To be audible or not to be...
Sound is one of our sensory perceptions and complex patterns of sound waves are labeled noise, music, speech, background, etc. Noise is defined as unwanted sound and any imbalance in a soundscape caused by intruding or disrupting sound of any kind, including ambient background, must be considered as sound pollution. As a community, we are concerned with the impacts of all pollutants and we should not arbitrarliy disregard noise.
However, is it reasonable to claim "I can hear such and such noise, therefore it is a hazard or a nuisance"?
Should new industrial installations in rural settings be inaudible? Or should noise be scientifically measured, and a threshold of hazard/nuisance be enshrined in public law?
It might be argued that inaudibility is not necessary, is difficult to enforce, is far from being precise and might even be entirely unreasonable. If a sound is audible then it can be measured - portable and highly accurate SPLMs (Sound Pressure Level Meters) have been available since the 1970s, so why persist in using subjective criteria?
A and C weightings for sound analysis
The only reason that 'inaudibility' continues to have validity in sound assessment is that the standard 'A weighted' SPLM  will invariably fail to register infra-sound. Amplified musical "bass beat" may be used as an example - the 'A weighted' SPLM display stays steady (at x dB LAeq y mins ) whilst the bass beat can be clearly heard - the A weighted meter totally ignores the green section of the first graph we show here. And so, instead of searching out the reason for this anomaly, the convenient recourse to a subjective assessment - audibility or lack thereof - has gained some popularity.
A, C and G weightings
In recent years, particularly following the publication of G.P. van den Berg's thesis, the use of 'G-weighting' has improved infrasound research; but there still remains (green area in the second image) an indisputable level of infrasound that is not being recorded. Additionally it should be noted that many environmental surveys done for planning/regulatory processes still use A weighting.
Human hearing suffers a significant reduction in sensitivity over the frequency range below about 250 Hz and can reach an extreme where vibrations can be felt before sound is "heard" - returning to musical levels, most humans in loud night clubs report feeling the bass rather than hearing it. Virtual inaudibility as an end result is often necessary and has often been endorsed in Europe. However, it is a subjective criterion and is capable of being misused.
The balanced approach to avoiding such misuse is available: the instrumentation and the accompanying skills have been available for many years but have not been taken seriously. The solution is to use developments of 'C' or more probably of 'G weighted' analysis - and actually measure the noise level in the green areas of the graphs.
If we accept that there is a potential risk to human health, caused by low frequency noise; and if we accept that the science to measure low frequency noise is available; then why refuse to carry out scientific analysis of low frequency noise in areas surrounding certain industrial installations?
If the green areas of the graphs constitute undesirable or harmful noise polution, so be it. The wind turbine industry appears reluctant to carry out such analysis, even in the face of mounting clinical evidence of possible hazards. If the risk to human health is non-existent, then we may disregard the question of whether the noise is inaudible to the human ear.
But if scientific analysis confirms potential health hazards, then we need to be aware of 'inaudibility'. It makes no sense at all to tackle one environmental problem by creating another.
[back] "A-weighted" is a standardised frequency weighting designed to represent the frequency response of the human ear; of the four international standards (A, B, C and D) A is the least sensitive to low frequency sound, C the most sensitive.
[back] Equivalent Continuous Noise Level (LAeq) is averaged over a time period of minutes or hours.
[back] "The sound of high winds: the effect of atmospheric stability on wind turbine sound and microphone noise", G.P. van den Berg, 2006; available as pdf at the University of Groningen website.