Amherst Island Wind Info
Noise Frequency Weights
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Amherst Island Wind Info |
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As most of us know, the human ear can at best detect sounds roughly in the 20hz to 20,000hz range. We also know that as we get older the range decreases, especially at the high end. Acoustic engineers have long measured the human response to different freqencies and know that it is not linear, even within the range of what we can detect. Generally speaking, the ear is most responsive to sounds in the our mid-range, roughly 3000-6000hz. The equipment (assuming it's high quality) used to measure sound can do so in a very linear fashion. To make what it measures more indicative of what we hear frequency weights are applied. The two most important are "A" and "C" weights, which are shown below, along with the B and D weights.
The most widely used weighting is the "A" scale, shown in green above. When it is applied to either decibels or sound levels, an "A" is appended, i.e. dBa, or LA. It's purpose is to de-emphasize those frequencies that are not so well detected by the average human ear. As you can see from the curves above, a major part of what it does is decrease low-frequency sounds. In the second chart you can get an idea of what kinds of sounds a turbine actually produces. The major reason the A scale is used seems to be historical. It was originally designed for use at low sound levels, but is now used for more general purposes - including annoyance measurements, where it is not appropriate at all.
As you can see, wind turbines produce a lot of low frequency noise, which is de-emphasized by the A scale. Bill Palmer has written a very readible explanation of these differences. The World Health Organization recognizes this failing, and recommends using the C scale whenever there is a lot of low-frequency sound present. As you can see, the C scale above does not de-emphasize low frequencies nearly as much as the A scale does.