Wind Farm Flicker Expert or mathematician needed

[coldraven]

As for the flicker rate you'd see, that really depends. The rotation of the blades are highly unlikely to ever be synchronised, so you can't simply sum the frequencies. In reality you'd see an asynchronous flicker if you were looking through several swept areas.

35dB is very quiet, btw. I wouldn't worry about that

OK I see what you mean about asynchronous flicker but I am trying to prove that the proposer's figures are incorrect.
If the figures for flicker are wrong then most likely the figures for sound levels are also wrong.

So if all three turbines were rotating at the same speed would the flicker rate be 1.5 x 2 x2 = 6Hz or 1.5 x 2^3 = 12Hz ?
This is what I would really like to know, I cannot figure it out myself and I was hoping someone would.

I have just checked the projected sound contours and my house is almost on the 40db line. With amplitude modulation this could get much louder.

Thanks for continuing to read this post.
Free beer to anyone who comes up with an answer, collection only, but I will give you a tour of the hills

[Paqman]

So if all three turbines were rotating at the same speed would the flicker rate be 1.5 x 2 x2 = 6Hz or 1.5 x 2^3 = 12Hz ?

9 blades at 34rpm would occlude the sun 306 times per minute, so your flicker would be at 5.1Hz. Each turbine on its own would be 1.7Hz at 34rpm. In reality the blades are likely to be turning slower than their maximum for most of the time. I would take the 1.5Hz figure as very reasonable. There might be a spot where you could be exposed to flicker from more than one turbine, but given that flicker vertigo is only going to happen within 10 diameters the footprint of that spot will be small, that where it was possible to see flicker through three turbines would be minute. You'd have to be very unlucky to have your house in that spot, and even then you'd only be exposed for a very brief period of the day for part of the year. What compass bearing is your photo taken on? The position of the sun changes during the year, you may find that any potential window of exposure is only for a small part of the day at a specific time of year. If that's during winter you're much less likely to have direct sunlight at all.

I have just checked the projected sound contours and my house is almost on the 40db line. With amplitude modulation this could get much louder.

40dB is still really quiet. Human speech is about 70dB. This page has a useful table of how loud various sounds are. 40dB you'd have to go outside to hear, you wouldn't hear it at all if the wind was blowing the wrong way.

Remember that sound pressure is a logarithmic scale. That means that 70dB is 32 times more sound pressure than 40dB.

[coldraven]

9 blades at 34rpm would occlude the sun 306 times per minute, so your flicker would be at 5.1Hz.

Thanks Paqman, I think that you are correct. PM me for your free beer.
As for alignment, the view you see is looking West so the Sun will set there around the equinox. The turbines are in a straight line with my house so some of my sunsets will be spoiled.
I'll mark this as solved, thanks to all who replied.

Edit: What happened to "mark thread solved" in the thread tools?
Edit2: OK I found it under "Go Advanced"

[tgalati4]

However 40dB at 6Hz at 3 am will be uncomfortable. Nothing in nature (except whales perhaps) generates that level of sound at that low a frequency. I agree, 40dB is a mouse fart in the normal hearing range, but at 6Hz that is 1/2 of your respiration rate and equal to your bowel movements. So when you feel the rumbling in your abdomen at night you will be thinking: is it me or is it those blasted turbines.

Plus, wind is both smart and lazy. Put too many turbines in one spot and the wind will just go around them. Your property will have more wind as a result of the backpressure created by turbines when you are windward.

A little light reading: http://en.wikipedia.org/wiki/Sonic_weapon

http://en.wikipedia.org/wiki/Infraso..._to_infrasound

[Paqman]

However 40dB at 6Hz at 3 am will be uncomfortable. Nothing in nature (except whales perhaps) generates that level of sound at that low a frequency.

I think a lot of stuff does. Elephants do (and they feel it through their feet IIRC), seismic events do, etc, etc. Besides, just because something "doesn't exist in nature" doesn't mean it's bad for you.

equal to your bowel movements. So when you feel the rumbling in your abdomen at night you will be thinking: is it me or is it those blasted turbines.

Pretty sure that whole thing about the "brown note" is a Cold War tall tale told by submariners, and not actually real.

[Horbo]

Thanks Paqman, I think that you are correct.

He is, but that is only the maximum possible flicker, under perfect conditions (all three perfectly in sync at 34 rpm, with the sun shining directly through the blades of all three). You aren't likely to to experience it much (if at all).

Even if you did experience that (I'd be amazed), 5.1Hz is right down at the very, very bottom edge of the range that is said to be able to trigger seizures, so is unlikely to affect most photo sensitive epileptics. In fact 5.1 Hz is BELOW the trigger range according to some epilepsy societies: http://www.epilepsy.org.au/about-epi...itive-epilepsy
The effect peaks in the range from 15-20Hz.

However 40dB at 6Hz at 3 am will be uncomfortable. Nothing in nature (except whales perhaps) generates that level of sound at that low a frequency.

And human ears only hear down to around 20Hz, so 5.1Hz isn't going to be heard.

I agree, 40dB is a mouse fart in the normal hearing range, but at 6Hz that is 1/2 of your respiration rate and equal to your bowel movements. So when you feel the rumbling in your abdomen at night you will be thinking: is it me or is it those blasted turbines.

6 times per second is half your respiration rate? No, it isn't, not even close.....

Plus, wind is both smart and lazy. Put too many turbines in one spot and the wind will just go around them. Your property will have more wind as a result of the backpressure created by turbines when you are windward.

I promise you the engineers who design and place them are far better versed in this than you are! They are well aware of how to place them to maximise output.

[tgalati4]

I meant to say a multiple of your respiration rate (12-14 cycles per minute). I deal with subsonic frequencies in my work, so I know the effects on body functions. Let's just say if you are hit with a 6 Hz wave, you will feel it and it will make you uncomfortable.

[cindy7543]

Just doing a quick calculation of the frequency. If the blades are rotating at 34 rpm you'll get a frequency of approximately 1.7 Hz. Slightly higher than the 1.5 Hz you were told. However, you shouldn't really have any issues with the turbines.

[Paqman]

I meant to say a multiple of your respiration rate (12-14 cycles per minute). I deal with subsonic frequencies in my work, so I know the effects on body functions. Let's just say if you are hit with a 6 Hz wave, you will feel it and it will make you uncomfortable.

Can you back that up? Link us up to a reputable source. Because everything I've seen about epidemiological studies of the populations around wind turbines (like this) indicates that they only begin to report physiological anomalies after there're told that wind turbines make you ill, and the types of things that are reported (eg: insomnia) are so ridiculously common that it's impossible to ascribe them to any particular cause.

In other words, it's all just hyperchondria. See also "fan death" in Korea.

[Horbo]

Let's just say if you are hit with a 6 Hz wave, you will feel it and it will make you uncomfortable.

Maybe, but not at 40 dB....

In other words, it's all just hyperchondria. See also "fan death" in Korea.

That's a new one for me!

I did have a Chinese girlfriend though who refused to sleep with a fan running if the blades were metal...she was convinced it would do terrible things to her health. Metal blades while awake were (just barely) acceptable, and plastic or wooden blades were fine even while sleeping...!