Sunday, 3 February 2019

Infra-sound from wind power plants travels quickly and far

The infra-sound from wind power plants got finally into discussions in Finland in autumn 2016. Instead of focusing on the harmful effects the industrial infra-sound has to human health, many newspapers focused on denying the lethal effect on bats. It was, of course, a shock also to the media to hear that the whole of western Finland with its inhabitants and animals was, and still is, exposed to a constant, uncontrolled infra-sound from industrial wind turbines.

However, from the point of view of an exposed person, the media showed lack of discretion. By denial the effects, the situation will not change nor will the harmful health effects be removed.

A more sensible approach would have been to ask what infra-sound actually is.

Infra-sound is a technical phenomenon in physics and acoustics. Acoustics is a science of sound. It is a study of mechanical waves, such as vibration and sound. [1-2] The basic concepts have been explicitly explained, for example, in Noronen's (2015) thesis in welfare technology.

Sound waves are longitudinal waves whose particle’s movement is in the same direction as the wave propagation velocity. Sound waves can only proceed in a medium, such as liquid and gas as longitudinal waves, or in solid material as transversal waves, or plasma. [3] Sound does not proceed in the vacuum [4].

Gas, such as air, can conduct sound at a high frequency scale. The frequency of the audible sound is about from 20 to 20,000 Hz; the frequency of the ultrasound, used even by bats, is over 20,000 Hz and the frequency of the infra-sound is less than 20 Hz. Infra-sound can be compared to low-frequency light, i. e. to infrared. It is invisible to the human eye. [3]

In addition to the frequency, the sound waves have other features, such as wavelength, oscillation extent, i. e. amplitude and propagation velocity [3, 5]. Wavelength is calculated by dividing propagation velocity by frequency. The unit of the wavelength is meter (m) and lambda is often used as its symbol. [5]

Sound intensity is described by sound pressure or sound pressure level [4]. Sound pressure is the instantaneous pressure variation caused by the sound wave in a ratio to static pressure. It is measured as pascals. [6] The unit of the sound pressure level is decibel. It describes the volume of the sound compared to the selected reference value. [4, 7]

Thus, infra-sound below the frequency of 20 Hz is sound. It propagates at the speed of sound of 344 m/s, when the air temperature is +20 degrees Celsius near the ground. [3]

Low-frequency sounds have a wider wavelength than high-frequency sounds, such as audible sound. By calculating, it is apparent that the wavelength of the 0.5 Hz infra-sound is 688 m in the described conditions. The wavelength of the 1 Hz infra-sound is 344 m.

Therefore, the wavelength of the infra-sound is really long. This is why it fades slowly and can travel huge distances almost without dampening. [3]

The 1 Hz infra-sound needs only 2.9 seconds to the distance of one kilometer and about half a minute to 10 kilometers. It travels one hundred kilometers in 4 min 51 s. The forward or reverse wind also affects to the traveling time, so the propagation velocity of infra-sound either accelerates or slows down. The rise in temperature increases the propagation velocity of infra-sound as well. [3, 8-9]

Wind turbines generate infra-sound, which propagates in the surrounding medium, i. e. in the air, quickly and far [3, 10-11]. It also proceeds in solid medium and liquid, i. e. it travels through buildings, in water and in humans [3]. There is 70% liquid in humans.

Just like the sea is not a vacuum - it is water where sound waves and noise propagate far [12-13] - air is not a vacuum either, but a substance to which the wind turbines constantly produce vibrating, long, slowly dampening and far traveling pressure waves – industrial infra-sound. This kind of waves cannot be created by any other industrial structure than by large wind turbine plants. It is not irrelevant in what kind of air humans and animals live and what they breathe.

There is infra-sound around the wind farms, at least at the distance of tens of kilometers, enough for all animals and people, for all living and working in those areas. The critical question is how does it affect health - and how fast?