Some wind turbines spend their lives spinning in peace, while others generate more complaints than electricity. So what differentiates the troublesome turbines? As yet, there’s no good answer for that question. And that’s what Steve Ambrose says has him so interested.
Last spring, Ambrose and fellow acoustic engineer Robert Rand conducted a brief study of sounds produced by a privately-owned wind turbine in Falmouth, MA. One neighbor has complained vehemently about the turbine, which is of the same make and model as the embattled town-owned turbines located less than a mile away at the wastewater treatment plant.
Of course, there have been previous sound studies. Two consultants – one hired by the town, and one hired by disgruntled abutters – conducted sound studies of Falmouth’s Wind 1 in fall of 2010. But those studies, as with most sound studies, focused on audible sound (in tech-speak, A-weighted sound or dB(A)). Meanwhile, the debate about whether and how wind turbines cause the health woes of nearby neighbors has largely come to focus on the possible role of infrasound, sound waves that are too low-frequency to be heard by most people.
So Ambrose and Rand set out to measure the low-frequency sounds generated by one of the turbines. They set up shop at the home of Sue Hobart, who has been vocal in her complaints about the privately-owned turbine near her house, and prepared to spend a couple of long, fairly mundane (they’ve done this plenty of times before, mind you) days and nights with their equipment. Instead, they almost immediately began to suffer symptoms similar to motion sickness – headaches, nausea, lethargy, and anxiety – that they say made it difficult to even do the job they came to do.
Much of the report, released in late December, focuses on the personal experiences of Ambrose and Rand, and that is what has generated the most comment from both sides of the debate (see here and here for examples). But, in the end, what I found most interesting – potentially game-changing – were their sound data.
Wind turbine noise has generally been described as broadband – white noise with no particular pitch or tone. But Ambrose and Rand documented two distinct tonal sounds, one at 129Hz which may be audible outdoors, and – the real kicker – one at 22.9Hz. While 20Hz is the official cut-off for infrasound, sounds with frequencies just slightly above that line have to be quite loud to be heard. Ambrose’s and Rand’s data suggest that the 22.9Hz tone would not be audible, but that it could trigger the ear’s infrasound sensors, known as outer hair cells. That was true whether the measurements were taken inside or outside the house.
Although it wasn’t included in this report, Ambrose says he and Rand have documented a similar, though not identical (slightly different wavelength), very low frequency tone produced by large wind turbines in Vinalhaven and Mars Hill, ME – two other communities where there has been an outcry about negative health effects. A detail for the technically inclined: the turbines in Maine are manufactured by GE, whereas the ones in Falmouth are all Vestas turbines.
Actual evidence of tonal low-frequency sound piqued my interest – seemed like a significant departure from the current conversation – so I ran it past Dr. Alec Salt, an otolaryngologist (a.k.a. ear guy) at Washington University who has gotten interested in the whole wind turbine issue.
I think the issue of whether low frequency energy is distributed as noise across many frequencies or whether it is predominantly a tone at a specific frequency (as in the 22.9 Hz component they reported here) doesn’t change the situation very much. The ear transduces and responds to every cycle of a sound, so the way sounds show up in spectra (which are averaged across a time window) is sometimes difficult to reconcile. But I think the ear will be responding to the low frequency tones that they measured here.
When I spoke with Dr. Salt last spring for our series, The Falmouth Experience, he explained that he thought infrasound could be triggering subconscious alert systems in the brain, disrupting sleep and causing chronic stress. So I was also curious to see what he had to say about Ambrose’s theory that infrasound is essentially causing motion sickness.
As you point out, what is interesting is that they reported symptoms appearing very quickly, obviously not involving sleep deprivation or chronic stress. I can’t say that I have no explanations of their symptoms, but I am certainly not confident enough to go “on the record” with a possible explanation. If low level, low frequency tones routinely caused seasickness by some mechanism (such as by stimulating the sacculus) then I would have thought it would have been widely documented by now. We are currently starting a project looking at how high frequency sounds (those you can hear) suppress the ears response to low frequencies. So it may be that in this particular house, if A-weighted sound levels were low, then the ears were responding more strongly to the low frequency tones. But if that is true, you should be able to induce seasickness in subjects in a quiet test booth with low frequency sounds. And that hasn’t been reported to my knowledge, so it probably doesn’t happen. So there is no real way to make sense of this yet. It is one of those observations that I log in my mind and maybe we’ll be able to explain it later on.
So no smoking gun, but a possible suspect. That, in and of itself, is a development.