Infrasound

Infrasound refers to sound waves with frequencies below 20 Hz – the lower limit of what the human ear can normally perceive as sound. Although we cannot ”hear” infrasound in the traditional sense, these low-frequency oscillations can affect the body in other ways. When the frequency is high enough and the sound pressure level strong enough, infrasound can be perceived as pressure changes, vibrations, or pulsations.

Unlike audible sound, infrasound is dampened very little by the atmosphere and can therefore propagate over long distances. It can also penetrate buildings and other obstacles more effectively than higher frequencies. Infrasound can arise from both natural sources such as thunder, earthquakes, and ocean waves, and from human activities such as heavy machinery, ventilation systems, and large wind turbines.

In built environments, infrasound can create resonances in buildings and rooms. When the wavelength of infrasound coincides with a building’s or room’s dimensions, standing waves can occur that amplify certain frequencies. This can cause infrasound to be experienced with different intensity in different parts of the same room or building.

Measurement and evaluation of infrasound is technically challenging. Common sound meters with A-weighting filter out most of the infrasound, which means that standardized noise measurements often miss this aspect. To correctly measure infrasound requires special equipment and specific measurement methods that can handle these low frequencies.

How infrasound affects humans is still the subject of research. While some studies indicate possible effects on health and well-being during prolonged exposure to strong infrasound, the relationships are complex and often difficult to isolate from other environmental factors. Individual sensitivity to infrasound also varies considerably between different people.

A particular challenge with infrasound is that it can be difficult to locate the source, as the long wavelength means that the sound can be reflected and conducted in complex ways in the environment. This can make it problematic to implement effective measures to reduce disturbing infrasound.