In last month’s newsletter, we discussed how ultrasound can be used in medical applications such imaging and non-invasive procedures. Ultrasound has a long-wavelength sibling in infrasound – sound from 20 Hz and below – which can create as many challenges as ultrasound solves. Read on to learn more about the sources of infrasound and its effects on people.
Like low-frequency audible sound, infrasound has long wavelengths (longer than 57 feet), and this means that it propagates in ways that may be surprising or unintuitive to those outside of the scientific community. Sources of infrasound are (usually) small with respect to the wavelength of the infrasound they produce, which leads to the sound propagating in all directions evenly, or omnidirectionally. Furthermore, long wavelength sound tends to diffract or bend around small obstacles, increasing the difficulty of creating barriers to attenuate it. In lightweight structures, such as wood-framed houses with windows, infrasound can pass right through the façade, disturbing the residents inside. Additionally, infrasound waves propagate more like plane waves than spherical waves, so spreading losses are much less pronounced than for audible and ultrasound; the Mt. Etna eruption in 2016, for example, created infrasound that only decayed by 20% after traveling more than 750 miles! All these factors combine to make infrasound a formidable acoustic foe.
Animations courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State
A well-known source of infrasound is wind turbines. As the blades of a turbine pass through the air, molecules of air are displaced, and the pressure waves propagate as sound. The periodic passage of the blades sets up the conditions for infrasound at the blade pass frequency, usually around 1 Hz; this is well below the range of human hearing, which is between 20 Hz and 20,000 Hz. Other sources of infrasound include volcanic or seismic activity, large animals such as elephants and whales, and large pieces of mechanical equipment. There is growing evidence that data centers, which have extensive building systems to keep computing equipment cool, are significant sources of infrasound. Since these facilities are being built at an ever-increasing rate, the significance of the infrasound they produce may become a concern in the coming years.
Some might wonder that if we can’t hear infrasound, who cares? Well, just because we don’t perceive these waves as audible sound does not mean we don’t perceive them at all. While the exact mechanisms by which infrasound is perceived are not well understood, researchers hypothesize that the vestibular system in our inner ears play a role, as does our auditory system and our general nervous system. The effects of infrasound on our bodies mirror those of high levels of audible sound: annoyance, fatigue, sleep disturbances, and increased cortisol levels, to name a few. Additional effects include nausea and feelings of awe or fear. In fact, some researchers believe infrasound may be responsible for some ghost sightings and other supernatural phenomena.
Infrasound and its sources, effects, and mitigation measures remain areas of active research, and as humans build more devices and facilities that create these massive sound waves, more information will undoubtedly come to light. In the meantime, just chalk up that spooky apparition sighting to some nearby source of infrasound and move on!