Acoustical Effects of Windscreens on Infrasound Measurement
May 29, 2017
By Todd Busch, P.Eng., Pinchin Ltd.
The scientific research on the interactions of infrasound with wind screens does not necessarily explore what is occurring at the lower end of the infrasound spectrum.
Two municipalities in Ontario, Kincardine and Grey Highlands, have sponsored measurements of infrasound in the vicinity of future wind-turbine sites.
These measurements were motivated by public concerns over exposure to infrasound. These concerns are at odds with the conventional wisdom on the subject found among people in the scientific realm and engineering professions.
Although the sponsored work demonstrated that infrasound energy is present, both natural and man-made, the final reports did not attempt to associate increased infrasonic energy with increased complaints and adverse health effects. Following is a discussion around how accurate measurements can be made to contribute further knowledge to this unresolved public-health question.
Infrasound is very low-frequency sound, in the range from 0 Hz to 20 Hz, that is outside of the normal audio range of human hearing (i.e., audible sound), which extends from 20 Hz to 20,000 Hz. Such low-frequency sound has been associated with public-health complaints in the vicinity of operating wind turbines. These complaints typically associate the acoustical energy at the blade-pass-frequency of a wind turbine’s rotating blades as they pass the wind-turbine mast as the source of the adverse health effects. Such blade-pass frequencies will vary with the rotational speed of the turbine as they respond to fluctuating wind conditions and are typically within the frequency range from 0 Hz to 1 Hz.
We have reviewed the technical requirements for doing such work and have found that an essential issue for accurate measurement involves the selection of a so-called “wind screen.” A wind screen is typically an open-cell foam sphere that is placed onto a microphone in order to reduce the deleterious effects of wind blowing across the microphone capsule. The wind itself in this case can create electrical signals within the instrumentation that mimic sound waves that are incident onto a microphone.
Conventional spherical wind screens are often either 60 mm or 90 mm in diameter. In the absence of a wind screen, inaccurate measurements of sound can result due to the interaction of moving air with a microphone, since the passing wind creates “phantom” noise data that is not representative of the sound present at the microphone.
A wind screen may be effective at mitigating the generation of phantom noise but also presents itself as a barrier to sound waves reaching the microphone, which is another problem related to measurement accuracy. The wind screen itself creates reductions of the apparent amplitudes of the real sound waves that are present. We have noted that conventional wind screens may be suitable for measurements of audible sound while being inadequate for acquiring reliable infrasound data.
This is due to the progressive worsening of phantom wind-induced noise at low frequencies where errors can be much worse at 1 Hz than, for example, 100 Hz, with the result being an inaccurate measurement.
We have reviewed the alternatives to conventional wind screens and found three custom-fabricated options in the scientific literature on the subject that have documented performance in terms of infrasound measurement:
• Semi-porous shrouds developed by NASA;
• “Double” wind screens comprised of two layers of open-cell foam separated by an air void; and
• Techniques for more-or-less burying the microphone in the ground and covering the hole with a permeable material.
Although these approaches are all suitable for reducing the creation of phantom noise, the known techniques themselves, once again, obstruct the arrival of sound waves at a microphone. Furthermore, the scientific research on the interactions of infrasound with such wind screens does not necessarily explore what is occurring at the lower end of the infrasound spectrum at frequencies of significance to the blade-pass frequency of a wind turbine.
Although there are notable improvements in performance for reducing the generation of so-called phantom noise at a microphone there is still the obstruction that the wind screens and alternative techniques present to sound waves themselves. This implies that the accuracy of the infrasound amplitudes that are present will be compromised.
Furthermore, among the effective approaches for improving the accuracy of infrasonic measurements, including either a wind screen or alternative techniques, there remains the technical problem of whether or not sound at audible frequencies can be measured concurrently.
At this time, the answer to that question would seem to be “no” due to questions of accuracy: although improved infrasound measurement is possible, the implementation of these options for reducing phantom noise do not allow for concurrent measurement of audible sound. As such, separate microphones and instrumentation must be used that implement different methods of installation with different wind screens.