What you cannot hear CAN affect you.

The American Wind Energy Association state that a scientific panel they sponsored concluded that :

Subaudible, low frequency sound and infrasound from wind turbines do not present a risk to human health.”

The erroneous assumption made by this committee was that effects of low frequency sound can only be mediated through hearing, so that if infrasounds could not be heard then they could not possibly affect human physiology.

Such an assumption is absurd and does not apply to other sensory systems of the body.

We know there are countless things we cannot TASTE that can harm us. Tetrodotoxin (puffer fish poison), salmonella and cholera toxin are some examples of things we can't taste that can sicken or kill us.

We know there are many things that we cannot SMELL that can harm us. The lethal dangers of breathing carbon monoxide or carbon dioxide are well established.

We know there are things that we cannot SEE that can harm us. It is well established that ultraviolet light is invisible, yet it can make someone's life miserable, with skin burns and eye damage if they are exposed to excessive amounts.

So, one wonders what was the scientific basis of the assumption that sounds that could not be heard could have no influence on the body. According to Alves-Pereira and Castelo Branco (Prog Biophys Mol Biol 2007; 93:256) the statement that “What you can't hear, won't hurt you” is attributable to a 2001 newspaper article by a sound engineer (Campanella) related to the “Kokomo Hum”. The point is that this concept was not established by a medical doctor or researcher with specific knowledge of the ear but was introduced by someone with limited knowledge of inner ear physiology. As such, this concept represents an unfounded speculation. Nevertheless it has been repeated often enough to become accepted in some circles as a proven fact.

What the illustration below summarizes is our knowledge of the physiologic pathways in the ear that show sounds CAN affect the brain by pathways that are unrelated and are more sensitive than hearing.

Click for a pdf version of this image

This graphic presents our current understanding of how low frequency sounds affect the brain. While the inner hair cells (IHC), which mediate hearing, are insensitive to infrasound, the outer hair cells (OHC) are stimulated by it. There may also be a number of mechanisms that actively eliminate infrasounds from conscious hearing. It has been shown that nerve fibers (Type II afferents) connect groups of OHC in the cochlea to the granule cells of the cochlear nucleus, so their activity can be represented at the brainstem level. The fact that we cannot hear infrasounds suggests that stimulation of these pathways does not give a conscious percept. Indeed, the low frequency sound at the level of the granule cells feeds back though what has been called a “self cancellation circuit” to the fusiform cells that mediate hearing. This is likely part of a mechanism so these sounds are not heard. Activation of the afferent pathways by the OHC could also give rise to symptoms such as ear fullness, pressure, discomfort and tinnitus. The granule cells of the cochlear nucleus are also involved in “attention state” and “alerting” so it is quite possible that stimulation of this pathway could wake you up and disturb sleep. Note that all this could happen with low frequency sounds that you cannot even hear.

It is also known that OHC provide mechanical amplification of the high frequency sounds that you normally hear. The presence of low frequency sound can alter the amplification of high frequency sounds which causes amplitude modulation and harmonic distortion of the high frequency sounds. This is probably the cause of the amplitude modulation that some people report to be very disturbing. Note that this type of amplitude modulation is completely biological in origins and cannot be demonstrated with sound level meter measurements.

As physiologic pathways and mechanisms exist between the OHC and the brain, the possibility of physiologic disturbances by inaudible sounds cannot be dismissed.

It should also be stressed that the cochlea (the hearing organ) is not the only pathway by which infrasound could affect the body. Although no-one has ever reported measurements of vestibular (e.g. saccular) receptor responses to acoustic infrasound it remains possible that such responses could occur. It is known that vestibular hair cells are maximally “tuned” to infrasonic frequencies, so the possibility that they could respond cannot yet be excluded until such measurements are actually made. Similarly, infrasound-induced alterations of ion transport and cochlear fluid status could give rise to physiologic disturbance following prolonged exposure. Many such non-auditory mechanisms remain to be studied and cannot be dismissed just because no one has looked for such effects.

So, we conclude that in view of the known physiology of the ear, the idea that sounds you cannot hear can have no effect on the body is totally incorrect.