Page 31 - Volume 9, Issue 3

CONCERNS ABOUT INFRASOUND FROM WIND TURBINES
Geoff Leventhall
150 Craddocks Ave Ashtead, Surrey KT21 1NL United Kingdom
“It appears that concerns over infrasound and low frequency noise have found a place deep in the national psyche of a number of countries and lie waiting for a trigger to bring them to the surface.”
Infrasound
Infrasound has been defined as: “Acoustic oscillations whose fre- quency is below the low frequency limit of audible sound (about 16 Hz).” (IEC 1994)
However, sound remains audible at frequencies well below 16 Hz. For exam- ple, measurements of hearing threshold have been made down to 4Hz for expo- sure in an acoustic chamber (Watanabe and Møller 1990) and down to 1.5 Hz for earphone listening (Yeowart et al. 1967).
The limit of 16 Hz, or more com-
monly considered as 20 Hz, arises from
limit for which the standardized equal loudness hearing con- tours have been measured, not from the lower limit of hearing. From the subjective point of view, there is no logical reason for terminating a continuous process of hearing at an arbitrary fre- quency, so that from about 10Hz to 100Hz could be taken as the low frequency range. It may also be argued that there is no log- ical reason for terminating at 100 Hz, and the range is some- times extended to about 200Hz and down to 5Hz. However, objectors to wind turbine developments are now requesting that measurements are made down to below 1Hz.
Atmospheric infrasound
This is a well-established discipline, studying frequencies from about one cycle in 1000 seconds up to, say, 2Hz and higher (Bedard and George 2000). Atmospheric infrasounds are caused by weather variations, turbulence, meteorites, dis- tant explosions, ocean waves interacting (microbaroms) or waves breaking on the shore, practically any occurrence which puts energy into the atmosphere over a relatively short period of time and any process with a low repetition rate. The attenuation with distance is small and propagation can be complex. Monitoring of atmospheric infrasound is an essen- tial part of ensuring the success of the Nuclear Test Ban Treaty, since explosions in the air generate infrasound, and there are about 60 monitoring stations around the world.
Of course, it is important to realise that our evolution has been in the presence of naturally occurring atmospheric infrasound, which overlaps the lower end of wind turbine infrasound.
The Apollo Space Programme
Early work on low frequency noise and its subjective effects was stimulated by the Apollo space programme. It was known that large launch vehicles produce their maximum noise energy in the infrasound region. Furthermore, as the vehicle accelerates, the crew compartment is subjected to
boundary layer turbulence noise for a few minutes after lift-off. Experiments were carried out in low frequency noise chambers on short term subjective tol- erance to bands of noise at levels of 140dB to 150dB in the range up to 100Hz (Mohr, Cole et al. 1965). It was concluded that subjects who were expe- rienced in noise exposure, and who were wearing ear protection, could tol- erate both broad¬band and discrete fre- quency noise in the range 1Hz to 100Hz at sound pressure levels up to 150dB. Later work suggests that, for 24 hour
exposure, levels of 120-130dB are tolerable below 20Hz (von Gierke 1973, von Gierke and Nixon 1976). These high, long- term limits were set to prevent direct physiological damage. It was not suggested that the exposure is pleasant, or subjec- tively acceptable, for anybody except those whose occupation requires them to be exposed to the noise.
Work was also in progress in the UK (Yeowart, Bryan et al. 1969, Hood and Leventhall 1971) and France (Gavreau, Condat et al. 1966, Gavreau 1968) from the 1960s and in Japan and Scandinavia from the 1970s (Møller 1980, Yamada 1980). Japan and Scandinavia are now the main centres for work on infrasound and low frequency noise. A review of studies of low frequency noise has been given by Leventhall (Leventhall, Benton et al. 2003)
Origins of the Concerns
The early American work was published from the mid- dle 1960s and did not attract public attention, but a few years later infrasound entered upon its “mythological” phase, echoes of which still occur, currently in relation to wind tur- bines. The main name associated with the early phase is that of Gavreau from CNRS Marseille, whose work was in progress at the same time as that of the Apollo space pro- gramme. (Gavreau, Condat et al. 1966, Gavreau 1968). Infrasound at 7Hz from a defective industrial fan, which may have been operating at an unstable point on its characteristic, led to investigations of the problem and to the design of high intensity low frequency sound sources. The frequencies and levels reported in Gavreau (1968) were:
the lower frequency
Frequency Hz
2600
340
196
37
7
Reported level dB
Not given 155dB 160dB Not given Not given
30 Acoustics Today, July 2013