Piezo audio devices go micro, revitalize output in a big way

Article By : Bill Schweber

The paper-thin, very flexible loudspeaker can turn any surface into an active audio source, giving new meaning to the phrase "wall of sound".

The piezoelectric effect is an extraordinarily useful and versatile phenomenon which engineers have adapted to countless transducer applications. In some of these, the applied voltage is transformed into a mechanical strain output for use in nano-positioning actuators and even as a basic sound source. In the complementary mode, mechanical stress is applied to the piezo material, which then acts as a rugged sensor and produces a voltage. It may sound trite, but there seems to be no limit to how creative and clever minds have figured out how to use this effect and manufacture rugged, reliable and low-cost devices of all types by exploiting the unique properties of suitable materials.

As sound sources, piezo-based speakers provide a mix of attributes. They can be made fairly thin and create relatively high sound pressure level (SPL), but audio quality is limited due to mechanical and physical-material issues. Now, a team at MIT has devised a way to create a dense array of tiny piezo-based dome loudspeakers, which offers a new physical embodiment of this classic analog function. The result is a paper-thin, very flexible loudspeaker that can turn any surface into an active audio source, giving new meaning to the phrase “wall of sound” (Figure 1).

Figure 1 The array of densely-packed piezo-based loudspeakers has the look and feel of a flexible piece of paper. Source: MIT

In conventional thin-film loudspeakers, the film must be allowed to bend freely to produce sound, as firmly mounting them onto a surface would dampen the vibration and attenuate their output as well as limit their frequency response.

The MIT team approached the dilemma in a different way. Rather than having the entire material vibrate, their design uses tiny domes fabricated on a thin layer of piezoelectric material, where each can vibrate individually. These domes, about 15 μm in height, move up and down only about half a micron when they vibrate. Each dome is a single sound-generation unit, so it takes thousands of these tiny domes vibrating together to produce audible sound. The basic loudspeaker weighs only 2 grams, is 120-μm thick, and can be manufactured at low cost using what they maintain are standard processes (Figure 2).

Figure 2 The six-step fabrication sequence uses an innovative arrangement and layering of common materials with standard processing steps (a); the flexibility of the resultant piezo-sheet is clearly visible (b); and a microphotograph shows the density of the dome array (c). Source: MIT

The domes are surrounded by spacer layers on the top and bottom of the film that protect them from the mounting surface while still enabling them to vibrate freely. These same spacer layers also protect the domes from abrasion and impact during day-to-day handling, enhancing the loudspeaker’s durability.

Researchers used a laser to cut tiny holes into a thin sheet of polyethylene terephthalate—better known as PET, a standard plastic used for a wide range of applications, including beverage bottles—and laminated the underside of that perforated PET layer with an 8-μm thick film polyvinylidene fluoride (PVDF), a common commercial and industrial coating. Then they applied vacuum above the bonded sheets and an 80°C heat source underneath them.

Since the PVDF layer is so thin, the pressure difference created by the vacuum and heat source caused it to bulge. As the PVDF can’t force its way through the tough PET layer, the tiny domes instead protrude in areas where they aren’t blocked by PET and these protrusions self-align with the holes in the PET layer. The researchers then laminated the other side of the PVDF with another PET layer to act as a spacer between the domes and the bonding surface.

Among their extensive tests, they measured a sound pressure level of 66 dB at 30 cm with a 1-kHz/25-Vrms excitation and 86 dB at 10 kHz at that same drive voltage, regardless of the rigid surface on which it is bonded.

Another interesting attribute of this fabrication scheme is that it allows for tunability within certain limits by changing the size of the holes in the PET. Domes with a larger radius displace more air and thus produce more sound, but these larger domes also have lower resonant frequency.

The technique is not just suitable for audible-range sound. Since it’s just those tiny domes which are vibrating rather than the entire film, the loudspeaker has a high-enough resonant frequency that it can be used effectively for ultrasound applications such as medical imaging.

“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,” noted Vladimir Bulović, the Fariborz Maseeh Chair in Emerging Technology, leader of the Organic and Nanostructured Electronics Laboratory (ONE Lab), director of MIT.nano, and senior author of the academic paper. He added that it can even be applied as “wallpaper” and so be a sound source.

The work is described in the paper “An Ultra-Thin Flexible Loudspeaker Based on a Piezoelectric Micro-Dome Array” published in IEEE Transactions on Industrial Electronics; they have also posted a 40-second YouTube video of the speaker playing “We are the Champions” by Queen.

Have you ever fabricated your own custom piezo-based transducers, especially ones with extremely small size and/or high density? Or do you think this is a lab-only innovation which will have little commercial impact?

This article was originally published on Planet Analog.

Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.


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