Infra-sonic Sound – Rumblings Between the Centre and Fringes of Science

As far as physical phenomena go, infrasound has probably been implicated in more mysterious events than just about anything else on the planet. From ghost sightings and sleep deprivation, to a sense of impending doom, infrasound has been offered as an explanation for a myriad of strange occurrences.

In terms of pure physics, there is nothing mysterious about infrasound.  It simply describes soundwaves which occupy a region of pitch below the lower limit of human hearing. For most of us, frequencies below 20Hz (Hertz or cycles per second) are  undetectable to our auditory receptors.

Very low frequencies are termed long waves, as they cycle slowly and can propagate over vast distances. Infrasound is also very resistant to deflection, allowing it to  resonate through a variety of different media with very little loss of volume. The reason distant lighting takes on a deep thunderous rumble is because the lower frequencies are the only pitches which reach us with sufficient magnitude to be heard. If lightning strikes close by however, we hear the sharp crack associated with the higher frequencies, which then decay rapidly.

Other natural events are capable of producing infra-sonic sound, such as geologic disturbances, large waves, storms and avalanches.

To communicate with submarines, the military has utilized the properties of very low frequencies to delve into the deepest depths of the ocean where conventional radio waves do not penetrate.

Infrasound is also used by certain animals to communicate over long distances. Elephants produce vocalizations as low as 10Hz to maintain contact within herds over several kilometres. They also possess a fatty layer in their feet to enhance detection of sound through vibro-tactile means. Even more extra-ordinary is the ability of blue whales to communicate over  hundreds of kilometres with infrasonic calls.

Some argue that humans are capable of detecting infrasound through mechanisms other than hearing. In a kind of cross-stimulation of the senses (kinesthesia), it is claimed our bodies are affected by very low frequencies in ways we do not ordinarily associate with sound. Studies have shown infrasound is capable of eliciting negative feelings, from mild uneasiness and anxiety to fear and revulsion, which is why it has been offered as an explanation for allegedly haunted sites.

Some of the controversy surrounding wind farms has centred on the potential for large fan blades to produce drumming in the infra-sonic range. Complaints of sleep and balance disturbances and mood alterations have been blamed on suspected low frequency output from wind farms. Whether true or not, the blades of today’s wind turbines are nonetheless designed to minimize low frequency resonances.

Even more bizarre, infrasound has been proposed as an explanation for ghost sightings. It is suggested that sounds in the 18Hz region (resonant frequency of the human eye) are capable of producing hallucinations through a distortion of the light sensitive retinal cells of the eye.

Over the years, townships around the world have reported hearing inexplicable humming. The most famous of which occurs in Taos, New Mexico. The “Taos Hum” was investigated by lead scientists in 1997, without any firm conclusions. Heard by approximately two percent of its citizens, it is described as a pervasive and modulating drone occupying a 48 kilometre radius. Is it some unidentified geologic event, or merely tinnitus? Maybe the power of suggestion?  In other towns at least, “The Hum” has been traced back to industrial installations, which were able to be re-tuned, much to the relief of highly-perceptive citizens!