The phrase "frequency sound" refers to the rate or number of vibrations or waves that create a sound. It describes how high or low a sound is, or how many times it vibrates per second. Full definition
Basically the earth emits low frequency sounds which have a unique signature when passing through oil that can be detected on the surface. (wexboy.wordpress.com)
To obtain graphene, the researchers exposed its parent compound, graphite, to a high frequency sound wave. (sciencedaily.com)
The high - frequency sounds of earthquakes can then be heard by humans at the surface. (sciencedaily.com)