Infrasound

Infrasound is sound below the audible range of human hearing, which is approximately 20 Hz – 20,000 Hz. Some sounds below 20 Hz may be audible to people provided the sound level is sufficiently high. Sound at these low frequencies can also excite structures and these structures in movement may radiate some sounds in the audible range.

Examples of such sounds would be:

combustion turbine exhausts
HRSG structures and exhausts
jet engine test cells
wind turbines