What are the noise levels I can expect and what do they mean?

Each turbine will produce different noise levels and different tones and sounds. The main sources emitted are the sound of the blades cutting through the air and the sound of the generator and shaft turning. Noise may be produced inadvertently by a brake pad rubbing or a squeaky yaw bearing. This can be dealt with quickly and easily with correct assembly, erection and routine maintenance. Noise issues experienced with gearboxes have largely been dealt with as the majority of turbines at this scale do not use them.

The noise the blades, rotors or generator of any wind turbine produces is dependent on the speed of the wind. As the wind speed and the revolutions per minute (RPM) of the rotor increase background noise levels also increase. The increase in noise levels may be cancelled out due to the fact that background noise from the wind itself increases with speed also. Vegetation, which rustles in the wind, can help minimise the noticeable noise emissions from a turbine.

According to the planning regulations the level of noise at your premises or the nearest neighbour's premises (whichever is closer) should not be above 43dB(A) - or 5dB(A) above the background noise.

40dB(A) is comparable to the noise levels in a quiet rural area, quiet library or ¼ as loud as ordinary conversation. 50dB(A) can be approximated as equivalent to the noise levels in a quite suburban area, in an average home or with a dishwasher running in the next room. 50dB(A) is roughly ½ as loud as normal conversation. Conversational speech at 1m distance equates to noise levels of 60dB(A).

The specifications a turbine manufacturer supplies should include figures for noise levels to be expected at different speeds at a given distance from the turbine. Ideally these figures should have been certified by an independent or third party test facility.

Noise levels and the concept of dB(A) can be hard to grasp so visiting a supplier's installed turbine is probably the best way to gauge the noise levels to be expected.

The relevant international standard in relation to noise emissions from a turbine is EN61400-11. Evidence of this certification should be sought when choosing a turbine. Only accredited test laboratories' certificates can be given any credence.

Blade tip speeds below 75 m/s are advisable as speeds above 80 m/s cause a noise level which, depending on the site and proximity to dwellings, may be an issue. It is possible to calculate the tip speed by using the RPM of the turbine shaft and the diameter of the blade swept area. These specifications should be readily available from the supplier or manufacturer.

The speed or velocity of the blade tip (VT) is equal to the RPM multiplied by the diameter divided by a constant (19.1):

VT = RPM x D/19.1

Taking an example where the blade diameter is 5.5m and the rated RPM is 200 (i.e. at rated output the turbine will be turning at 200 RPM) we can calculate the blade tip speed at rated output (VTrated) thus:

VTrated = (200) x (5.5/19.1) = 58 m/s

So when this turbine is producing its rated output the blade tips will be moving at 58 m/s.