MIT engineers have developed a paper-thin loudspeaker that can turn any surface into an active audio source. MIT News reports:
This thin-film loudspeaker produces sound with minimal distortion while using a fraction of the energy required by a traditional loudspeaker. The hand-sized loudspeaker the team demonstrated, which weighs about as much as a dime, can generate high-quality sound no matter what surface the film is bonded to. To achieve these properties, the researchers pioneered a deceptively simple fabrication technique, which requires only three basic steps and can be scaled up to produce ultrathin loudspeakers large enough to cover the inside of an automobile or to wallpaper a room.
A typical loudspeaker found in headphones or an audio system uses electric current inputs that pass through a coil of wire that generates a magnetic field, which moves a speaker membrane, that moves the air above it, that makes the sound we hear. By contrast, the new loudspeaker simplifies the speaker design by using a thin film of a shaped piezoelectric material that moves when voltage is applied over it, which moves the air above it and generates sound. [...]
They tested their thin-film loudspeaker by mounting it to a wall 30 centimeters from a microphone to measure the sound pressure level, recorded in decibels. When 25 volts of electricity were passed through the device at 1 kilohertz (a rate of 1,000 cycles per second), the speaker produced high-quality sound at conversational levels of 66 decibels. At 10 kilohertz, the sound pressure level increased to 86 decibels, about the same volume level as city traffic. The energy-efficient device only requires about 100 milliwatts of power per square meter of speaker area. By contrast, an average home speaker might consume more than 1 watt of power to generate similar sound pressure at a comparable distance.
Researchers develop a paper-thin loudspeaker
The flexible, thin-film device has the potential to make any surface into a low-power, high-quality audio source.
Adam Zewe | MIT News Office
Publication Date:April 26, 2022
A typical loudspeaker found in headphones or an audio system uses electric current inputs that pass through a coil of wire that generates a magnetic field, which moves a speaker membrane, that moves the air above it, that makes the sound we hear. By contrast, the new loudspeaker simplifies the speaker design by using a thin film of a shaped piezoelectric material that moves when voltage is applied over it, which moves the air above it and generates sound.
Most thin-film loudspeakers are designed to be freestanding because the film must bend freely to produce sound. Mounting these loudspeakers onto a surface would impede the vibration and hamper their ability to generate sound.
To overcome this problem, the MIT team rethought the design of a thin-film loudspeaker. Rather than having the entire material vibrate, their design relies on tiny domes on a thin layer of piezoelectric material which each vibrate individually. These domes, each only a few hair-widths across, 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. The same spacer layers protect the domes from abrasion and impact during day-to-day handling, enhancing the loudspeaker’s durability.
You might be wondering how the ultrasound turns back into audible sound: if you can't hear Mona Lisa when you stand to one side, why can you suddenly hear her if you walk through the sound beam?
The speaker array actually produces a modulated wave made of two separate ultrasound waves. One of them is a steady, reference tone of a constant 200,000 hertz (Hz) frequency (the carrier wave) and the other is the signal that fluctuates between 200,200 Hz and 220,000Hz (the modulating wave). Although they're combined, it's easiest to think of them as two separate waves traveling out in parallel straight lines through a column of air without overlapping. If they meet an obstruction (such as your curious head), they suddenly slow down and mix together so they interfere constructively (by adding together) and destructively (by subtracting from one another). By the principle of wave superposition, two ultrasound waves of those frequencies can subtract from one another to produce a third wave with a much lower frequency in the range 200–20,000 Hz—and that's in the frequency range that your ears hear. An electronic circuit attached to the piezoelectric transducers constantly alters the frequency of the two waves so they produce the correct lower, audible frequency when they collide and "demodulate." (It also encodes the signal in a unique way that ensures there's less noise and distortion when it separates out in the listener's ear.) The process by which the two ultrasound waves mix together is technically called parametric interaction, which is why speakers that work this way are sometimes called parametric loudspeakers.
