A laptop's little speaker can't make deep sounds. Play a cello through it, and the lowest frequency of its low notes never comes out. That lowest frequency is the one that matches the note's pitch. So what should you hear? By rights, the cello ought to sound wrong. Its notes should jump, or the tune should come apart. Neither happens. The sound gets thinner, and you still hear the right notes in the right order.
A deep voice on the phone does the same trick. The phone doesn't carry the lowest frequency of the voice either, and you still hear exactly how high or low the voice is. You do this every day without noticing.
The frequency that matches the pitch never reaches you, and you hear the pitch anyway. Something in the sound that does arrive is enough. That puzzle is where we begin, and the question behind it is a plain one. What is your brain actually hearing when you hear a note?
We'll answer it in two halves. The first half takes a note apart: what your ear does to it, and a quarrel in the 1840s over a missing frequency. The second half puts it back together: how your brain builds a pitch, and why remembering an exact pitch is more common than naming one.
Start with what reaches you. Each eardrum moves to and fro with tiny, fast changes in air pressure. That's all your brain gets from the air. In a busy restaurant, the voices, the music and the plates all arrive at each eardrum as one single wave. Everything you hear has to be pulled out of that wave.
One of the most influential accounts of what's inside that wave came from a man drawn to sound by its mathematics, not its music. Georg Simon Ohm is the Ohm of Ohm's law, the rule for electric circuits he published in 1827. In the summer of 1843 he published a paper on what a musical note is. In it he used a piece of mathematics from the study of how heat spreads. It breaks any repeating wave into a sum of simple waves, each with a single frequency.
Ohm applied it to the sound of a siren. A siren in his day was an instrument for experiments on sound: a spinning disc with holes in it. Air blown through the holes came out as a fast string of puffs, and the faster the puffs came, the higher the pitch. Ohm concluded that a musical note is a mixture of simple waves. He said the pitch belongs to the lowest of them, and that this lowest wave has to be physically there in the sound. Ohm ran no experiment of his own. He worked entirely from another man's published measurements.
In the 1850s and 1860s, Hermann von Helmholtz carried Ohm's idea into the body. He proposed that the inner ear does Ohm's mathematics for real, splitting every sound into its simple waves. His theory gave each pitch its own place in the ear.
Helmholtz was right about that first step: the ear does split the sound. A long membrane runs through your inner ear, and it works like a prism. A prism spreads light into its colors, and the membrane spreads a sound into its frequencies, each to its own place along the membrane. Every point answers best to one frequency. High frequencies peak near the end where sound comes in, and low ones toward the far end. The membrane changes in thickness and stiffness from one end to the other, so each stretch is tuned a little differently. This has been measured directly, on human ears after death.
Tens of thousands of nerve fibers carry the result to the brain. Each one is wired to a short stretch of the membrane, and is deaf to most of the rest. The layout survives the trip, with the order from low to high kept through the brainstem and right up to the hearing cortex. That's the part of the brain's outer layer where sound first arrives. Call this layout the place map. It's the basic way your hearing is organized, and we'll lean on it from here on.
The fibers carry a second clue as well. They tend to fire at the same point in each cycle of the wave, at least for lower frequencies, so the timing of their signals follows the sound. That gives the brain two things to read: where on the membrane a sound lands, and when the fibers fire.
Now send a musical note through it. A violin playing the orchestra's A makes a wave that repeats 440 times a second. That rate is the note's lowest frequency, called its fundamental. The wave also carries energy at 880, at 1,320 and at 1,760. Those are two, three and four times the fundamental, and the series goes on up from there.
Those higher frequencies are the note's harmonics, stacked above the fundamental at whole-number multiples of it. Their mix is part of what makes a trumpet sound like a trumpet on the same A. On the place map, the violin's A arrives as a row of separate peaks, one for each of the lower harmonics. And yet you hear one note, with one pitch, at 440.
Ohm had the explanation ready. The 440 is in the stack, so the pitch is the 440. His rule was that the pitch is the lowest simple wave, and that it has to be there. It's a tidy rule. The man whose measurements Ohm had borrowed was not convinced.
That man was August Seebeck, an acoustician who worked with sirens. Four months after Ohm's paper, Seebeck replied. He described a siren with three spinning discs. It gave a clear pitch, and yet the frequency of that pitch didn't seem to be in the sound at all.
Seebeck would not accept a theory that went against his own two ears. He asked how else you could "determine what belongs to a tone, if not with the ear." His own idea of a note was broader than Ohm's. For Seebeck, any pattern of puffs that repeats regularly makes a note, whatever the shape of each puff.
Ohm couldn't answer on those terms. In 1844 he replied that he could not check the matter by listening, because "nature has altogether denied me a musical ear." So the man who had just defined a musical note said he couldn't judge one by ear. Ohm never published on sound again, and Seebeck died in 1849.
The verdict came from Helmholtz in 1856, two years after Ohm's death. He reviewed the whole dispute and found for Ohm. In time, he also put Seebeck's pitch down to distortion in the ear. Famous hearing scientists still held that view well into the twentieth century, and Seebeck's observations were almost forgotten.
They shouldn't have been. By the middle of the twentieth century, it was firmly established that a note keeps its pitch with its fundamental gone. You can predict the result before I give it. Go back to the violin's A, and filter out the 440 entirely. The lowest frequency left is 880, twice as high, so on Ohm's rule the pitch should leap up to match it. It doesn't. Listeners still hear the A at 440.
Look at what's left. The remaining harmonics still add up to a wave that repeats 440 times a second. Only the energy at 440 is gone. That's awkward for a theory where each pitch has its own place, because nothing in the sound now sets the matching spot on the membrane moving. This is called the missing fundamental: you hear a note's pitch even when its fundamental has been taken out of the sound. It works when the fundamental is drowned in noise, too.
That's the cello on the laptop, and the voice on the phone. It also makes sense for hearing to work this way. Suppose a pitch vanished every time noise covered its lowest frequency. In a noisy world, it would be little use. You still see a whole cup when a hand hides half of it. In the same way, you still hear a note when its bottom is missing.
So each side of the old quarrel had half the truth. Ohm and Helmholtz were right that your ear takes a note apart and lays its harmonics out along the place map. Seebeck was right that a pitch needs no simple wave of its own, only a pattern that repeats. That leaves the obvious question. Your ear has split the note into pieces, so what reads the repeating rate back out of them?
Helmholtz had an answer, and it deserves a fair hearing. He said distortion in the ear adds a new frequency at the difference between two others. The arithmetic works. The neighboring harmonics 880 and 1,320 differ by exactly 440. So maybe the ear quietly puts the 440 back.
A study aimed at a different theory answers it, I think. It came from Adrianus Houtsma, a pitch researcher at MIT. He played trained musicians two-note melodies, and asked them to name the interval, the musical step from the first note to the second. Each note was made of just two neighboring harmonics. In the notes made of higher pairs, there was nothing at the fundamental. The pitch they heard was the missing fundamental's. The musicians could do it. Then Houtsma split each note between the ears, sending one harmonic to the left ear and the other to the right. Now each ear got a single frequency, and nothing else.
Think about what that does to Helmholtz's answer. Distortion in the ear is real. It's made inside the inner ear when two frequencies reach it together, and here no ear got two. So Helmholtz's account needs the pitch to vanish. Instead, the musicians did about as well as when both harmonics went to both ears. Houtsma published the result in 1972 with a co-author, J. L. Goldstein. Houtsma later wrote that the findings were taken as strong evidence that the pitch of a missing fundamental must come from "some central, neural process." In other words, the brain. Neither ear had the pitch to give.
The same experiment showed which harmonics matter. The musicians did best when the two harmonics were the lowest ones, and worse and worse as the harmonics went higher. Many studies agree that the low harmonics set the pitch. They're also the ones the ear keeps apart. The low harmonics sit far enough apart that each one lands on its own spot. Higher up, the harmonics crowd so close that several share a spot and blur together. So the brain can get a clean reading of each low one.
This, then, is what your brain does with a note. It takes the separate low harmonics and works out the one repetition rate they all fit. The violin's A has low harmonics at 880, 1,320 and 1,760. Every one of them is a whole-number multiple of 440.
The wave they make repeats 440 times a second, and that's the pitch you hear, whether or not the sound has any energy at 440. How the brain reads those harmonics is still argued. It may go by their places on the map, by the timing of the nerve signals, or by both.
A bell shows the brain doing this with a real instrument. A tuned bell rings at several frequencies at once, and the note you hear matches a weak one called the Prime. Houtsma's group took a recording of one bell from a church carillon in Gouda, and shifted its frequencies one at a time. Retuning the Prime left the pitch alone. Shifting three strong frequencies above it moved the pitch. Those three sit at two, three and four times the Prime, like harmonics. The brain treats them as harmonics and hears their fundamental, which happens to sit right on the weak Prime.
Where in the brain this happens is disputed. A team recording from a few marmoset monkeys found neurons at the edge of the hearing cortex that answer to a pitch. They fire whether or not the fundamental is in the sound. One such neuron answered best to 182 times a second. It ignored each of the note's harmonics played on its own. It fired when they played together, with nothing at 182.
Babies are a puzzle of their own. Three-month-olds can pick out a changed note in "Twinkle, Twinkle, Little Star" with every fundamental removed. They were tested on a caregiver's lap in a sound booth. At that age the hearing cortex isn't fully developed, and the team that tested them thinks lower parts of the brain may be enough. But a study that recorded babies' brain waves found the cortex responding to the pitch of a missing fundamental only from four months, not at three. The two infant results don't agree.
Your brain builds a pitch from a pattern. A pitch the brain makes might also be one it keeps. Does it keep it, and can it name what it keeps?
In 1994 Daniel Levitin tested the first question with a shelf of CDs. He set out well-known pop and rock albums in front of forty-six students at Stanford. Each student sang two songs they knew well. For each, they held the album, closed their eyes and sang the song from memory. Pop songs suit this test because most exist in a single recording. People have heard them, maybe hundreds of times, always in the same key.
Levitin compared the first notes they sang with the notes on the record. A pure guess would land on the right note one time in twelve. With two songs, guessing would hit at least once about one time in six. Forty percent of the students hit the record's exact pitch on at least one of their two songs. Their misses bunched around the right note, too, and musical training made no difference.
Six labs later repeated the test with 277 people. The memory showed up again, but weaker, and it varied a lot from lab to lab. A quarter of them hit the exact pitch at least once, against Levitin's 40 percent. That's still above what guessing gives.
Now set that beside absolute pitch, what many people call perfect pitch. Give most musicians one note and its name, and they can name the notes around it by how far away they are. What most of them can't do is name a note heard on its own. That is absolute pitch: naming or singing a note with no other note to go by.
An anonymous letter to a newspaper in 1763 describes the seven-year-old Mozart doing it. He listened from another room and named each note as it was played. He could even name the note of a bell, a clock or a pocket watch as it struck.
It's often said that fewer than one person in ten thousand has absolute pitch. A team of researchers traced that number to a 1955 estimate. Its author had found 123 musicians with some absolute pitch around Chicago, and assumed there must be many more. He seems to have set that guessed number against the four million or so people in the region. Absolute pitch is rare in the general public. Nobody has a reliable count of how rare.
Levitin's idea is that absolute pitch is two abilities. One is pitch memory, holding on to the actual pitch of a sound. The other is naming, tying that pitch to a label like C-sharp. His shelf test suggests the memory is the more common of the two. Diana Deutsch studies music perception and has absolute pitch herself, and she sees the same puzzle. There are only twelve note names to learn, so the mystery is why so few people learn them. She compares the rest of us to people who can see colors perfectly well but can't put names to them.
The naming seems to come most easily to people who learn it young. Deutsch tested first-year students at two music schools. One was the Central Conservatory of Music in Beijing, and the other the Eastman School of Music in the United States. Look only at students who had started lessons at four or five. About 60 percent of them in Beijing passed her test, against 14 percent at Eastman.
The Beijing students all spoke Mandarin, a language in which pitch is part of a word. Say "ma" on a high, level pitch, and it means mother. Say it on a pitch that dips and then rises, and it means horse. Deutsch's hypothesis is that children like these are tying pitches to names from infancy. Others credit early piano lessons, and nobody has settled it.
Some of the naming can be learned later. In a 2025 study, a dozen adult musicians practiced naming notes online for about twenty hours. Every one of them learned to name at least a few notes quickly and reliably. Two learned all twelve, and one of those had taken up music only as an adult. It was a small study, with no comparison group.
So, back to the cello on the laptop. Its low notes reach you without their bottom frequency. What does reach you is a row of harmonics. Your inner ear lays them out in order along the place map, and that order is kept all the way to the hearing cortex. The nerve fibers carry their timing, too. From the lowest few, the ones the ear keeps apart, your brain works out how often the whole pattern repeats. That rate is the pitch you hear, and it's still there when the bottom frequency is gone.
The pitch your brain builds is also steady enough to keep. In Levitin's test, many people with no special training kept a song's exact pitch. Naming is a separate step, tying a pitch you hold to a word like A or C-sharp. It seems to come most easily to those who learn it young.
Two things are still open. Nobody knows yet exactly where in the brain the rate is worked out, or whether it's read from places or from timing. And nobody has settled why naming comes so easily to some children. Deutsch links it to speech, to languages where pitch changes what a word means. That raises a larger question, and it's the next lecture's. Does your brain treat music as a thing of its own, or as a kind of language?
If you keep one thing, keep this. The pitch of a note is a conclusion your brain draws from a repeating pattern. The frequency that matches it doesn't even have to be there.