Let me introduce you to Domino, who agreed to star in this video in which we'll study the science of purring.
We'll be covering sound analysis, babies, astronauts,
guitars, bones, violins, anatomy, survival,
vibrations, osteoporosis...
And, because this is YouTube, cats.
The analysis of the cat's purr begins with the recording of it.
Domino exclusively purrs in the early morning, so that's when I get up.
The way I'm going to do this is I hide my microphone inside of my sleeve
so that Domino doesn't try to catch it.
Big failure. I never managed to get the cat to purr on camera -
but I did off camera. Check it out:
Let's begin the sound analysis.
I selected two breath cycles (inhalation and exhalation *2).
For more precision, we get this graph of the sound wave's intensity as a function of its frequency.
Thus we get interesting frequency peaks.
Short reminder: sounds are nothing more than vibrations. When you play a cord on your guitar,
it vibrates at a certain frequency, causing the air around it to start vibrating;
the vibrations that reach us are the sounds we hear.
To each musical note correspond many different frequencies.
The first 'original' one is called the fundamental frequency.
The fundamental determines the pitch of the note. A G will have a different fundamental to that of an F or an A.
The other frequencies are called harmonics. The cord vibrates to them, too, but they are mere multiples of the fundamental frequency.
Now let's go back to the cat's purr.
The fundamental frequency here is 24Hz, which corresponds to a G.
So Domino purrs in G - a very low one which can't even be played on the piano: the lowest-pitched key gives a 27 Hz frequency.
Compare this to your average man; when he talks (not when he snores), his fundamental frequency is between 85-180 Hz.
An average woman's fundamental frequency is somewhere between 165-255 Hz.
You would think that, if Domino purrs in G, another cat may purr in E and another in A...
But that's not the case! Research has found that all cats purr in G - no matter their age, size, race or weight.
Cats are very resourceful: they can adjust the way they purr to us.
Can you hear a difference between this purr...
and that one?
If you are a maternal or paternal soul, you may hear that the second purr sounds much less pleasant than the first.
When your cat wants something -typically, either food or going outside-, he'll add a special frequency (~380 Hz) to his normal purr.
That frequency corresponds to that of a crying baby!
Yes, they totally manipulate us.
Let's go back to our graph again.
It's hard to see the evolution of the purr without any indication of time, so we're switching to this graph:
It's a bit more complex: we're given frequency,
time,
and intensity as modeled by the colors.
What's cool to see here is that, even in between inhales and exhales, the underlying purr does not stop.
The purr itself is a succession of sharp noises, like an engine's.
This is different from a guitar's sound, which is continuous and uninterrupted.
An engine-like sound. How do cats do it?
The vibration of the purr is done by the diaphragm and the larynx.
The glottis, where you find the vocal cords, voluntarily opens and closes.
The dark streaks of color there correspond to the closing of the glottis.
The brighter streaks correspond to the opening of the glottis.
So why do cats purr in the first place?
We don't know much about how cats activate their purr, but we know when they do.
Of course, cats purr when they're happy, but they also purr when they're hurt,
when they're stressed, or even when they give birth.
In those difficult moments, the fact that the cat spends energy purring must mean that it's linked to their survival instinct.
Around the year 2000, in a completely different context, researchers have put chicken on vibrating plates -
...
- and they found out that frequencies around 20 to 140 Hz were very therapeutic to the chickens.
They're good for bone growth and repair, for muscle growth and repair, and respiratory relief.
Since then, this finding has been applied to (a small group of) humans.
In space, astronauts' bones aren't stimulated by their weight anymore;
their bones thus thin down and their mass decreases of 1% per month.
The NASA are now working on machines that would help keep astronauts' bones intact by getting them to vibrate.
In short, frequencies between 20-140 Hz are beneficial to people, too.
On the purr graph, you can see that the main frequencies (both fundamental and harmonics) fit exactly into that range.
Vets have known for decades that cats need a much shorter recovery time when they're hurt than dogs do.
All of this leads us to the conclusion that cats purr in order to help themselves heal and stay fit.
Of course, this is still a hypothesis because such a thing is very hard to prove,
but it does align with our findings - especially considering that a cat doesn't need
to go running everyday in order to stay healthy, like dogs do.
From astronauts to kitties, researchers are now studying people with osteoporosis -
people who are mostly of the female kind.
And with good reason: a woman's bone mass will be at its peak (i.e. strongest) around her 30th birthday,
which is the equivalent of an 80-year-old man's bone mass.
Researchers are currently working on developing devices that work with vibrations for people with osteoporosis.
Who knows, it may be a good treatment for the future.
In the meantime, you can always get your cat to come and purr on your lap!
Thank you so much for watching this video until the end, I had a lot of fun making it
even though Domino hasn't been very collaborative.
I'd like to thank Serge Barbier in particular, who guided me in the analysis of the cat's purr,
and thank you to Le Café des Sciences for getting me in touch with him.
That's it, the video's yours: subscribe, share, like, don't like, whatever you want.
In any case, I'll see you next time!