- [Dianna] The flames were shaken so hard by the sound waves coming out of the speakers that the flames flattened out and became horizontal.
I did not expect that.
I'm having Adam sweep around 19 hertz to see if we can feel it in our eyes at all.
(laughs) But I don't know if the speaker system is gonna be able to do a high enough amplitude to feel it or if there even is a resonant frequency of your eyeballs at 19 hertz.
Hey, I'm Dianna and you're watching Physics Girl.
The other day I was at U Street Music Hall in DC with my friends from SoundField.
You know when that bass drops and you're like, boom, and you're like, oh, I feel it.
- Feel it.
- Charged up.
- Charged up.
- Nahre and L.A. hooked it up to go see these giant speakers at the nightclub.
Physics Girl, nightclub, where is this video going?
(club music) - We were given access to a 140 decibels of potential speaker power.
That's a hundred million times the power of the sound coming out of my mouth.
So to get a sense for how powerful those are, we aimed the speakers at some candles.
(club music) The flames were shaken so hard by the sound waves coming out of the speakers that the flames flattened out and became horizontal.
I did not expect that at all.
The thing is, though, I'm still not sure whether the flames were actually flattening out or whether they were moving forward and backwards so fast that my camera couldn't pick it up.
Let me know what you all think is happening in the comments.
And then as soon as we turned up the speakers, the candles started to blow out one by one.
These speakers were able to put out fire.
We have an entire infrastructure for that.
(honkey-tonk piano music) And we were doing it with speakers.
So with this sound power (sings), I wanted to test whether these speakers could make different body parts resonate.
- So that stage is kind of like a drum.
- The stage was rumbling and I had a bottle of water here.
And it says no drinks here and the bottle of water was jumping around.
- If you were to play those bass tones, you could literally lay on that stage, and you would feel it.
It's like a physical massage.
So, I didn't want to see whether we could resonate the stage and feel that.
That's what Will was describing.
I wanted to see whether we could make your whole body resonate with just the sound waves.
To get my whole body to resonate rather than the stage, we'd have to match its exact natural frequency.
Resonance is a super common phenomenon.
It's the reason why wine glasses can break when you sing at them loud enough (high pitched tone) and with the right note.
It's the reason why the Tacoma Narrows Bridge collapsed in 1940 as high winds caused the bridge to swing and stretch at a frequency that matched its resonant frequency.
So basically, it's a response.
It's something moving more and more and more in response to something driving it at the right frequency.
It's resonant frequency.
Now, standing directly, like right in front of these super powerful speakers turned up to the max with safety considerations in mind, I did not feel anything at the whole body level, even though your body has a resonant frequency, like the bridge does, or the wine glass does.
(glass shattering) The problem is, it's at five to 10 hertz, which is far below the level of human hearing.
I'm gonna flash something right now at the rate of five hertz.
So, seizure warning.
Okay, here we go.
It's not that fast.
It's below human hearing.
In fact, there's a five hertz tone playing under this video right now.
Here's a 15 hertz tone, which is about the lower limit of the subwoofers we were using.
Can you hear that?
Probably not, because the limit of human hearing is around 20 hertz.
Here's 20 hertz.
Can you hear that?
This is kind of a trick, because whether you can hear it also depends on whether your speakers or headphones can even play these notes.
The lower limit of those AirPods that are so popular is around 65 hertz.
Can you hear that?
So, yeah, even the subwoofers couldn't play five hertz.
So, I wasn't able to get my whole body to resonate.
But what about individual body parts?
- [Will] I'm going up by like 2 at a time.
- There was this military study that came out in the 1970s showing that eyeballs have a resonant frequency around 18 to 19 hertz.
They can put out that frequency.
I don't really hear it (laughs) I was hoping my eyeballs were gonna just shake and pop out of my eye sockets but.
- [Will] Well, I'm sorry.
- It was disappointing.
(laughs) (groans) I'm so frustrated.
I'm gonna jump soon to the actually cool demos.
The ones that were successful.
But first I wanna tell you what I learned about why this was unsuccessful.
When audio engineers make speakers, including subwoofer speakers that are meant to play between 15 and 100 hertz, they don't care about the notes that you can't hear as much.
So, I was never gonna be able to get enough power out of these speakers to resonate my eyes at 19 hertz.
YouTubers often cut out the boring stuff, like the stuff that doesn't work, but I just want you to see that experiments don't always work out as planned.
(slow music) Now for the actually interesting stuff.
Here are the googly eyes.
(intense music) We were playing music and sweeping up through the resonant frequencies of the trashcan so that googly eyes sitting on top of the trashcan were feeling the resonance of the trashcan.
(driving music) It turns out the trashcan was the thing in the club that had the best resonance.
Kind of fun to think of a DJ droppin' the bass and trash flying out of the trashcan.
That doesn't happen.
Anyways, then we put oobleck, which is a non-Newtonian fluid that sometimes acts like a solid, sometimes a liquid, which is made of a mixture between flour and water.
(speaker buzzing) And you can see the cool resonance and standing waves patterns on the trashcan.
And then the oobleck had the creepiest reaction once you got strong resonance.
(speaker buzzing) So the experiments weren't a total failure in the end.
Those are some sick speakers.
And the thing is, there is a frequency you can feel on one of your body parts.
And you guys probably know which one it is.
When you feel the bass, you often feel it in your chest.
The subwoofer speakers were strong enough to put out the resonant frequencies of your lung area, which occur at about 25 to 50 hertz.
So when you feel that bass note in your chest, I know we've all experienced this with some strong music, it's not just the power of the speakers, it's actual resonance.
And believe me, this is not gonna be the last video I make about sound and music because I love me some tunes.