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Hey-yo.
It's Dianna, and you're watching Physics Girl.
Let's just get right to it.
Is there like a note, like ooo or aaah?
MIKE BOYD: Yeah, I think a [HIGH PITCH NOTE]
an E. E is the best note.
So try not to touch your lips on the glass.
OK.
MIKE BOYD: Yeah, that's it.
OK-- [HIGH PITCH NOTE] I should have mentioned it was
going to be really annoying.
I was in New York City at the YouTube space in December
with fellow YouTuber Mike Boyd, who shared a video with me
that he'd made about something that I've
wanted to do for years.
You beat me to it, Mike Boyd!
He probably didn't know I had dibs on the topic.
But the topic that I had in mind was
to figure out whether a nonprofessional singer can
break a wine glass with just their voice.
Because I'd seen this done before-- breaking a wine
glass with sound at MIT, where that broke a wine
glass with audio speakers, and then
the Mythbusters did it with a professional singer.
But the raw, untrained human voice?
Well, Mike brought some good news with him, too,
which is that he was able to do it!
And I'll put a link to his learning
video in that description.
But he brought a wine glass to the space for me to try.
MIKE BOYD: You're really looking to feel the vibration.
Yeah, yeah, down here?
MIKE BOYD: Yeah.
OK.
MIKE BOYD: And then just reduce the sweeps until you're like--
feel like you're right on it.
[HIGH PITCHED NOTE]
MIKE BOYD: Yeah, you got it!
You got it!
It's exhausting.
MIKE BOYD: Mike was telling me to try these sweeping noises up
to the right pitch.
[AWKWARD PITCH NOTE] Eh, I don't know.
MIKE BOYD: All right, time three.
[HIGH PITCH NOTE]
[CHEERS]
FEMALE SPEAKER: Yes!
Oh my gosh!
[MUFFLED LAUGH] I think this is one of the few cases in life
where it's easier to learn a trick if you understand
the physics.
Also, I learned some major tips along the way,
including the obvious ones like wear safety goggles
and don't try this at home.
The reason this is so challenging
is that the note that you have to hold
has to be the glass's exact resonant frequency.
We'll get to resonant frequency.
But first, we need to think of the glass not as something
that you drink out of but as an oscillating system.
An oscillating system is a system
that goes back and forth, like a playground swing.
I know that back and forth is a vague definition,
but there are too many different types of oscillating systems
to get more specific than that.
Electrically magnetically oscillating systems,
economically oscillating systems--
like an economy that goes up and down, yeah?
But mechanically, these are all oscillating systems.
So when you flick a wine glass, it vibrates back and forth,
pushing the air, and that's what makes
the ringing note that you hear.
Now back to the playground swing.
The swing is a totally overused analogy
for explaining resonance, but it's just so good.
It's an oscillating system, and it has a resonant frequency.
You can describe resonant frequency two ways.
One is that it's the frequency your swing will keep swinging
at if you just let it go.
In the wine glass, that frequency
is the note it vibrates out if you flick it and let it ring.
By the way, musical notes can always
be described by their frequency or frequencies.
Now another way of describing resonant frequency
is that if you drive your oscillating system
at that frequency, it will have the maximum response
in amplitude, meaning it will have the maximum response
and distance between the back and forth.
So if you push the swing at just the right moment--
or in other words you match its resonant frequency,
the amplitude will increase--
I know you've done this-- and it will increase more than if you
push it at any other frequency.
Now the wine glass analog is that if you
sing at the glass at its residence frequency,
your vocal cords are vibrating the air molecules that
vibrate the glass and drive it at its resonant frequency.
And then just like the swing will go higher and higher,
the glass will vibrate or shake harder and harder.
And if you shake something hard enough,
it will eventually break.
So that's why this is so tricky.
You have to sing at the exact right frequency
and hold it really loudly.
I can't break this particular glass no matter how hard I try,
because I can't hold that note loud enough.
So it helps to know physics and also
to be able to match the right pitch.
I should probably disclose that I did some musical theater
and acappella in my day-- but hey, it helped with this!
[HUMMING SOUND]
Goal accomplished!
Except-- this story is not finished.
There was a slight difference between the glass that I broke
and the one that Mike broke in his video.
Mine had a tiny scratch in the top to make it easier to break.
When MIT did this demo for our lecture,
they told us you could do the same thing.
You put a tiny little scratch in the top
to give the crack some place to start.
But when Mike broke his glass, he
used a new, perfect, unblemished glass.
And I wanted to see if I'd be capable of doing that, too.
So while I was home for the holidays-- can
I try to break your wine glasses with my voice?
Ready?
[HIGH PITCH NOTES] I can feel it.
MALE SPEAKER: That was loud.
FEMALE SPEAKER: I'm afraid if I have a sip of my drink,
my glass is gonna break.
[HIGH PITCH NOTE]
MALE SPEAKER: You wanna break the other one?
Bring the other one.
I'm gonna pass out.
MALE SPEAKER: You know what--
What?
MALE SPEAKER: --we could do?
You could be going [AWKWARD PITCH NOTE] Now,
shoot a pellet gun at it.
MALE SPEAKER: Yeah, BB gun.
Easy.
You won't notice when the glass goes shooting that way.
[HIGH PITCH NOTE]
[HIGH PITCH NOTE] You'll break it.
DIANNA: OK.
DIANNA'S FATHER: So Dianna?
Yes?
DIANNA'S FATHER: I'm trying to print a PDF file.
And every time I go to print it, it puts it small on the page.
Is there any way to increase that?
[HIGH PITCH NOTE] Wow.
You hear that?
[HIGH PITCH NOTE]
MALE SPEAKER: Aw, I thought you were gonna get it!
I thought you were gonna get it!
I thought I had it, too!
MALE SPEAKER: I was like zooming in for dramatic effect right
then, you know?
I was trying to do it with whatever
wine glasses my mom had, which were made of glass.
But what you really want is a glass that-- when you flick it,
it rings, which physically means that it's not damped.
Which is kind of like if you greased the top of your swing
so that it doesn't slow down, and then
you also want one with a thin rim, which means it'll deform
wore.
The glasses that meet these requirements
are typically made out of crystal,
which is totally misleading because they're not actually
crystalline in the way that chemists think of crystals, as
in they don't have a nice, repeating molecular structure.
They're just glass with some lead or something else in them.
I braved the post-holiday shopping for you guys,
and I bought a couple of crystal glasses.
So-- [GASP] These say crystal.
They have a good ring.
[HIGH PITCH NOTES]
So promising and so painful.
[VARYING HIGH PITCH NOTES] [HIGH PITCH NOTE]
[HIGH PITCH NOTE]
[EXCITED SCREAM] Like, part of the glass just leapt off!
I compared my experience later with Mike
about having to sing forever at these glasses,
and he had found the same thing.
For new glasses with no little scratch,
you have to just sing at the glass for a really, really
long time.
This is interesting.
Why?
Why would you need to keep singing at it.
Is it weakening the glass over time?
Well, I asked my engineering friend Kyle.
The issue in question, actually,
is why the glass breaks at all when you sing at it?
Theoretically, the glass should actually
be this incredibly strong material--
like, you should be able to take your wine glass
and like bounce it off a floor and catch it.
Like, that's how strong it should be.
But when we do experiments measuring
the strength of glass, what we measure is about 100 times less
than the natural strength of glass.
What causes that is defects in the glass--
very, very small defects that we call cracks.
And that's what starts the fracture.
Everything starts with these cracks.
So you're singing at the wine glass,
and it's vibrating back and forth like this,
and it's going to put a little bit of tensile stress
on the small crack.
So it's going to stretch it.
And every time it does that, maybe the crack grows
by a little, tiny bit, right?
And you do that again, and again,
and again thousands of times until finally, the crack
gets to a critical crack length, and then boom!
You get brittle fracture-- catastrophic failure--
glass is done.
So that's one possible explanation.
Another idea is actually that it takes you
maybe a long time, 10 minutes, to actually get
the perfect pitch.
So you have to be able to hit this resonant frequency
for long enough.
And when you get that pitch, the resonance
is so great that's the tensile stress
exceeds the critical tensile stress, even
for a small crack, and then it just--
it could be that you're not accurate enough.
Exactly.
I mean, you sound beautiful, but I don't know.
I can't tell.
Like a beautiful, dying bird.
So no matter what, you need some blemish.
You need some kind of crack in the glass.
And putting a little one at the top
makes it so you don't have to go through that process
of weakening and lengthening the crack in the glass over time.
All right, engineering mystery solved.
That's all for me.
Thank you so much for watching, and happy physicsing.
Last thing-- I'd like to think brilliant.org for supporting
PBS Digital Studios.
One thing that I miss about taking physics classes
is actively doing problems.
Brilliant.org is a unique learning site
in that it is problems first, which
is a good thing in this case.
You can try out practice problems about things
that you might already be interested in like,
could you build a sphere around the sun
to harness all of its solar energy?
Well, you can do the calculation for that
and be guided by brilliant.org to understand the concepts
behind this type of problem.
In summary, brilliant.org is a problem-solving website
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