Blog: Mobile phones charged by power of speech

In the search for alternative energy sources there's one form of energy you don't hear much about, which is ironic because I'm referring to sound energy. Sound energy is the energy produced by sound vibrations as they travel through a specific medium. Speakers use electricity to generate sound waves and now scientists from Korea have used zinc oxide, the main ingredient of calamine lotion, to do the reverse – convert sound waves into electricity. They hope ultimately the technology could be used to convert ambient noise to power a mobile phone or generate energy for the national grid from rush hour traffic.

Piezoelectrics are materials capable of turning mechanical energy into electricity, and can be substances as simple as cane sugar, bones, or quartz. Much research in this field has been focused on transforming the movement of a person running, or even the impact of a bullet, into a small electrical current, but although these advanced applications are not yet available in consumer products, scientists have been using piezoelectric materials in environmental sensors and speakers for years.

The Korean researchers were interested in reversing this process however. "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, authors of the article in journal Advanced Materials.

Piezoelectrics create an electrical charge under stress, and thus zinc oxide, the main ingredient of calamine lotion, was bent into a field of nanowires sandwiched between two electrodes. The researchers subjected the sandwich to sound waves of 100 decibels which produced an electrical current of about 50 millivolts.

On average, a mobile phone operates using a few volts, and as a normal conversation is conducted at about 60-70 decibels it's clear the technology falls some way short of being genuinely useful yet, but the researchers are optimistic that given time they can improve the electric yield. They hope future applications could include mobile phone charging from conversations, or sound-insulating walls near highways that boost the national grid using energy generated from rush hour traffic noise. However, with the increasing popularity of near silent electric vehicles there might be a decreasing window of opportunity for that particular application.