13 Sounds Humans Can't Hear
There is a world of sound that you cannot hear. Most of these sounds are made by things in nature, but there are quite a number that are human-made. You can hear some sounds and not others because of how the human body has evolved to interact with sound.
Sounds travel in the form of waves, which make up different kinds of frequencies measured in units called hertz. The human ear can only hear frequencies from between 20 hertz to 20,000 hertz (or 20 kilohertz). Frequencies above the range of human hearing are called ultrasound, and frequencies below that range are called infrasound. Think of infrasound as deeper than the deepest bass-baritone, while ultrasound is higher than the highest mosquito hum.
All around you, noises are being made in infrasound and ultrasound that, while you cannot hear them, other species can. Perhaps the reason why your pet suddenly perks up its ears in an unsettling way is that it is hearing something in a frequency you cannot. Let's look at some of those sounds, both natural and human-made, which enter into this mysterious range of frequencies.
Dog whistles
It is generally well-known that your pet pooch has far better hearing than you do. It has been shown that dogs can hear incredibly soft noises — even whispers of between minus 5 to minus 15 decibels. For perspective, a volume of zero decibels is considered the lowest limit of human hearing. Additionally, dogs can hear frequencies as high as 45,000 hertz, which is about double what a normal human ear can detect.
People have realized that their cute canine companions have this formidable ability and have capitalized on it. For example, The Beatles knew this when they inserted sounds only dogs could hear in the song "A Day in the Life" on the seminal "Sgt. Pepper's Lonely Hearts Club Band."
Others without the same musical aptitude as the Fab Four invented the dog whistle. In fact, it was just one person, Sir Francis Galton, who invented it in 1876. These tiny and usually cylindrical tools seem to make no noise when you blow on them, but to your pet labradoodle, it is a clarion call. Dog whistles emit ultrasound at up to 40,000 hertz, and because it is silent to people, it is a training tool of choice for those working with police, herding, and hunting dogs.
Bat echolocation
Bats have a remarkable ability to use ultrasound to find their food. These flying mammals emit squeaks and clicks up to 160 times per second, which range in frequency from 20 to 200 kilohertz. These sounds rebound off possible prey, mainly insects, and are detected by the bat — an adaptation called echolocation. As the bat gets closer to the prey, it emits a faster series of signals to provide greater resolution. By the echo, bats can determine not just the location but also the size and texture of their prey. Echolocation not only gives the bat the ability to hunt effectively at night, but it is also a tremendous navigation aid.
Bats are not the only animals that use ultrasonic senses. Nocturnal birds have also developed some echolocation, although they are admittedly not as adept as bats. Even some prey creatures have used unheard sounds to their advantage. One study published by PNAS shows that a number of moth species throughout the world have developed anti-bat, sonar-jamming capabilities to avoid becoming the latest midnight snack.
Acoustic weapons
Humans have weaponized sounds we cannot hear. Armin Krishnan reported in "Military Neuroscience" that acoustic weapons may emit audible and inaudible sounds, and have been a source of fascination for military researchers. In his analysis, Krishnan referred to a study from the U.S. Air Force that states infrasonic weapons could cause nausea, vomiting, organ damage, and perhaps even death. He also pointed out, however, that there are claims about these weapons that may be unfounded. More recent research shows that these weapons may simply impact mood.
Nevertheless, these types of weapons are under study and development. For example, one theoretical device is the so-called "Voice of God," which could potentially send messages directly to the brain, causing the victims to think they are hearing some sort of supernatural entity — think of Lex Luthor in "Superman."
Some weapons that are already in use do not exactly play the dulcet tones of James Taylor. For example, in 2019, it was widely reported that China had invented an infrasonic weapon resembling a rifle that could be used to disperse crowds. Another class of acoustic weapons, called long-range acoustic devices (LRADs), have been used by police in the United States to disperse protesters by inflicting pain, nausea, and loss of balance in the targets.
Tomato plants
Plants are usually considered to be fairly quiet organisms. At least, you wouldn't expect your fern to start screaming, "Water me!" Curiously enough, though, tomato plants may just do that. A study published by Cell in 2023 reported how researchers placed tomato and tobacco plants under various levels of stress, such as denying them water or cutting them. They found that those plants which were under more stress emitted sounds.
Does this mean that the tomato plants in your garden are trying to tell you to turn on the sprinkler? Maybe — but the problem is that the plants are inaudible to people, since the sounds are ultrasonic. However, other animals, such as moths and mice, can hear them suffering. What is unknown is whether the animals respond in a way that either helps the plant survive, or simply serves as an ultrasonic dinner bell. While there is clearly lots more research to be done, the authors of the study argued that ultrasounds produced by plants could potentially be used as a means to monitor crops, especially if other species of plants are found to exhibit this behavior.
Whale song
Whale songs are otherworldly, and have undoubtedly enchanted seafarers since they took to sailing. What is remarkable is that these animals can create sounds ranging from ultrasound all the way down through audible sound to infrasound. In a way, when a human listens to a whale, it is like listening to Led Zeppelin with only one headphone plugged in, since you are not getting the whole song. For the whale, meanwhile, the sounds are vital for survival. For example, infrasound travels immense distances underwater, giving whales the ability to call to one another across an ocean.
Whales even use sound to help them obtain food, since the density of water allows for powerful sounds to be generated. For example, it has been discovered that sperm whales use their enormous heads to help propagate tremendous sounds — up to 236 decibels – to locate prey through sonar. In comparison, a jet engine is about as loud as 150 decibels. Toothed whales use ultrasonic clicks to echolocate food, and some scholars have hypothesized they can even use these inaudible sounds to disable prey.
A couple of human singers
One singer sings so low that even he cannot hear it. Guinness World Records officially lists Tim Storms as having the deepest voice in the world. In an interview with Futurism, he recalled that during an examination by an ENT, the doctor said, "[his] vocal cords were almost twice as long as what he's used to seeing." This has made him one of the most niche vocalists in the world.
So how deep is Storms' voice? On March 30, 2012, at the CityWalk studios in Branson, Missouri, Storms was recorded on specialized recording equipment as two music professors and an acoustician watched. The witnesses couldn't listen since Storms dropped his voice to an incredible 0.189 hertz. To give this some perspective, this is 8 octaves under the lowest G note of a piano. Erik Vance in "Suggestible You" reported that the famously deep-voiced Barry White's lowest note descended to a respectable, but still audible, 90 hertz.
If human-produced infrasound is possible, what about ultrasound? Guinness World Records attributes the highest note by a male vocalist to Amirhossein Molaei of Iran, who reached 5,989 hertz. The female ultrasonic record was set on August 18, 2004, by the Brazilian singer and dancer Rossana Monti, who uses the stage name Georgia Brown. Guinness World Records reported that she achieved a G10 note, which is over 25,000 hertz – out of the range of most human ears.
Tiger roars
If you are trekking through the northern Asian wilderness, one sound you do not want to hear is the roar of a Siberian tiger. And if you did hear one, you'd likely stop dead in your tracks — and not just figuratively: A tiger's roar is known for paralyzing its victims. Yet there may be more to tiger sounds than just what humans hear. According to the American Institute of Physics (via Science Daily), studies of tigers have shown that they produce not just the audible growls that we hear, but also infrasound down to 20 hertz.
It is thought that these carnivores use infrasound to maintain their territory since, while a tiger's roar can carry nearly 2.5 miles, infrasound will go much further, passing through mountainous terrain and dense forest. Presumably, this means that tigers who can hear in the infrasound range know that a territory is claimed. Considering a Siberian tiger's territory is over 1,000 square miles, this way of silently warning off rivals is a very handy capability.
Elephant rumbles
Anybody who has watched "Jumanji" should recognize the trumpeting call of an elephant. But there's more to this familiar sound, since a number of sounds made by pachyderms fall in the inaudible infrasonic range of between 1 and 20 hertz. Ann Downer in "Elephant Talk" related that when elephant infrasound was played back at high speed, they were comparable to the noise made by "mooing cows."
These unheard calls seem to be most often used by family groups to communicate. The theory is that since elephants require tremendous amounts of food to survive, they need to break out from their herds to cover a wide territory so that each individual can get nourishment. Elephant infrasound, which can be detected by herd members as far as 115 miles distant, is an excellent means of staying in touch. The discovery of the elephant's use of infrasound in the 1980s also helped to clear up some long-standing puzzling behavior, such as how elephant herds ranged out over the large areas they forage during the day, and regather into the herd by night. The phenomenon is still under study, and scientists are trying to figure out the meaning behind their secret language.
Waterfalls
The roar of a waterfall is one of the most stirring sounds of nature. Indeed, these natural wonders are so powerful that they generate sounds beyond what humans hear. These rumblings descend into the infrasonic range, which is unsurprising considering the amount of power waterfalls create. A case in point is Niagara Falls, which one paper from The Journal of Comparative Physiology A reported emits powerful infrasound ranging from 5 to 20 hertz.
Perhaps the most interesting thing about waterfall infrasound is how nature puts it to use. A waterfall would be a continuous source of unheard noise since the water (usually) never stops flowing. To nonhuman ears, it is like having a skyscraper-sized boom box playing the deepest blast of death metal for eternity. Birds can hear the sounds from hundreds of miles away, and it is argued they use these sounds to help navigate during migrations, sort of like an auditory lighthouse. Homing pigeons, for example, have long been known to detect infrasounds as well as shorebirds. Waterfalls, therefore, are inadvertent sonic navigation aids.
Lightning bolts
Lightning bolts are associated with thunder, but these powerful natural phenomena also produce sounds that are outside the range of human hearing. In one 2018 study published by Geophysical Research Letters, researchers flew balloons to a 20-mile altitude in thunderstorms to measure the sound of lightning. They found that the lightning was generating infrasound in the 0.1-to-20-hertz range.
Thunderstorm infrasound, according to Thomas Farges in "Lightning: Principles, Instruments, and Applications," is also produced by sprites. These are red and orange flashes that occur far above a thunderstorm in the mesosphere, and are referred to by laypeople as upper atmospheric lightning. What is more, one article in Frontier Earth Science reports that these thunderstorms, in general, will produce infrasound. The Journal of Geophysical Research: Space Physics reported that this might be due to the convective forces of the atmosphere rather than how audible thunder is produced, which is by the lightning bolt itself.
The infrasound produced by lightning is still under study, with the biggest question being what causes it in the first place. Perhaps if researchers keep flying balloons into the sky, they'll find out.
Volcanoes
Volcanoes are such a powerful natural phenomenon that they release sound in the infrasonic range when they erupt. The eruption causes vibrations in the ground and pressure waves in the air. Those in the earth are seismic waves, but the pressure waves in the air translate to sound.
In order to keep watch over these potentially dangerous natural hazards, scientists have established monitoring stations to keep tabs on the infrasound emitted by erupting volcanoes. The data, when used in combination with seismic information, can often be a matter of life or death. For example, the Alaska Volcano Observatory (AVO) is responsible for volcano monitoring, and sets infrasound sensors near known active volcanoes in the state. Based on the quality and pattern of the sound, observers can see what kind of eruption is occurring and whether there is an imminent danger.
Information gleaned from a rumbling volcano can provide real-time warnings to people. AVO notes that this is especially important for aircraft that understandably would want to avoid flying into a plume of volcanic ash.
Giraffe noises
To the human ear, giraffes are typically quiet animals. Anne Innis Dagg, a giraffe expert, stated in "Giraffe: Biology, Behavior, and Conversation" that in her many years of experience with these majestically long-necked beasts, they hardly ever made a sound. Some may snort, others may bleat, and then there are coughs and sneezes, but this hardly constitutes a unique animal voice. It is only once in a while that a giraffe may provide a loud sound in objection to ill-treatment by, say, a zookeeper. Yet, on the whole, you can't expect to hear much out of a giraffe.
What is somewhat puzzling is that giraffes do have a well-developed larynx that would support vocalization. It seems that giraffes have difficulty providing the needed airflow to create the necessary vibration to make noise. However, they are quite capable of generating infrasound using their anatomy. This they produce by stretching their long necks in an upward arc until the head is pointing up. Researchers suppose that giraffes use infrasound to keep in touch with one another over long distances. What they might be saying, however, may never be known.
Mouse squeaks
Everybody knows that mice squeak, but what they may not know is that many of their squeaks go unheard by human ears. Research published in Nature shows that the common house mouse can squeak in the ultrasonic range, from 30 to 110 kilohertz. These sounds only seem to occur when there is an interaction between mice; the researchers noted that these sounds occur during fights or playing. It is hypothesized that the noises are also used during mating, with males emitting the ultrasounds to determine sexual partners, and show that they are ready to breed by demonstrating how healthy they are. Females seem to use the signals for similar purposes, but also for socialization with other female mice.
While it is still under study, curiously, our knowledge of inaudible mouse sounds may benefit humanity. Another article from Nature argued that understanding rodent vocalizations can help us determine the spread of disease. Ultrasounds also have been experimentally used for the treatment of a variety of medical conditions, which one article in Neuroscience and Biobehavioral Reviews says includes autism.