Tuesday, September 27, 2022

A paper-thin loudspeaker

The flexible, thin-film device can make any surface into a low-power, high-quality audio source.

A typical loudspeaker found in headphones or an audio system uses electric current inputs that pass through a coil of wire, thereby generating a magnetic field. This magnetic field moves a speaker membrane- that moves the air above it- and makes the sound we hear. Ultra-thin, lightweight, high-performance, low-cost, and energy-efficient loudspeakers that can be deployed over a wide area have become increasingly attractive to both traditional audio systems and emerging applications such as active noise control and immersive entertainment.

MIT engineers have devised an ultra-thin flexible loudspeaker that can turn any surface into an active audio source. This new paper-thin loudspeaker produces sound with minimal distortion while using a fraction of the energy required by a traditional loudspeaker.

Unlike typical loudspeakers, this new thin-film loudspeaker simplifies the speaker design by using a thin film of a shaped piezoelectric material. The material moves with applied voltage, which moves the air above it and generates sound. It can generate 86 dB sound pressure level (SPL) at a 30-cm distance with 25-V (RMS) excitation at 10 kHz.

The proposed loudspeaker also exhibits high bandwidth, extending its prospects into the ultrasonic range. It weighs only 2 g, is 120 m thick, and can be manufactured at a low cost.

Engineers designed the loudspeaker by using a deceptively simple fabrication technique. Their technique requires only three basic steps and can be scaled up to produce ultrathin loudspeakers large enough to cover the inside of an automobile or wallpaper a room.

Features of this paper-thin loudspeaker:

  • Active noise cancellation in a clamorous environment by generating the sound of the same amplitude but opposite phase.
  • Flexible for immersive entertainment.
  • Provides three-dimensional audio.
  • Lightweight, hence requires such a small amount of power to operate.
  • Well-suited for applications on smart devices where battery life is limited.
  • High quality, low power.
  • Energy-efficient: Requires about 100 milliwatts of power per square meter of speaker area.

The design of this loudspeaker relies on tiny domes on a thin layer of piezoelectric material which each vibrates individually. These domes, which are only a few hair widths wide, are protected from the mounting surface by spacer layers on the film’s top and bottom while allowing them to vibrate freely. The same spacer layers protect the domes from abrasion and impact during day-to-day handling, enhancing the loudspeaker’s durability.

Using a laser, engineers made tiny holes into a thin PET sheet. They then used an extremely thin film of a piezoelectric material called PVDF to laminate the underside of the perforated PET sheet (as thin as 8 microns). They then applied a vacuum above the joined sheets and an 80°C heat source beneath them.

Because the PVDF layer is so thin, it bulged due to the pressure difference induced by the vacuum and heat source. Because the PVDF cannot make its way through the PET layer, tiny domes protrude in areas where PET does not block them. The perforations in the PET layer self-align with these protrusions. To function as a spacer between the domes and the bonding surface, the engineers laminate the other side of the PVDF with another PET layer.

Lead author Jinchi Han said, “This is a simple, straightforward process. It would allow us to produce these loudspeakers in a high-throughput fashion if we integrate them with a roll-to-roll process in the future. It could be fabricated in large amounts, like wallpaper to cover walls, cars, or aircraft interiors.”

The domes are 15 microns in height, and they only move up and down 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.

Another benefit of this process is tunability: Engineers can change the size of the holes in the PET to control the size of the domes. Domes with a larger radius displace more air and produce more sound, but larger domes also have lower resonance frequency.

After completing the fabrication, engineers tested the loudspeaker by mounting it to a wall 30 centimeters from a microphone. They measured the sound pressure level and recorded it in decibels when 25 volts of electricity were passed through the device at 1 kilohertz (1,000 cycles per second). The speaker produced high-quality sound at conversational levels of 66 decibels.

As mentioned above, the loudspeaker generated an 86 dB sound pressure level at 10 kilohertz.

Han said, “Because the tiny domes are vibrating, rather than the entire film, the loudspeaker has a high enough resonance frequency to be used effectively for ultrasound applications, like imaging. Ultrasound imaging uses high-frequency sound waves to produce images, and higher frequencies yield better image resolution.”

Bulović said, “The device could also use ultrasound to detect where a human is standing in a room, just like bats use echolocation, and then shape the sound waves to follow the person as they move. If the vibrating domes of the thin film are covered with a reflective surface, they could be used to create patterns of light for future display technologies. If immersed in a liquid, the vibrating membranes could provide a novel method of stirring chemicals, enabling chemical processing techniques that could use less energy than large batch processing methods.”

Vladimir Bulović, the Fariborz Maseeh Chair in Emerging Technology, said“The device could also use ultrasound to detect where a human is standing in a room, just like bats do use echolocation, and then shape the sound waves to follow the person as they move. If the vibrating domes of the thin film are covered with a reflective surface, they could be used to create patterns of light for future display technologies. If immersed in a liquid, the vibrating membranes could provide a novel method of stirring chemicals, enabling chemical processing techniques that could use less energy than large batch processing methods.”

“We can precisely generate mechanical motion of air by activating a physical surface that is scalable. The options of how to use this technology are limitless.”

Journal Reference:

  1. Jinchi Han et al. An Ultra-Thin Flexible Loudspeaker Based on a Piezoelectric Micro-Dome Array. DOI: 10.1109/TIE.2022.3150082
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