You've probably heard of renewable energy sources like solar, wind and geothermal, but have you heard about sound energy? Researchers at Sungkyunkwan University in Seoul, South Korea, have developed a novel method of turning sound into electricity, and the technology could soon be used to charge our mobile phones with nothing more than the power of the human voice, reports the Telegraph.

Phones equipped with the technology could generate power from any kind of background noise — meaning the phone could be charging even when you're not speaking directly into the phone. The need to carry around an outlet charger could be supplanted by time spent listening to music, or perhaps even the sound of the wind roaring past your car window.

The technology is made possible by tiny strands of zinc oxide that are sandwiched between two electrodes. A sound-absorbing pad atop the device vibrates to ambient noise, causing the zinc oxide wires to compress and release. This movement is what generates the electrical current.

"The sound that always exists in our everyday life and environments has been overlooked as a [power] source. This motivated us to realize power generation by turning sound energy from speech, music or noise into electrical power," said Dr. Sang-Woo Kim, one of the developers. "Sound power can be used for various novel applications including cellular phones that can be charged during conversations and sound-insulating walls near highways that generate electricity from the sound of passing vehicles."

The latter development — using sound from a noisy highway to generate power, perhaps for streetlights — is just one potential use of the technology. Conceivably, anywhere that's noisy could be used as a power source. What was once considered noise pollution could soon become a green source of energy.

A prototype has already converted sound of about 100 decibels into about 50 millivolts of electricity. That's not enough to charge a phone, but it's an encouraging test, and researchers are optimistic that the technology could soon be developed into something more powerful.

At the very least, by combining this technology with other renewable energy technologies like kinetic energy generators, it may not be long before all of our mobile electronic devices are powered on the go — and for free.