Hey.

I'm Dianna, and you're watching Physics Girl.

So the other day, I was holding a tube.

And I thought to myself, I can find the resonant frequency of this tube by singing into it and sliding up the scale of frequencies-- like this-- [VOCALIZING] until it resonated... because physics.

But something super weird happened.

I don't understand.

MAN: I was so happy when I saw that you were still doing this.

[LAUGHS] [VOCALIZING] There were some notes I could not sing, where my voice just cracked.

I asked some of my musician friends what they thought was going on, and some of them thought that it was maybe psychological.

So I tested it on them.

You have to hold it against your mouth.

GRANT: Like I'm singing through it.

[VOCALIZING OFF-KEY] [LAUGHTER] [VOCALIZING OFF-KEY] [LAUGHTER] So if you want to be able to sound like a dying elephant-- [NOTE TURNS OFF-KEY] [LAUGHTER] What if you just do it like this without the-- [VOCALIZING SCALES] I don't have any problem.

I'm not faking it.

I think you're faking it.

It's a really weird and specific thing to fake.

That's not what I meant.

Everyone was experiencing the same thing.

A cracking voice, the inability to sing certain notes, and on the same notes.

[VOCALIZING OFF-KEY] Now, that's just weird.

[WOBBLY VOCALIZING] There is something weird going on there.

There's something weird going on!

If you want to try it for yourself, I will include your best dying animal noises in a video.

So I'll leave instructions on how to get me your videos at the end of this video.

So what was I trying to do in the first place?

And why didn't it work?

What is going on?

OK, so you know how sometimes when you blow into a bottle, you can get it to whistle?

[WHISTLING] I'm going to pass out.

[WHISTLING] The note that it whistles is the natural frequency of the space inside of the bottle.

Most things have natural frequencies.

Pipe organs-- the note that each pipe plays is one of its natural frequencies.

Wine glasses-- when you flick a wine glass, you can hear it ring with a specific note.

That's one of its natural frequencies.

Yeah, I know.

Most musicians don't call notes frequencies.

But I'm a physicist.

And a sound wave is basically just air moving back and forth at a specific frequency.

So frequencies it is.

Something notable about natural frequencies is that if you push something at its natural frequency, it'll have a huge response.

It'll resonate.

If you sing at a wine glass at its natural frequency, your sound pressure waves will match the glass's resonant frequency, and it'll vibrate more and more and more and bigger and bigger until it breaks.

[SINGING HIGH PITCHED NOTE] [GLASS BREAKS] You are welcome to check out that video where I sang at a wine glass for two weeks straight.

Here's another example.

If you push a swing at its resonant frequency, it'll go higher and higher.

By the way, natural frequency is what we call it if you just ping it once and let it ring.

And resonant frequency is what we call it if you drive it at that frequency.

They can be the same frequency.

Subtle distinction, kind of pedantic.

Now, I thought if you sing into a tube at its resonant frequency, it should get loud.

It should suddenly resonate.

What I didn't expect was the total inability to sing that note.

So to figure out what was going on, I reached out to a scientist in Australia who studies almost this exact problem-- weird.

It's possible that even the experienced singers don't deal with it.

Even experienced singers would experience something weird here.

What's going on is that your vocal chords or your vocal folds are dealing with the pressure pushing back from the tube.

And that push is very different below the resonant frequency, at resonance, and above resonance.

Professor Wolfe explained using a spring as an analogy.

Here's a mass on a spring.

So here we've got a system which at its natural frequency exerts a large oscillating force here.

DIANNA: At resonance, we get a large force.

But well below that, its force is very small.

The oscillatory force is very small.

Well, above it, it's very small.

DIANA: Below resonance and above resonance, we get a small force.

But at resonance, the force is high.

So that could be one factor.

The force at resonance is really high to oscillate the spring back and forth.

There's no spring, you're pushing with your vocal folds, but there's air inside the tube vibrating back and forth.

And the force to push that air back and forth at the resonant frequency is higher.

But force isn't the only factor.

And also, they have different phase relationships.

This is really interesting too.

The air in the pipe and a spring system will respond completely differently if you move them really fast versus really slow.

So far below resonance, if you're moving the spring really, really slowly, then the motion of the hand moving the spring is in phase with the motion of the mass at the bottom of the spring.

If you move really fast, it will respond much more like a spring according to Hooke's laws, and it'll move out of phase in the opposite direction of the force.

This is what Professor Wolfe meant when he said-- They have different phase relationships.

So below resonance, you're going from the air being in phase with your vocal folds to above resonance-- the air is out of phase with your vocal folds.

And that's a completely different load, we would call it, on your vocal folds.

Your vocal folds are not prepared for that kind of switch over.

That's another one of the factors contributing to why you can't sing this note.

So we've got the force being really big at resonance, and then the fact that you're switching over to a different kind of load on your vocal fold.

So it's not psychological.

But hold on.

There's more.

Your vocal tract is a pipe.

So that means it has a resonant frequency.

Your vocal tract is the whole complex system of your larynx and all the tubing in here.

And since it's a pipe, it has a resonant frequency.

So what happens if you try to sing through the resonant frequency of your vocal tract?

Well, my experience with the PVC pipe would make me think that it's impossible.

But Professor Wolfe and his colleagues did some research on professional singers, particularly sopranos, and what happens when they try to sing through the resonant frequencies of their vocal tract.

Every soprano whom we've ever measured, whether trying to or not, knows to tune the vocal tract in that high range.

This is so weird.

It turns out that soprano singers tune their vocal tract so that the resident frequency of it matches the note that they're singing.

It's like the opposite of avoiding the resonant frequency of the tube.

They tune their tract by changing the shape of their mouth and throats as they're singing, which is why a lot of singers will often open their mouth really wide when they're singing a note really, really high.

Researchers Laura Wade and Professor Wolfe found that soprano singers will tune their vocal tract to match the note they're singing, which probably helps them project without using as much energy because they get that resonance.

Fascinating.

Mystery solved.

Thanks so much for watching, and happy physics-ing.