Do you ever have trouble sleeping because of the noisy world outside? Perhaps you live by a busy highway, or it's Spring, and the birds won't give it a rest. If so, then perhaps a recipe for better sleep is appreciation of the fact that it could be worse. Lucky you, your hearing range is limited.

There's a whole world of sound, called infrasound, that your ears can't pick up. Infrasound consists of long sound waves that travel at a frequency of below twenty hertz. The lower the frequency, the farther these sounds can travel without dying off. That means that an earthquake in one part of the world can potentially be detected on the other side of the globe.

Infrasound is produced by a variety of sources, from hurricanes and the northern lights to elephants and rhinoceroses to nuclear device testing. The monitoring of these sounds is a relatively new science. Scientists are still discovering new sources of infrasound, as well as still learning how to match particular sound signatures to their sources.

Infrasound detection gained heightened interest in the beginning of the Cold War, when nuclear testing began to be closely monitored. Today, interest in infrasound is as varied as its sources. Some scientists are interested in the prospect that animals such as elephants may use infrasound to communicate over long distances. Others are interested in how infrasound technology may lead to better forecasting of the weather, or the ability to predict occurrences like avalanches and volcanoes.