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I made a video a while back, called a better description of entropy,
because I felt that the bog standard description of entropy wasn't great,
and I thought I could do better.
It was a really fun video to make so I thought it could become a series,
and this is the next one, a better description of resonance.
The bog standard description of resonance goes something like this.
If you ping a wineglass or pluck a guitar string,
then the note that you hear is the frequency that that object likes to vibrate at.
It's that object's favorite frequency, it's the resonant frequency of that object, and that's resonance.
It's okay as a description, but I think I can do better.
The best way I found to describe resonance is using this thing.
It's called a Rubens' tube. It's a hollow aluminium tube.
At one end it's blocked off with a piece of wood, at the other end it's blocked off with rubber membrane,
and if I tap that membrane with my finger
then that will send a pulse of pressure through the tube traveling at the speed of sound.
When the pulse reaches this end, some of it gets reflected
and it travels back through the tube to here,
and then when the pulse gets to here some of it gets reflected and it travels back in this direction.
So when I tap the rubber membrane, a pulse of pressure
bounces backwards and forwards inside the tube getting weaker and weaker and weaker until eventually it fades away to nothing,
But! What if I repeatedly tap the rubber membrane? So...
I tap the rubber, the pulse
passes through the tube, it bounces off here. It comes back to here, [and] just as it reaches this side
I tap the Membrane again, and now the pulse is even bigger than it was before because I've added to it.
So now a larger pulse is traveling through the tube. It bounces back, gets to here. Just when it gets to here,
I tap it a third time, and now the pulse is even bigger again.
And if I keep doing that, if I keep tapping the rubber membrane, at just the right time,
the pulse will get bigger and bigger and bigger.
And that's resonance. Resonance is...
getting the timing just right,
getting the frequency just right,
so that the energy you put into the system adds to the energy that's already there.
That's resonance.
So the frequency of taps that I need to...
That I need to get, that's the resonant frequency of the column of air inside this tube,
but it's not the only resonant frequency of the column of air. There are infinitely many.
So for example, if I tap the rubber membrane and a pulse goes into the tube and then I tap it a second time,
so now there are two pulses inside the tube [and] they cross over like this,
and then, the third tap happens when the first tap gets back.
Or I could tap the rubber membrane twice. So you know one goes in, another one goes in there, maybe,
and then a third one goes in like that, and then the fourth tap doesn't happen until the first one gets back.
But I'm still getting the timing just right, so that the energy I put in adds to the energy that's already there,
It's just that I've waited for a couple of taps to go in first.
So what's that like when you've got pulses coming in from this direction and the reflected pulses coming back,
and they're crossing over each other? It looks like this.
So the waves, coming from both directions, they cross over each other and they add together,
and when you add them together, you get something called a standing wave, where you have nodes and antinodes,
the nodes are the points where there's no movement and the anti nodes are the points where there's lots of movement.
So how fast do I need to tap this membrane to see the effect?
Well much faster than I can tap with my finger. In fact it's an audible frequency,
so instead of using my finger, I'll attach a speaker to the end,
like this.
So if I can drive the speaker, at the resonating frequency of this tube,
then the energy that I put into the system adds to the energy that's already there
and we should hear that as an increase in the amplitude of the sound.
We should hear the volume go up, so let's try that...
Okay, so this isn't one of the resonating frequencies of the tube, if I just bump up the frequency a little bit...
Okay...
so the volume went up.
I found the resonating frequency of this tube,
the speaker is "tapping" the rubber membrane at just the right speed so that the next tap of the speaker is
meeting one of the returning pulses from this end and...
we're adding to the energy that's already in the system. That's resonance. The problem is,
you can't really see what's happening because
it's happening inside the Tube and...
air is invisible.
Which is why the Rubens' tube is designed to be set on fire.
Before I set anything on fire, I have actually switched out the Rubens' Tube,
the first one I showed you was one I built myself, and it was a while ago,
and I didn't know what I was doing, and it's a little bit worse for wear.
so I wasn't comfortable setting that one on fire. This is one that I bought, the only problem with this one is
that the speaker assembly is completely enclosed,
and I really wanted to show you the rubber membrane, and the whole tapping thing.
With this one, I'm comfortable to set it on fire.
So we pump in butane through here,
and then we set light to it.
The first thing you have to do is fill the Tube with butane.
Basically you have to get past fifteen percent butane because that's a dangerous mixture.
It's a stoichiometric ratio, and it's explosive so we just have to make sure
Mm.
we get- I can smell percentages. So.
So hopefully you can see the result of the standing wave in the flames.
So these points are the nodes...
here and here. These are the anti nodes.
So these points, the nodes (where the flames are small)
those are the points where the pressure isn't varying very much at that point. The tall flames,
those are the anti nodes those are the points where the pressure is varying a lot, at that point, and-
let's see if we can find some higher ones.
I swept through a few there, let's see what we've got.
And that is the Rubens' Tube.
So what's all this got to do with the sound that something makes when you hit it, or pluck it, or blow over it?
Well any scenario like that you come across,
needs its own explanation. So let's look at one particular example. Let's look at wind instruments.
So I've got my wind instrument here,
it's a bottle and I've filled it with water and I can blow over the neck like this, *Bottle makes a nice note*
And when I do that, I get a nice note.
So why does that happen and how does it relate to resonance?
So when you make that noise with your mouth, that kind of *Whoosh*
noise, that's white noise,
And you may have heard white noise being described as
a mixture of all frequencies within a range at the same time.
So when you get that kind of hissing, whooshing, sound it's a whole range of frequencies all jumbled up together.
So when you blow over the neck of a bottle, you're
driving the body of air inside at a whole range of frequencies.
But because of what we learned with the Rubens' Tube
It's actually only the resonant frequencies of the body of air inside that get amplified.
So those are the ones that you hear. Interestingly when you hear a wind instrument,
you perceive the lowest resonating frequency,
and all the other frequencies that are being generated when you blow over the neck of a bottle like this
just add to the timbre of the sound, they don't change the perceived pitch of the sound.
So the quality changes, the timbre changes, depending on the balance of the resonating
frequencies that are generated, and
it's this balance that helps us to distinguish one instrument from another. [In] the case of
a guitar string being plucked is a bit more k- it is a guitar actually, I'm just massive.
The case of a guitar string being plucked is a bit more complicated, it involves things like
Fourier analysis, and stuff like that. It's really interesting and I may cover it in a future video but for now
I'm going to leave you with the Rubens' Tube
responding to music. But before that, if you're interested in learning more about wave dynamics and
resonance and sound and that sort of stuff then I can recommend an online course it's called physics and our universe and you can find
it over at thegreatcoursesplus.com
I'm grateful to the great courses plus for sponsoring this video. If you want to see
physics and our universe - the course, for free - then you can sign up for a free trial at
or click on the link in the description. If you use that url, or the link in the description then they
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*Playing At Rest, by Kevin MacLeod*
I hope you enjoyed this video, if you did don't forget to hit subscribe and click on the notification bell, and I'll see you next time.
Check out these other videos that feature resonance, and the original better description video about entropy.