Aside from that proverbial tree that falls in the forest, there are many sounds going on around us that our ears just don't register. It could be argued that these really aren't sounds because they don't correspond to our ear and brain "hearing" the vibrating air molecules. But something usually hears them, right?

Let's start with ultrasonic sounds. These are high frequency sounds just above the upper threshold of human hearing. One surprising new study found that plants emit ultrasonic sounds, and when under stress, even more. Researcher Lilach Hadany at Tel Aviv University in Israel reasoned that because some plants respond to sounds, plants themselves may make sounds. Hadany and her team noted that plants already produce visual, chemical, and tactile cues that other organisms respond to. But sound emanating from plants had not been thoroughly studied.

TAU scientists found that when plants were under-hydrated, damaged, or experiencing other types of stress, they produced ultrasonic popping sounds detectable in both a controlled acoustic box and in an open greenhouse, even from several meters away. The test plants (tomatoes and tobacco) were able to transmit their physiological state to surrounding organisms. Whether or not this is intentional is unknown, but animals, insects, and other organisms that are capable of hearing the sounds could respond.

Many animals have hearing ranges well beyond us lowly humans (we hear up to about 20 KHz, i.e. 20,000 cycles per second). For instance, cows and horses can hear up to 40 KHz, dogs to 46 KHZ, rodents to 100 KHz, and bats, whales and dolphins beyond a whopping 150 KHz.

The animal world uses ultrasonic sounds for communication, guidance, and as a tool, like echolocation. Some bats using echolocation emit a 160 KHz sound to locate food. The greater wax moth, a delicious treat for bats, can hear and communicate at twice that range to about 300 KHz. That helps them stay off the dinner plate.

In the ocean blue, dolphins, porpoises, and toothed whales also use echolocation, emitting calls in the 175 KHz range. Interestingly, they receive and amplify the echoes with their melon, an oval-shaped fatty organ on their heads. Now that's using your melon! Other creatures that use ultrasonics to communicate are the tiny primate Philippine tarsier, adult sloths, a rainforest katydid, a Chinese toad, and the common rat.

We've (sort of) heard the highs of the world around us, now let's go down low. Any sound below 20 Hz, the lower threshold of human hearing, is considered an infrasound. This is the range where sound is felt rather than heard. Machinery, sub-woofers, ocean waves, explosions, atmospheric and geological events, and some animal communications are in the infrasound range. Infrasonic sound waves can penetrate structures, earth, and water and can travel hundreds of miles unimpeded. In other words, they're a big deal.

Some of the animals that communicate this low are elephants, hippos, rhinos, giraffes, alligators, and whales. Sumatran rhinos can produce infrasounds as low as 3 Hz, and a tiger's roar can be felt down to 18 Hz and lower. The humpback whale's song is as low as 3 Hz, baleen whales from 10 - 30 Hz, and elephant calls from 15-35 Hz. Those low frequencies can come in handy for animals communicating great distances – around six miles for elephants and hundreds of miles for whales.

It's theorized that for navigation, some birds use infrasounds created from natural air turbulence around mountain ranges. Many animals may sense impending danger from infrasounds created by natural events like earthquakes, tsunamis, and tornadoes. In fact, scientists are experimenting with infrasound sensors in attempts at early detection of earthquakes and tornadoes.

Now let's really get out there – as in outer space. NASA's Perseverance rover on our neighbor has microphones on board so that we can now listen to the Martian landscape. The atmosphere is very thin on Mars, so most sounds are very weak and lack definition. Celestial bodies are noisy in one way or another, we really don't hear them because some of the sounds are just so low and slooooooow. To hear them, the natural vibrations are pitched up into our audible range, like the singing comet recording made by the European Space Agency in 2014. The original "sound" had a vibration in seconds (~3.3) instead of fractions of a second. And though not technically "sound," stars and planets emit radio frequencies that when slowed down, fall into the audible range, like this recording of Saturn that NASA released from the Cassini probe. It's right out of a 1950s science fiction movie. And the granddaddy of all space sounds is the Big Bang itself, which is still echoing as cosmic microwave background radiation. Slowing this down into the audible range creates a sound PhysicsWorld describes as resembling a Kraftwork track.

So, do ultrasonic or infrasonic sounds have an effect on the human body? Ultrasound imaging technology has been used for some time in medical diagnostic procedures with no harmful side effects. Ultrasound has many other medical uses, such as for healing injuries or breaking up clots. Repeated or prolonged exposure may have detrimental side effects, so these procedures are in the hands of professionals. Some studies on the effects of ultrasound exposure on human hearing indicate no harmful side effects after ten or twenty minutes.

What about infrasounds? This is where things get really interesting. One study found the ultra low frequencies can upset our balance and create sea sickness. A couple of studies with a focus on wind turbines found that populations of people may experience irritation, fatigue, insomnia, fullness, and tinnitus. And workers that spend time near large turbine arrays have measured negative changes in EEG readings.

Being near large subwoofer arrays at a very loud concert can cause lung collapse, and a London student died of sudden arrhythmic death syndrome after complaining that loud bass notes were "getting to his heart." These are extreme cases, but studies have shown at least some discomfort, raised blood pressure, and feelings of revulsion and fear in individuals exposed to infrasound tones.

Stepping out of this world and into the paranormal world, some scientists believe that infrasonic sound vibrations may be behind some reports of ghost sightings and paranormal experiences. While working in a laboratory that was deemed haunted, a professor at Coventry University in England began experiencing anxiety, followed by a vision of a gray blob in the corner of one eye. While working on a fencing foil the next day, he noticed it vibrating wildly while clamped in a vice. Upon further investigation, he discovered that an air handler for the lab was vibrating at 18.98 Hz. This is very close to the 18 Hz resonant frequency of the eyeball and similar to experiences that NASA discovered while researching the effects of high-powered rockets on humans during launch. It could be that many unexplained paranormal activity experiences can be linked to something that can't be heard, like infrasound.

The more we study our physiology, the more we discover how truly inferior we are to the rest of the animal kingdom. Our vision is limited, our smell unremarkable, we lack feelers or tentacles for enhanced touch , and we hear but a fraction of what all the dogs, cats, squirrels, fish, and bugs around us do. So why do we say that humans rule the earth?

Tags: sound wave, infrasound, Ultrasound, Animals