MIT has invented a very promising paper-thin speaker

MIT has invented a very promising paper-thin speaker

MIT has produced an impressive ultra-thin speaker whose applications could go well beyond entertainment.

At the prestigious Massachusetts Institute of Technology (MIT), researchers have developed a prototype of extremely thin speakers that can produce sound from any surface.

In practice, the device takes the form of a thin membrane that can theoretically be applied to any surface. The prototype presented by the researchers was a square measuring approximately the size of an adult hand, and lighter than a penny. According to its designers, despite its finesse, the machine would be able to produce a very convincing sound with “minimal distortion”.

It’s a remarkable feeling to take what looks like a sheet of paper, attach two clips to it, plug them into your computer’s audio jack, and start listening.”, explains Vladimir Bulović, lead author of the study”. “It can be used anywhere, and it takes a miniscule amount of energy to power it”, he rejoices.

In a standard loudspeaker, an electric current passes through a coil to generate a field magnetic. The latter serves to move a magnet whose movements are synchronized with the electrical signal. We then obtain a kind of small magnetic plunger whose role is to vibrate the diaphragm.

This has the effect of generating pressure waves that travel through space; once they reach the eardrum, they are interpreted by the brain as hiswhose height depends on the vibration frequency; the higher the vibration frequency, the higher the perceived sound will be, and vice versa.

Here, no magnetic piston as on traditional loudspeakers; the diaphragm is directly set in motion using an electric current. © Josh Sorenson – Unsplash

A piezoelectric membrane as a diaphragm

In this new ultrathin speaker, the reasoning is broadly the same. On the other hand, superfluous steps have been eliminated and the overall design has been simplified. To produce this film, the researchers chose to use a so-called piezoelectric material. This generic term designates a material capable of generating an electric charge when it is subjected to mechanical stress, that is to say when it is deformed.

Here, the researchers used an inverse piezoelectric effect. This means that the material deforms when an electric current is applied to it. This membrane therefore acts as a diaphragm whose movements can be controlled by applying an electric current to it.

This is a concept that has already been explored many times. But often with a prohibitive limit: these loudspeakers must usually be separated from any surface to be able to deform, and therefore produce sound. A limit which considerably reduces the practical interest of this technology.

So the MIT researchers changed the approach slightly. Here, there is no question of making the entire membrane vibrate; they produced a layer of piezoelectric material littered with thousands of small domes six times thinner than a human hairand all capable of vibrate individually. The layers which include these protrusions are then separated from the fixing layer by thin layers of air; all these little domes can thus vibrate together to produce audible sound.

Countless qualities

First advantage: this architecture allows the material to vibrate freely, even once applied to a surface. Moreover, thanks to the combination of thousands of small subunits, this loudspeaker is apparently formidable in terms of audio fidelity. It also appears that this technology is extremely energy efficient, with about 100 mW/m²! For comparison, the researchers explain that a standard loudspeaker can easily consume more than 1W for “produce similar sound pressure at a comparable distance”.

Another great quality of this technology: these membranes are extremely simple to produce in large quantities. Very briefly, it suffices to build a sandwich of adhesive and piezoelectric material. It only remains to heat the whole thing, then to pass it under a vacuum chamber through a mold; this allows the domes to form spontaneously under the effect of the pressure difference.

It is a very simple manufacturing process, without frills”, explains Jinchi Han, a postdoc in Bulović’s lab. “This makes it possible to envisage high-speed production, and that we could therefore use it to cover entire walls, car interiors, aircraft cockpits, etc.”, he suggests.

And the icing on the cake is that this technology could even find very interesting applications that are very far from the usual uses of loudspeakers. Since the domes are extremely thin and small, the researchers believe they could be ideal for applications related to ultrasound, which are neither more nor less than very high frequency sounds. This could have interests in several fields, in particular in medical imaging.

Not just for music

The researchers also believe that it would be possible to use these domes in the opposite direction. Instead of making them vibrate, it is also possible to wait for an outside sound to set them in motion. We can then measure this vibration; we then end up with a system ofecholocationsuch as that available to bats or whales.

In addition, unlike a traditional loudspeaker, this film is by nature fully waterproof. This means that it would be possible to use it in liquids with very little constraint. In this context, thanks to its mechanical properties, sound could, for example, be used to mix dangerous chemicals away from them.

Even more impressive: this system could even one day be used to create brand new screens !”If vibrating domes are coated with reflective material, they could be used to create light patterns for future display technologies”, explain the researchers.

In summary, this small membrane is already overflowing with potential even though it has only just been born. “We now have the ability to generate air movement by activating an expandable surface…the options are virtually limitless”, Bulović breathes.

It will therefore be very interesting to follow the evolution of this very promising technology; we could soon find it in the cabin of our cars, in concert halls, and even in the industry of tomorrow.

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