Comparing Sound From Wind Turbines and Other Sources
“Note that the wind turbine is louder than the air ambulance for frequencies below about 10 Hz.”
6th International Meeting
Wind Turbine Noise
Glasgow 20-23 April 2015
Wind Turbines – A Changed Environment
William K.G. Palmer TRI-LEA-EM 76 Sideroad 33/34 Saugeen, RR5, Paisley, ON N0G 2N0 Canada.
This paper gives examples of the sound from wind turbines in the outdoor environment, and in the indoor environment. These are compared to other sounds occurring in the environment, such as road traffic or overhead aircraft, and to the sounds produced in a typical municipal library and by a typical refrigerator. In summary, the paper shows that wind turbines do alter the acoustic environment, both outside homes and inside homes presenting a greater difference at low frequencies than other sound sources normally met.
Classical problem solving for systems suggests that when a working system experiences a failure, look for changes in its environment. As an example, if an engine that has worked well for some time suddenly experiences distress, look for what has changed. Was the oil change schedule altered? Has a bearing reached end of life? This paper applies a similar approach to look for what has changed when distress occurs in the human system.
When wind turbines are installed in the environment of humans, a common finding based on face-to-face interviews conducted by the author, with many people, is that some report discomfort, at varying degrees, ranging from mild annoyance, to severe adverse health impacts. The healthy human system experiences distress. Interviews reveal when people leave the wind turbine environment, their distress diminishes, but when they return, so does the impact. What change is causing the distress?
Rather than trying to discount the discomfort, this paper looks for changes in the environment wind turbines create based on research into the sound and it’s special characteristics as received where the humans live. The sound from wind turbines is compared to other sounds in the environment to examine the differences based on analyzing recordings of actual sound monitoring. We will look at sound from wind turbines, vehicular traffic, aircraft, wind, and people, to identify differences. We will look too at the way the sound is monitored, to see if that can have an impact.
Finally, through examination of the special characteristics of the sound generated by wind turbines that are different from the sound from other sources, a reason for the discomfort people experience is offered. The links between the changes in the environment wind turbines create and the human condition is explored.
2.1 What do we mean by “the sound” from a wind turbine? Questions of how much sound wind turbines emit, and how that sound compares to other sound sources has been around for a long time. Almost everyone who has been following the information cloud surrounding wind turbines has heard the common expressions:
- The sound from wind turbines at your home is less than from your refrigerator or air conditioner.
- The sound from a wind turbine is comparable to a quiet library.
- The background noise of the wind “masks” the sounds emitted by wind turbines.
- The sound level produced by typical wind farms is so low that it would not be noticeable in most residential areas.
So, what does it mean when we speak of “the sound” from wind turbines? It is often represented by a single value, representing the amplitude in the range our hearing is most sensitive, the A-weighted value. But, should we not also consider the “quality” of the sound and it’s special characteristics? The nature of human hearing is that we respond to a very wide range of sound inputs, and often it is differences in sound, and differences in the characteristics of the sound that gives them a recognizable signature. We can hear a whisper of a companion or a whimper from a restless baby. We can recognize the voice of someone we know in a crowded room. Most of us can whistle a familiar tune that sticks in our head. Our hearing responds better to differences than just to volume, and our mind responds to specific tones, or repetitive patterns more than to a random sound. Sound is far more than “volume.”
2.2. What have we learned already? From previous work, we have learned that:
- At distances of more than 500 metres to a kilometer, the sound from wind turbines are rich in low frequency sound (sound less than 200 Hz) and infrasound (sound less than 20 Hz), while the higher frequencies are attenuated to be comparable to background,
- low frequency sound travels longer distances than high frequency sound
- low frequency easily passes through most building materials, even while higher frequency sounds are attenuated
- WHO states low frequency sound warrants special consideration
- the special characteristics of the sound from wind turbines makes them recognizable even when the volume is low
Further adding to the confusion is the fact that most regulators base sound level limits on A-weighted values, often found by considering only the octaves centred from 63 Hz to 8000 Hz. All this means sound levels at different octaves are adjusted as follows:
- sound from octaves at frequencies below 63 Hz is ignored
- 63 Hz – measured sound reduced by 26.2 dB
- 125 Hz – measured sound reduced by 16.1 dB
- 250 Hz – measured sound reduced by 8.6 dB
- 500 Hz – measured sound reduced by 3.2 dB
- 1000 Hz – measured sound considered as is
- 2000 Hz – measured sound augmented by 1.2 dB
- 4000 Hz – measured sound augmented by 1.0 dB
- 8000 Hz – measured sound reduced by 1.1 dB
- sound from octaves at frequencies above 8000 Hz is ignored
As a result low frequency and infrasound are reduced or ignored by most regulators, as are higher frequencies, on the assumption that those sounds are not considered part of the normal hearing range as used in spoken communication.
2.3 What will we not do? What this document will resist doing is to identify a single value of the sound intensity for any of the sources. Generating a single value by somehow adding together the octave contribution across the spectrum of sound produced by a source neglects the impact of the special characteristics of the sound. If a sound is cyclical (displaying a repeating pattern) or tonal (with a discernable pitch at one or more frequencies) it is more recognizable than a sound that is constant, and evenly distributed across all frequencies. Generally regulators recognize that if a sound has special characteristics of recognizable tonality, a cyclical nature, or impulsiveness (like a hammer blow or a gunshot) then the sound is penalized, yet, some regulators specifically do not consider sound from wind turbines which have a recognizable repetitive “swoosh” which modulates the sound at all frequencies as being cyclical. Yet, it is the cyclical, repetitive nature of sound from wind turbines that seems to make them most recognizable.
To those who hoped this document would produce a simple answer to the question about how the sound from a wind turbine compares to other sources, an apology must be given in advance. Instead, this document proceeds at a somewhat “pedestrian” rate, trying to give the reader a better understanding of why there is no simple answer, and why a response must be conditioned with “it depends.”
3.0 The CasesPlease read on