Propagation of Low Frequency Noise
This article contains information on the Propagation and Measurement of Low Frequency Noise (LFN) and Infrasound (ILFN)
LFN can travel considerable distances and still affect peoples' health. Because wind farms are typically sited on the tops of hills and mountains LFN emitted by wind farms has no barriers and can travel considerable distances, depending on wind direction, atmospheric conditions and local topography.
Higher-frequency noise is less of a problem, except to those people unfortunate enough to live in hearing range of a wind farm. The atmosphere filters out higher-frequency noise so it doesn't carry nearly as far.
Based on the personal experience of some members of this group there is very good reason to postulate that LFN emitted by large wind turbines can travel at least 30 miles. Due to the omnipresent nature of low-frequency noise, its source is extremely difficult (if not impossible) to pinpoint, so proving our hypothesis would be very difficult. However, we do have seven years of observations so we are as certain as possible that this hypothesis is correct and would like the opportunity to prove it in due course. Below is a graph produced from a noise recording in a rural Carmarthenshire home in June 2009 before Alltwalis wind farm was built. See attached graph: EG noise graph.jpg
Before elaborating on the propagation of LFN a bit of background in noise impact assessment requirements for wind farm applications is useful.
Measurement of Wind Farm Noise
Noise measurement guidelines, ETSU-R/97, were drawn up in the 1990s in collaboration between wind industry experts and government bodies. These guidelines were drawn up specifically to monitor and assess the potential impact of wind farms on local residents. At this time wind turbines were much smaller, and the low-frequency noise spectrum was much smaller.
The ETSU-R/97 guidelines only require the wind farm developers to monitor and assess potential noise emissions using an A-weighted filter (dBA), which was developed in the 1930s when noise monitoring equipment was less sophisticated. The A-weighted filter is designed to detect what the ears normally hear. This filter effectively removes nearly all the low-frequency component of the noise (LFN), and all the very low frequencies (ILFN).
As wind turbines get larger, the LFN and ILFN spectrum of the wind farm noise gets greater. The end result is that wind farm noise predictions effectively ignore the largest portion of the noise spectrum. The result is that wind farm applications cannot fail on the basis of noise 'predictions'.
Another point on noise prediction models used by the wind industry is that they assume spherical projection of noise from the turbines. As you will see below, this is a very idealised assumption, which reduces the predicted noise output.
Factors Affecting Low-Frequency Noise Propagation
- Normal mathematical models for predicting noise levels do not apply very well to LFN.
- Atmospheric factors can influence how far LFN can travel (most noise impact studies only assume spherical spreading of sound):
o Topographical Effects
Sloping land with increasing ground angles, especially in combination with atmospheric conditions, can cause sounds to combine (e.g. near Vancouver Airport, hills rising from a flat plain caused sound levels to be 20dB higher at 5500m than at 4000m).
o Atmospheric Effects:
During the day warming air rises, increasing turbulence and carrying noise aloft, scattering wind turbine noise.
The following factors can also affect LFN:
§ Stable night time atmosphere
At night there is normally minimal atmospheric turbulence and light winds. These factors enable noise from wind turbines to carry much farther than expected.
§ Inversion layers
When a temperature inversion layer forms above the height of turbines, it can reflect some of the sound back toward the ground and, with the ground, create a channel to facilitate longer-range transmission of sound.
Other factors can affect LFN readings
o Coherence
Coherence is interference patterns of noise from a cluster of wind turbines. For example, if you drop two pebbles at different spots in a pond, you would produce a chaotic pattern of ripples wave peaks combining, troughs combining, or even a wave and trough cancelling each other out. The same principles apply to sound waves in air.
o Resonance
Amplification of LFN inside an enclosed space, e.g. a house. The graph below shows simultaneous indoor and outdoor noise recordings at a house near Shirley Wind Farm, USA. Between the frequencies 14 and 28 Hz, the noise indoors is up to 20 dB higher, which is caused by the house resonating and amplifying the noise - like putting a vibrating tuning fork on a piano. See attached graph: Shirley WF LFN resonance.jpg