How Noise is Measured:

The noise from wind developments has been one of the most intensively studied impacts. Noise levels can be measured and predicted, but public attitudes to noise from wind turbines depends heavily on personal perception.

Sound is always associated with small scale change in pressure, which provides sensations (i.e. is ‘heard’) at the human ear. Due to the wide range of sound pressures to which the ear responds, sound pressure is an inconvenient quantity to use in graphs and tables so noise is measured on a logarithmic scale in decibels (dB). The decibel is a measure of the sound pressure level, i.e. the magnitude of the pressure variations in the air.

A change in sound level of 1dB cannot be perceived, except under laboratory conditions. Doubling the actual energy of a sound source or doubling the number of identical sound sources corresponds to a 3dB increase. A 3dB change in sound level is considered a barely discernible difference, outside the laboratory.

The noise that a machine such as a wind turbine creates is normally expresses in terms of its sound power level. Although this is described in dB(A), it is not a measurement of the noise level but of the power emitted by the machine, which then creates the sound pressure level which can be heard and measured using a sound meter.

Comparative Noise Levels:

Sources of Wind Turbine Noise:

There are plenty of detailed reviews of the sources and noise generation processes of wind farm noise, but in general, the sources of the noise emitted from operating wind turbines can be divided in to two categories, mechanical and aerodynamic.

Mechanical Noise:

Mechanical noise is transmitted along the structure of the turbine and is radiated from its surface. The hub, rotor and tower can all act as loudspeakers, transmitting the mechanical noise and radiating. It is associated with turning machinery; this noise can be heard at a distinct constant frequency, described as ‘tonal’.

Aerodynamic Noise:

The biggest contributor to the total sound power from a turbine is the aerodynamic noise which is produced by the flow of air over the blades. The proportion of noise from each source is typical of modern wind turbines. A large number of complex flow phenomena occur which can generate aerodynamic noise. There is much ongoing research into these phenomena.

Government statement regarding the findings of the Salford University Report in to Aerodynamic Modulation of Wind Turbine Noise:

In 2006 DTI (now BERR) published a study by Hayes McKenzie which investigated claims that infrasound or low frequency noise emitted by wind turbines generators was causing health effects. The report concluded that there is no evidence of health effects arising from infrasound or low frequency noise generated by wind turbines. The report went on to note that a phenomenon known as Aerodynamic Modulation (AM) was in some isolated circumstances occurring in ways not anticipated by ETSU-R-97.

Having taken the view that more work was required to determine whether or not AM is an issue which may require attention in the context of the assessment and rating advice in ETSU, the Government commissioned Salford University to conduct further work.

The objectives of this study were to:

• To establish the levels and nature of the reported noise complaints received across the UK relating to noise issues from wind farms, both historic and current, and determine whether AM can be predicted.

• To review and understand the level of knowledge/understanding that exists throughout the world on AM, and whether AM can be predicted.

The Salford University study has now been published. The study concluded that although AM cannot be fully predicted, the incidence of AM resulting from wind farms in the UK is low. Out of the 133 wind farms in operation at the time of the study, there were four cases where AM appeared to be a factor. Complaints have subsided for three out of these four sites, in one case as a result of remedial treatment in the form of wind turbine control system. In the remaining case, which is a recent installation, investigations are ongoing.

Based on these findings, Government does not consider there to be a compelling case for further work in to AM and will not carry out any further research at this time; however it will continue to keep the issue under review.

Government continues to support the approach set out in Planning Policy Statement (PPS) 22 – Renewable Energy. This approach is for local planning authorities to ‘ensure that renewable energy developments have been located and designed in such a way to minimise increases in ambient noise levels’, through the use of the 1997 report by ETSU to assess and rate noise from wind energy developments.

Low Frequency Noise:

Low frequency noise, with frequencies in the range of 20-100 Hz, is mostly associated with ‘downwind turbines’, with the rotor on the downwind side of the tower. It is caused when the turbine blade encounters localised flow deficiencies due to the flow around a tower. When a rotating blade encounters this, pulses of low frequency noise are generated. Turbines that have their rotors upstream of the tower, except in very rare circumstances, do not generate such pulses since there is nothing blocking the flow upwind of the rotor. When it does occur, because of the low rational rates of modern turbine blades, the peak acoustic energy radiated by large wind turbines is in the infrasonic range with a peak in the 8-12 Hz range.

Infrasound is generally defined as low frequency noise below the normal range of human hearing. The danger of hearing damage from wind turbine low-frequency emissions is remote to non-existent. Typically, except very near the source, people out of doors cannot detect the presence of low-frequency noise from a wind turbine.

http://www.dti.gov.uk/energy/sources/renewables/publications/pages31267.html

The method of assessing the impact of the wind farm locally is described in ‘The assessment and rating of noise from wind farms’, ETSU-R-97, by the Working Group on Noise from Wind Turbines (Final Report, September 1996). Since its publication, this report has been used to evaluate the noise from wind farms in the UK.

http://www.berr.gov.uk/files/file40570.pdf