There have been numerous objections to the erection of wind turbine generators (WTGs) in the UK because of tonal noise originating in the gearbox. The gearbox is essential in most designs of WTG: the turbine itself turns slowly (1-2 revolutions per second) whereas the electrical generator normally revolves much faster (typically1500rpm). The meshing of gear teeth generates the tonal noise.
Not much energy needs to escape from the gearbox to produce audible noise at a distance. A few milliwatts of acoustic noise, escaping from a machine generating hundreds of kilowatts of electrical power, is enough to be audible.
These very low levels of noise power make it difficult to track down and eliminate the pathways by which the noise is escaping. In a typical wind turbine, there are many potential paths. The gearbox can transmit vibration to the turbine blades, which, being large, can act as efficient sounding boards. The same is true of the nacelle which usually sits on top of the tower and contains the gearbox and generator (although it has a smaller surface area and is a less likely radiator). It can also transmit vibration to the tower on which the nacelle sits: this is usually made of steel and may be quite thick, so that it vibrates less, but its very large surface area makes it another potential radiator of sound. Noise from the gearbox can be transmitted to the air in the nacelle, and then out via gaps and ventilation grilles, or the airborne noise can transmit directly through the nacelle skin (if the nacelle is of reasonably light construction, which it usually is).
The complexity of the system is illustrated by a schematic diagram of the acoustic power flow through a WTG, produced for Mecon by Topexpress Limited using the AutoSEATM package:
Mecon uses two different approaches to the analysis of noise in WTGs. The first method is particularly suited to dealing with "rogue" machines which produce significant noise when most machines of the same class do not. Known as Transmission Path Audit (TPA), it analyses noise transmission through an individual machine at a single tonal frequency in great detail.
The second approach is to use Statistical Energy Analysis (SEA) technique, which analyses the statistics of noise transmission over a band of frequencies. It produces results that characterize the typical noise behaviour of a model of WTG, and is more applicable when that model is known to be noisy.
