, engineers plan to use these drivers to create the televisions of tomorrow.“If the vibrating domes are coated with a reflective material, they could be used to create light patterns for future display technologies ,” according to the researchers.