Researchers in Denmark are trying to establish what sounds are actually too low to hear. Using brain imaging, scientists determined that humans can "hear" sounds at much lower frequencies than previously thought.

The research was inspired by ongoing controversies over the installation of wind turbines. People living in the vicinity of wind farms have complained of disrupted sleep and other health problems. In addressing complaints, advocates for wind energy projects have denied the role of turbines and the low-frequency sounds, or infrasound, they produce.

"Neither scaremongering nor refuting everything is of any help in this situation," Christian Koch, a researcher with the German National Metrology Institute, said in a press release. "Instead, we must try to find out more about how sounds in the limit range of hearing are perceived."

To establish a true lower limit to the range of human hearing, researchers imaged listeners' brains while playing lower sounds through earphones. Functional magnetic resonance imaging (fMRI) results showed sounds as low as 8 hertz triggered brain activity in neural areas related to both hearing and emotions. Previously, 16 hertz was thought to be the lower limit of human hearing.

The imaging results were consistent with what listeners claimed to hear -- whether what they "heard" was tonal in nature or not.

"This means that a human being has a rather diffuse perception, saying that something is there and that this might involve danger," Koch said. "But we're actually at the very beginning of our investigations. Further research is urgently needed."

That research may offer a more accurate measure of how infrasound affects human health.