Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of the story. The models are simplified to show the idea.
Tap the buttons and slide the sliders.
Drag across the cat’s back. How much stroking keeps the purr going?
Stroke the cat and watch the purr build and fade. Switch on the sound.
Stroke the cat with your finger. Can you hear it purr?
Slow the twitches right down. Can you see each snap make a burst of sound?
Vary the cycle rate. Below about 8 per second you see single cycles; above it they blur.
Slide it slow, then fast. What happens to the gate?
Drag the chest to breathe. Which sound keeps going on the in-breath?
Compare a meow (out-breath only) with a purr through the whole breathing cycle.
Tap Meow, then Purr. Which one keeps going?
Change the pad size. Heavier folds wobble slower. What happens to the pitch?
A toy model: added mass in the vocal fold lowers the oscillation rate.
Pick a moment. Is the cat always happy when it purrs?
Contentment, nursing, solicitation, pain: the same sound in very different contexts.
Tap each picture. Is the cat always happy?
Turn the dial and switch on the sound. Where does a clicking motor turn into a purr?
Sweep the cycle rate from separate clicks to a fused tone and find the 20–30 Hz band.
Turn the dial. Can you find the purr?
When a cat feels cosy, it rumbles. That rumble is a purr. It comes from the cat's throat!
Deep in a cat's neck is its voice box. Inside are two soft flaps with a little gap between them. Tiny muscles squeeze the gap shut, then let go. Each time it opens, air rushes through and makes a little sound.
The gap opens and shuts super fast, much faster than you can count. All the little sounds join up into one long rumble.
A cat only meows when it breathes out. But a purr keeps going when it breathes out AND when it breathes in. That's why it hardly stops.
Cats mostly purr when they feel safe and happy. Kittens purr while they drink milk, and their mum purrs back. But cats can also purr when they are hurt or scared. It may help them feel calm.
Want more? See why cats sleep so much and how cats see. ← More explainers
A cat purrs with its voice box, the larynx. Tiny muscles in the voice box twitch, squeezing the gap between the two vocal folds shut and letting it spring open again. Each time the gap opens, air rushes through and makes a small burst of sound.12
This happens about 25 times a second, much too fast to hear the bursts one by one, so they blur into a smooth rumble.2 And unlike a meow, a purr keeps going while the cat breathes in and while it breathes out.12
The story isn't finished, though. In 2023 scientists found soft pads inside cats' vocal folds that may let the folds make the low purr sound by themselves, with less help from twitching muscles.37 How the two mechanisms fit together is still being worked out.
Deep in the neck, the larynx is a short tube that air passes through on its way to and from the lungs. Inside it are two soft flaps, the vocal folds (also called vocal cords). The gap between them is called the glottis.1
When a cat purrs, the small muscles inside the larynx work the glottis like a valve:16
What keeps the beat? Experiments going back to the 1970s point to a neural oscillator, a rhythm-making circuit in the cat's brain that sends the "twitch now" signals to the voice-box muscles, a bit like a metronome.14 Cats with a paralysed larynx don't purr, which is strong evidence that the voice box is where the purr is made.1
For a long time people had other ideas. One popular theory said the purr was the sound of blood rushing through a large vein in the chest (the inferior vena cava). Research has since pointed firmly to the larynx instead.16
Try meowing like a cat: you can only do it while you breathe out. Cats are the same. A meow, like almost every animal call, uses the out-breath only.1
The purr is special because it keeps going through the whole breathing cycle, in and out.125 The air changes direction but the glottis keeps opening and closing, so there's a burst of sound either way. That's why a purr sounds continuous, with only a tiny change each time the breath turns around. Recordings of purring cheetahs show the in-breath part is usually a little lower in pitch than the out-breath part.13
Normally, bigger animals have longer vocal folds and make deeper sounds, while small animals squeak. A house cat weighs just a few kilograms, yet it purrs at a very low 20 to 30 vibrations a second (20 to 30 hertz).73
In 2023, a team at the University of Vienna tested this. With their owners' permission, they took the voice boxes of eight cats that had been put to sleep because of illness, and blew warm, moist air through them. All eight made self-sustained vibrations at purring rates of about 25 to 30 hertz, with no brain signals and no muscles working.37
Under the microscope, they found soft pads of connective tissue, up to 4 millimetres across, embedded in the vocal folds.3 Extra weight makes a vibrating band wobble more slowly, like the thick, heavy strings on a bass guitar, so the pads may explain the low pitch.10 The researchers compared purring to vocal fry, the low, creaky voice some singers use.6
This doesn't mean the older muscle-twitch explanation was wrong. The authors say their results don't rule out that twitching muscles help in a living cat; a living cat's purr may well use both.37 The page's "pad" toy is a simplified model of this idea, not a measurement.
| Explanation | What it says | Evidence |
|---|---|---|
| Blood-flow idea (old) | Purr = blood rushing through a big chest vein | Abandoned; the larynx is the source1 |
| Muscle twitch | A brain rhythm makes voice-box muscles open and close the glottis ~25 times a second | Muscle recordings; no purr with a paralysed larynx14 |
| Vocal-fold pads (2023) | Heavy pads let the folds vibrate at purr rates on their own | 8 of 8 voice boxes purred with only airflow3 |
Scientists still don't know every reason cats purr, but they have watched when purring happens.12
| When | What the purr might mean |
|---|---|
| On a familiar lap, being stroked, greeting a friend, dozing somewhere warm | Contentment and "keep in touch": the most common purr2 |
| Nursing: mother and kittens both purr | "I'm here, I'm okay"; a bond between mother and kitten12 |
| Asking for food | A "solicitation purr" with a high cry hidden inside it86 |
| Hurt, frightened at the vet, giving birth, very ill | Probably self-soothing: calming itself down51013 |
Kittens learn to purr when they're only a couple of days old. Vets think the purr tells their mother "I am okay" and "I am here", and it's part of the bond between them.1 Mother cats purr while nursing, and kittens purr while they feed.2 A purr is quiet, which suits close-up conversations like these.10
Being stroked by a familiar person is one of the classic purring situations, along with rolling, rubbing and greeting.2 Purring is generally linked to friendly contact between cats and between cats and people.2 Some cats also use a special purr to get breakfast. In a 2009 study, cats asking for food hid a high-pitched cry inside their purr, and even people who didn't own cats could tell these purrs apart and heard them as more urgent, a bit like a baby's cry.86
Vets often notice injured cats purring, even after serious accidents, and cats can purr during birth and at the end of their lives.26 So a purr doesn't always mean a happy cat. Many experts think purring can be a way for a stressed cat to soothe itself.105 Some situations seem to switch purring off: fights, hunting and being around prey.2
You'll often read that purring heals bones. The idea comes from the fact that purr frequencies overlap with vibration frequencies used in some bone and tissue therapies for people.56 It's an interesting hypothesis, but it's unproven: there's no solid scientific evidence that a purr heals bones, in cats or in people.10 Some studies suggest that spending time with animals can lower stress hormones and blood pressure in people.12
The big roaring cats, lions, tigers, jaguars and leopards, don't make a true continuous purr.129 Their vocal folds are built as a powerful sound generator for roaring instead.11 Some big cats have friendly low sounds of their own: tigers, for example, "chuff", a breathy snort through the nose.9
Smaller wild cats are purring cats: cheetahs, pumas (cougars), lynxes, bobcats and ocelots purr just like a house cat, but can't roar.29 A traditional explanation is the hyoid, a chain of small bones at the base of the tongue: fully bony in purring cats, partly flexible cartilage in roaring cats.9 Scientists still debate how much the hyoid matters, and the snow leopard is a famous in-between case.13
More cat science: why cats sleep so much and how cats see in the dark.
Tiny muscles in the cat's voice box (larynx) twitch, squeezing the gap between the vocal folds shut and letting it spring open again, about 25 times a second. Each time the gap opens, air rushes through and makes a small burst of sound. The bursts are so close together that we hear one continuous rumble. A 2023 study found soft pads inside the vocal folds that help make the purr so low.
Most often when they feel safe and relaxed, for example on a familiar lap or while being stroked. Kittens and their mothers purr to each other while nursing. But cats also purr when they are hurt, stressed at the vet or giving birth, so vets think purring can also be a way for a cat to calm itself.
Probably, at least partly. Purring is started by a signal from the brain, and cats purr in some situations and stop in others: purring tends to switch off during fights, while hunting or around prey. Cats can also make other sounds, such as a cry, while they purr. But nobody can ask a cat whether it chooses to purr, so scientists aren't sure how deliberate it is.
Not really. Lions, tigers, jaguars and leopards are the 'roaring cats': their voice boxes are built for loud roars, and they don't make a true continuous purr. Smaller wild cats such as cheetahs, pumas (cougars), lynxes, bobcats and ocelots purr like a house cat but can't roar.
Not in the same way. A cat's purr keeps going while it breathes in and out, which is very hard for people to do with their voice. The closest human sound is 'vocal fry', the low creaky voice some singers use; the scientists behind the 2023 study compared the purr to it.
That's unproven. Purring happens at low frequencies that overlap with the vibration therapies used on human bones, so some people have suggested it might help cats heal. But there is no solid scientific evidence that a purr heals bones, in cats or in people.
Cats differ a lot: some purr constantly and some hardly ever, and a quiet purr can be hard to hear. A cat that doesn't purr isn't necessarily unhappy. If a cat that used to purr suddenly stops, or its voice changes, it's worth asking a vet, because problems in the voice box can affect purring.