Most of the sound around you goes unheard
Human ears catch only a middle band of all the sound there is. Here is what we miss, which animals hear it, and why the hums and whines from our own gadgets can still get under our skin.
Try this for a second. Sit still and listen to the room you are in. Maybe there is a fan, a car going past, the soft tick of a laptop cooling down. It feels like you are hearing everything. You are not. Sound is just the air shaking back and forth, and it can shake slowly or incredibly fast. A standard neuroscience textbook, Purves and colleagues' Neuroscience, puts the human range at about 20 to 20,000 shakes per second, and plenty of adults fall short of that top number. Below and above our window, the world keeps making noise. Here is what we are missing, who is listening in on it, and why some of the sounds we do catch can grind away at our nerves.
The slice of sound we get
Pitch is measured in hertz, which simply means vibrations per second. A deep drum note sits low on the scale, a whistle sits high. The Neuroscience textbook says humans detect sound from about 20 hertz to 20 kilohertz, or 20,000 hertz. Babies can hear slightly above 20 kilohertz, but we lose high-frequency sensitivity as we grow up, and the upper limit in average adults is often closer to 15 to 17 kilohertz. Anything below 20 hertz gets the name infrasound. Anything above 20 kilohertz is ultrasound. Both are completely real sound waves moving through the same air as your favorite song. They just fall outside the little band that our ears were built to pick up, so we mostly walk around without any idea they are there.
The top of our range also shrinks with time. The US National Institute on Deafness and Other Communication Disorders says age-related hearing loss comes on gradually, affects about one in three Americans aged 65 to 74 and nearly half of those over 75, and creeps in so slowly that people often do not notice. Someone turned this into a business. A device called the Mosquito plays a high, piercing tone to keep teenagers from loitering outside shops, and NPR reported that most adults over 25 cannot hear it. Its inventor, Howard Stapleton, told NPR he got the idea as a kid, when he walked into a factory room full of ultrasonic welding gear and turned right back around. The adults with him all asked the same thing: what noise?
Is there anything special about 432 hertz?
Scroll long enough and you will find videos promising that music tuned to 432 hertz is calmer, more natural or somehow in step with the universe. Here is what that number actually means. Musicians tune to a reference note, the A above middle C, and most of the world sets it at 440 hertz. That standard was agreed at an international meeting in London in 1939 and confirmed by the International Organization for Standardization in 1975. Tuning to 432 just drops every note a hair lower, less than a third of a semitone. Before the standard, orchestras used all sorts of pitches, and Verdi himself once pushed for an A around 432. So 432 is real history. It just is not a secret frequency.
The best known study, a small 2019 pilot in the journal Explore, reported a slightly lower heart rate with 432 tuning, but its authors called it a pilot and asked for bigger studies. So if a 432 track helps you unwind, enjoy it. Just know the calm probably comes from the slow, soft music most of those tracks are, not from eight missing vibrations per second.
Who hears what we miss
Plenty of animals live with a wider window. The veterinary school at Louisiana State University keeps a table of hearing ranges drawn from decades of research, and it is a humbling read. Humans come in at roughly 64 to 23,000 hertz by the table's cutoffs. Dogs reach about 45,000 hertz, cats about 64,000, mice about 91,000, beluga whales about 123,000 and porpoises about 150,000. The page itself warns that the studies used different methods, so the numbers are best treated as rough rather than a league table. Still, the pattern is clear. When your dog suddenly lifts its head at a silent room, it may be reacting to something perfectly audible, just not to you. You are the one with the limited hearing in that relationship.
Bats go further still. The Neuroscience textbook notes that some bat species are sensitive to tones as high as 200 kilohertz, while their lower limit sits around 20 kilohertz, right where young human hearing gives out. It also explains why size matters: small ear structures resonate best at high pitches and large ones at low pitches, the same reason a violin sounds higher than a cello. Elephants work the bottom end. Researcher Katy Payne was watching Asian elephants at a zoo in Portland, Oregon, when she felt a thrumming in the air, and her team went on to publish in 1986 that elephants make infrasonic calls. Cornell's Elephant Listening Project says those deep rumbles carry farther through dense forest, and wild elephants have answered recorded calls from 2 kilometers away.
The rumble under everything
Infrasound is not rare. It is the background of the planet. The Infrasound Laboratory at the University of Hawaii says the ocean continuously radiates infrasound, and that waves colliding at sea produce a steady hum called microbaroms, around 0.2 to 0.5 hertz. That is one wobble every two to five seconds, far too slow for any ear. It is so constant that the lab calls it a significant source of noise in the band used to monitor for nuclear tests. Storms add their own low notes. Researchers at the University of Mississippi's National Center for Physical Acoustics have studied the infrasound that hurricanes send out, in that same band. So while you sit in a quiet room, the ocean and the weather are rumbling below everything you can hear.
Here is the twist that most articles skip. Acoustics researcher Geoff Leventhall, in a 2007 review titled What is infrasound, points out that the popular idea that sound below 20 hertz is inaudible is not correct. If it is loud enough, people can detect it, and hearing thresholds have been measured down to 1.5 hertz. So the 20 hertz line is less of a wall and more of a very steep hill. He lists the sources as natural events, industrial installations and low-speed machinery. He also notes that infrasound has picked up decades of scary headlines, mostly based on media exaggeration, while people ignore how low the actual levels usually are. In other words, it is everywhere, and in everyday life it is usually very quiet.
The hum and whine in your house
Now for the sounds you can hear, and maybe wish you could not. Mains electricity in the US alternates 60 times a second. A physics reference from Georgia State University explains that the iron core inside a transformer or an old fluorescent light ballast physically stretches and shrinks with that magnetic field, an effect called magnetostriction. It changes shape twice per cycle, so what you hear is a 120 hertz hum, which the same source describes as familiar and sometimes annoying. That is the drone from the power brick, the wall wart or the box on the utility pole. It is low, steady and easy to tune out, until the room goes quiet at night and suddenly it seems to be all you can hear.
The high-pitched cousin is coil whine. The electronics maker TDK says laptops, tablets, phones and TVs sometimes make high-pitched sounds while running, because tiny coils and capacitors in their power circuits vibrate. Normally they switch far too fast to hear, at hundreds of thousands of times per second or more. But to save power they often run in bursts, and those bursts can drop the vibration into the audible range. Because a whine sits high, it lands right where adult hearing fades first, which is why a teenager can be driven up the wall by a charger that their parent swears is silent. Professor Tim Leighton at the University of Southampton has measured high-frequency sound from door sensors and speakers in stations and libraries, though he says the research is too thin to confirm or rule out harm.
Can a hum really wear on you?
Short answer: it can definitely annoy you, and that part is well studied. Leventhall's 2004 review in the journal Noise and Health describes low-frequency noise, roughly 10 to 200 hertz, as a special problem for sensitive people in their own homes. Annoyance climbs quickly as the level goes up, and the standard way of measuring noise tends to underestimate how bothersome low hums are. He estimates that about 2.5 percent of people may be at least 12 decibels more sensitive to low sounds than average, and he raises the idea that people can learn to dislike a hum over time, which feeds annoyance and stress. So if a drone bothers you more every week while your roommate hears nothing, that fits the research. You are not imagining the annoyance.
Lab work backs this up. Kerstin Persson Waye and colleagues in Sweden had people do office-style tasks while listening to ventilation noise at a fairly quiet 40 decibels, either heavy in low frequencies or evenly spread. The low hum was rated more annoying and more disruptive, and it interfered with proofreading, with sensitive people hit hardest. In a related study of 32 people, those sensitive to noise showed a smaller than normal drop in the stress hormone cortisol while working in the low hum. That is a real signal, but the authors themselves said longer exposure needs more study. The World Health Organization calls noise one of the top environmental health hazards, though that evidence is about traffic, aircraft and similar sources, not the hum of a charger.
So what do you do with a hum you cannot unhear? Start by finding it. Walk the room, turn things off one at a time, and see what stops. Chargers with nothing attached are a good first suspect, and Lawrence Berkeley National Laboratory already lists empty battery chargers among the things worth unplugging to cut standby power, so you save a little electricity too. If the culprit is something you need, moving it off the nightstand or across the room helps, since sound fades with distance. And if a teenager in the house insists something is whining, believe them, because their ears probably still reach frequencies yours no longer do. Meanwhile, far out at sea, the ocean is still humming along at about 0.2 hertz, well below anything any of us will ever hear.
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