Introduction
This topic covers infrasound (IS), the mechanical waves in air, water and solids with frequencies below the lower limit of human hearing, which is about 20 Hz. It describes the physical properties of these waves, where they come from, and how they act on the human body, including the effects of high and low intensities.
Physical properties
Infrasound is a longitudinal wave, like audible sound, and it travels at the speed of sound in the medium, about 340 m/s in air. Its wavelength is therefore very long: at 20 Hz it is about 17 m, and at 1 Hz it is about 340 m. The relation \lambda = v / f connects speed, wavelength and frequency.
- Low absorption in air: because the attenuation of sound in air increases with frequency, infrasound can travel for many kilometres with little loss.
- Penetration: the long wavelength means infrasound passes easily through walls, buildings and body tissues, and it is hard to shield against.
- Hearing threshold: the ear is much less sensitive at low frequency, so the sound pressure must be very high before it is heard. The threshold rises steeply below 20 Hz, and infrasound is often felt as pressure or vibration rather than heard as a sound.
Intensity is measured as sound pressure level in decibels. Infrasound is commonly assessed with the G-weighting, which gives the frequency response that matches human perception at low frequency.