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Hey, I'm Diana, 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.
Ah until it resonated because physics
but something super weird happened.
Don't understand.
- I was so happy when I saw that you were still doing it.
(laughs)
- 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 arm.
- Like I'm singing through it.
- Yeah.
(laughs)
- So if you want to be able to sound like a dying elephant.
(laughs)
- What if you just do it like this?
- Without the
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, I thought
- Everyone was experiencing the same thing.
A cracking voice, the inability to sing certain notes
and on the same note.
- Nah it's just weird.
- There is something weird going on.
- There's something weird going on.
If you wanna try it for yourself,
I will include your best dying animal noises
in a future 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?
Okay so you know how sometimes when you blow into a bottle,
you can get it to whistle.
I wanna pass out.
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 then 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.
You are welcome to check out that video
where I sang at a wineglass 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 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 is a mass on a spring.
So here we've got a system which at its natural frequency
exerts a large oscillating force here.
- At resonance, we get a large force.
- But well below that, it's force is very small.
The oscillatory force is very small
while above it, it's very small.
- 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 a 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 it really fast,
it'll respond much more like a spring
according to Hooke's laws.
And it will 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 vocals 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 folds.
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 track in their high range.
- This is so weird.
It turns out that soprano singers tune their vocal tracks
so that the resonant 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 track
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.
So if you wanna try the tube singing for yourself,
check in the description for a link.
It tells you how to send us a video.
And I will pick my best attempts
to feature in an upcoming video.
Thank you so much to all my friends who tried this,
you all are troopers.
And if you wanna know the physics
of how a didgeridoo works or how a flute works
or what the vibrational patterns of a violin look like,
Professor Wolfe has a wealth of resources
about acoustics on his website.
I'll put a link in the description as well.
Thanks so much for watching and happy physics'ing.
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(lighthearted music plays)