"Just as speakers transform electric signals into sound, the opposite process — of turning sound into a source of electrical power — is possible," said Young Jun Park and Sang-Woo Kim, the two corresponding authors of a new article in the journal Advanced Materials.
"Sound power can be used for various novel applications including mobile phones that can be charged during conversations and sound-insulating walls near highways that generate electricity from the sound of passing vehicles," the co-authors added.
Harvesting energy from phone calls and passing cars is based on materials known as piezoelectrics. When bent, a piezoelectric material turns that mechanical energy into electricity.
Lots of materials are piezoelectric: cane sugar, quartz and even dried bone creates an electrical charge when stressed. For decades, scientists have pumped electricity into piezoelectric materials for use in environmental sensors, speakers and other devices.
Over the last few years, however, scientists have made dramatic advances in getting electricity out of piezoelectric devices. Most of these devices, which are not yet available for consumer purchase, would generate power as a person walks, runs or, in this case, talks. The U.S. Army is even looking at partially powering some vehicles by channeling the physical impact of a bullet into a small electrical current.
The Korean scientists, however, want to harness a different kind of power source:
sound waves.
Using zinc oxide, the main ingredient in calamine lotion, Young Jun Park, Sang-Woo Kim and their colleagues created a field of nanowires sandwiched between two electrodes. The researchers blasted that sandwich with sound waves, which at 100 decibels were not quite as loud as a rock concert. A normal conversation is about 60-70 decibels.
The sound waves produced a mild electrical current of about 50 millivolts. The average cell phone requires a few volts to operate, several times the power this technology can currently produce.
The new research is interesting, said Michael McAlpine, a scientist at Princeton University who also builds energy harvesting devices.
"But the real question though is whether there is enough ambient noise to act as a power source as for a cell phone," said McAlpine. A consumer probably wouldn't want to attend a rock concert or stand next to a passing train to charge their cell phone.
The Korean scientists agree: 50 millivolts is not a lot of power, but they also say their research is proof of concept. As they continue their work, they expect to get a higher power output.
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