Wind Turbine Noise Levels

Blade Design

Early wind turbines suffered from high levels of noise. In the case of large wind farms, this was partially caused by gearboxes rumbling, but modern domestic generators are designed to work without gears

That leaves the swish of air passing the blades as the only remaining source of noise. In a well designed turbine, this should not be audible above the normal sound of wind in the trees from more than 100m away.

Some Chinese models have straight-cut blades – the blade has the same pitch throughout its length. These are the worst offenders, and if you come across a wind turbine that whistles, poor blade design is the most likely cause. Whistling sounds are high pitched and can travel over a long distance. We once put up a Chinese 300W turbine on an experimental basis – until one day I was visiting a neighbour about half a mile away and heard the whistling…

A properly designed blade has a pitch that varies along the length of the blade. This is because the outside of the blade is travelling a lot faster than the area closer to the hub. The wind is blowing at right angle to the direction in which the blade is spinning. Thus the “apparent wind direction” seen by the blade at its edge, is totally different to the apparent wind direction seen by a part of the blade closer to the hub.

We are working with a blade which has been designed using the latest CAD software for optimising blade performance by having the best possible attack angle along the entire length of the blade.

A side-effect of this is that the blade also runs more quietly.

Planning exemptions only apply to wind turbines that have a sound level below 43db(A), when tested at the nearest dwelling. They also must be no more than 5db(A) above background noise.

For some systems, there will be noise during start up. This is because the inverter on many systems does not cut in for the first three minutes, and the turbine over-spins during this time. We are working to minimise this effect by using an inverter with a low cut-in voltage, so this three minute period will happen during low wind speeds, minimising the effect.