What is the primary characteristic of infrasound waves?

Infrasound waves are characterized by their frequencies that are below 20 Hz. This places them in a category of sound waves that is not audible to humans, who typically hear sounds in the frequency range of approximately 20 Hz to 20 kHz. Infrasound is often produced by natural phenomena such as earthquakes, volcanic eruptions, and even certain weather events, as well as man-made sources like explosions.

The other options do not accurately capture the defining features of infrasound. In particular, humans cannot hear these low-frequency waves, and while some sound waves can indeed travel quickly, infrasound does not inherently move faster than all audible sound, as speed depends on the medium of transmission. Finally, although infrasound waves can travel through various materials including gases and liquids, they are not restricted to solids. Thus, the defining trait of infrasound is its frequency, confirming that the correct answer focuses on their classification based on frequency below 20 Hz.