Exploring Wind Turbine Noise

Wind Turbine Structure:

It is really important to become familiar with the basic structure of a wind turbine to understand the source/s of noises in this machine. A horizontal axis wind turbine is made of the following structures:

1) Blades (usually 2-3 in number) around a rotor

2) Shaft or tower (25-50 meter high)

3) Base unit (containing gearbox and generators)

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The noise people complain about can either from the turbine blades as they cut through the air and/or the base unit. The technology development in the blade design has minimized the noise coming from the blades (both design and positioning of the blade). The noise from the base unit is minimized using insulated material. Despite these attempts, one would definitely hear a swishing sound as the blades rotate as well as the sound from the generator if one were to stand close to the wind turbine.

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Human Hearing and Noise Pollution:

Humans hear noises in the audible range of 20 Hz (cycles per second) to 20 kHz (20,000 Hz) with individual variations. However, sounds around us are present in both in the audible range and inaudible range (infrasonic sound). Sound intensity is measured using decibels (dB) and some sound levels (or intensity) can be uncomfortable for human being. Lets look at the table below to understand the decibel levels of various environments around us:

Source/Activity * Indicative noise level dB (A)

Threshold of hearing 0

Rural night-time background 20-40

Quiet bedroom 35

Wind farm at 350m 35-45

Car at 40mph at 100m 55

Busy general office 60

Truck at 30mph at 100m 65

Pneumatic drill at 7m 95

Jet aircraft at 250m 105

Threshold of pain 140

* Information taken from The Scottish Office, Environment Department, Planning Advice Note, PAN 45, Annes A: Wind Power, A.27. Renewable Energy Technologies, August 1994

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Therefore, when we are just chatting we perceive the noise level to be at 0 dB. We are mostly comfortable between 20-60 dB noise levels and any further increase in dB might annoy us. One might feel a doubling of loudness with an increase of 10 dB. When it comes to environmental projects (construction, power plants, businesses) that involve human being, we measure the noise in dB(A) to account for correction for the sensitivity of the human ear. Business standards indicate that these projects (including wind turbine) noise levels should be within 5 dB (A) of the average existing evening or night-time background noise level.

Wind turbine noises and the debate around it !!!

There are two kinds of noises that people complain about wind turbines. The first one is in the audible range (higher decibel levels) due to close proximity of the wind turbines. However, due to the development in the technology these complaints are beginning to dwindle.

The second kind of wind turbine noise is also high decibels (over 90 dB) but it is in the inaudible range (infrasonic). This is where the debate gets interesting. It seems like a very small population is sensitive to the infrasonic sounds and our health can be affected (causes headaches, nausea, vomiting) while others seem not AT all affected by these infrasonic sounds.

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Apparently, infrasonic sound is causes by the rotors that support the wind blades. There used to be two kinds of rotors: upwind and downwind. Wind turbines in the past used downwind rotors and they did cause significant infrasound. However, most contemporary turbines now use upwind rotors (specially in utility-scale) wind farms that do not produce any infrasound.

Infrasonic sound is created in natural environment as well and it measures around 85d B or lower. Therefore, wind turbines are not the only source of this inaudible sound and research has indicated that humans usually don’t perceive infrasonic sound (either auditory or in-auditory) and it in most cases it doesn’t affect us adversely.

Therefore, one has to look at information such as:

1) What is the age of wind turbine?

2) How long ago was it installed?

3) What kind of rotor technology does it use?

4) Does it cause infrasonic sound (debatable)?

And

5) Does infrasonic sound affects your personally and you may not have known or recognize it earlier?

Therefore, keep these factors in mind when you make decisions about wind turbine noise and if you want to install one in your backyard or live close to a wind farm. Better still, visit one and see how you feel about the aesthetics, look, feel, and identify any discomfort you might have around wind turbines.

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