Monday, August 20, 2012

Infrasonic Noise

I have been having conversations with a retired acoustical engineer in upstate New York. He has been working with folks who have been disturbed by nearby wind farm towers producing infrasonic noise.

Recent studies show that infrasonic noise is detected by the inner ear. The main unknown is our understanding of how we react to the infrasonic noise that is detected by the outer hair cells in the cochlea. It is obvious that the inner ear is much more complicated than acoustical practitioners have been taught. The old saying, "if you can't hear it, the noise can't hurt you," just isn't true.

We know that we have been hard-wired to protect ourselves when we receive certain sensory perceptions. For instance, why does one third of the population get sea-sick or experience motion sickness? What is the root cause?

If you go to Wikipedia, you will find: We take all our information from the world through our senses, and many times that comes from multiple inputs. For instance, in the case of sea sickness, the brain is processing the inputs from the following senses:

Both inner ears monitor the directions of motion in three dimensions. Our eyes observe where our body is in relation to its surroundings as well as the direction of motion. Skin pressure receptors, such as those located in the feet and seat, sense in what direction the gravitational pull affects our body, in other words: “What side is up?” Muscle and joint sensory neural receptors report which parts of the body are in motion and in which relative direction.

When feeling motion but not seeing it (for example, in a ship with no windows), the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still. As a result of the discordance, the brain will come to the conclusion that one of them is hallucinating and further conclude that the hallucination is due to poison ingestion. The brain responds by inducing vomiting, to clear the supposed toxin.

In a similar manner, I don't think we understand why we react to excessive low frequency and infrasonic noise the way we do. One thing worth remembering is that in nature, when there is excessive low frequency or infrasonic noise, good things are not happening. When our forefathers and foremothers heard and felt the rumbling from thunder, tornados, and earthquakes, they knew it was time to head back to the cave. We are the survivors of those fast folks who made it back to the cave. Isn't this what happens in evolution?

As a note, most sound meters and smart phone apps can’t register infrasound.

Low frequency sound energy can cause some folks to react negatively. HVAC systems can cause low frequency sound (typically large air handlers and turbulence), leading to complaints, of which solutions are hard to come by. It is best to design systems right to avoid these kinds of problems. So to go back to the conversation with the engineer, if you have a nearby wind tower, you may understand why you or someone in your family may be bothered by its presence.

Authored by: Dan Int-Hout, Chief Engineer Krueger