This episode was written and produced by Jack Higgins.

What is sound, on a fundamental level? How did animals evolve to hear? And what happens when you go faster than sound itself? In this episode, we explore the physics of sound with the legendary Bill Nye.

MUSIC FEATURED IN THIS EPISODE

Runaway (Instrumental) by Katy McAllister
Segybabo (Instrumental) by Mello
Neo (Instrumental) by Ryan Teague
TwoPound by Muffuletta
Brek PKL by Limoncello
Crumbtown by Simple Alt Start
Meridian by Soular Order
Thirty Two by Utah
Light in Branches by Sound of Picture
Plastic Glare by Sound of Picture
The Dirty by Sound of Picture

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View Transcript ▶︎

You’re listening to Twenty Thousand Hertz.

[music in]

Over the years, we’ve explored the sonic universe from the depths of the ocean [SFX: underwater] all the way to the outer reaches of the solar system [SFX: blast off].

[SFX: key sound continues]

We’ve talked about sound design [SFX: Netflix], music [SFX: 808], language [SFX: hello montage] and history [SFX: Stradivarius], but one thing we’ve never really answered is what sound is, on a fundamental level.

To make an episode about the physics of sound, we knew we needed someone who could break down this complex subject in a way that’s fun and accessible. And there was one person who I thought would be perfect…

[music out]

Bill: Dallas! Greetings! Good morning!

Someone who inspired me back when I was a kid…

Bill: What happened? We're talking about sound. Yes.

His name is William Sanford Nye, but he’s better known as…

[SFX Clip: Bill Nye intro] “Bill Nye, the Science Guy!”

Bill: All right. Go ahead you guys, sorry. Let's get to work, people!

[music in]

Bill: So for us humans, sound is a pressure wave. Now, everybody has a sense of what a wave is, but stop and think about it. It means energy is moving through air, but the air is hardly moving.

When you snap your fingers [SFX: finger snap], it makes the air molecules by your hand vibrate back and forth, and bump into the molecules next to them [SFX: sound cascading from center to ears]. Eventually, this cascade of vibrating molecules reaches your ear, and vibrates your eardrum, so you hear a sound [SFX: snap].

As sound moves through air, it means these molecules are moving at the speed of sound and they bump into each other and produce these changes in momentum which is manifested, also, as a change in pressure which your ear can sense. It's a tiny, tiny amount of pressure change, and yet we animals are set up to detect it.

[music out]

To reach your ears, sound has to have something to travel through, which is called a medium. It could be air, it could be water [SFX: underwater VO], it could even a solid like metal [SFX: metal clink]. In places where there aren’t any molecules to create pressure, sound can’t travel at all. That’s why sound can’t travel through space. Some people like to point out that the space battles in Star Wars [SFX] should really be silent, [SFX] but that wouldn’t be any fun!

We measure soundwaves by their frequency, which means how many times the waves repeat every second. The scale we use to measure frequency is called Hertz, named after the German scientist Heinrich Hertz.

Bill: And so Hertz was a physicist who did quite a bit of work on waves and the relationship of wavelength to frequency.

If a sound has a frequency of 500 Hertz [SFX], it means that there are 500 waves per second. If we raise it to a thousand waves per second [SFX], or 1,000 Hertz, the pitch gets higher.

The [SFX: frequency sweep] lowest possible range of human hearing is about twenty Hertz. The upper range is about twenty thousand Hertz, which is where our name comes from.

Bill: The show is called 20,000 Hertz, which is 20,000 cycles per second. And that's above the range of most of our hearing, except little kids before their ears get beaten down by years of listening to that rock and roll music.

[SFX: Metal riff]

So, I’ll let you in on a little secret: No one here at Twenty Thousand Hertz can hear 20,000 Hertz. And if you’re an adult, you probably can’t either.

Bill: 20,000 hertz is long gone for me.

[music in]

So kids have better hearing than adults, but some animals can hear way higher than we can. For instance, dogs can hear up to around forty thousand Hertz.8 This is why dog whistles work.

Bill: If you've ever used a dog whistle, they're a metal tube, very small diameter, with a very sharp edge. Whether it's a referee's whistle [SFX: whistle] you whistling [SFX: person whistling] or a dog whistle, the sharp edge causes these little whirlpools or vortices [SFX: whirling wind sound, gradually pitched up and out of range] to shed, to spin off at a regular frequency and that's a frequency that dogs hear, that we can't hear.

Other animals can even hear lower frequencies than us.

Bill: Elephants can hear much lower frequencies than we do. Whales hear a whole range of sounds that we don't hear very well.

Elephants and whales both use these low frequencies to communicate. Elephants can hear each others’ low, rumbling calls when they’re over 6 miles apart, [SFX: low elephants] while some whale songs can travel thousands of miles. [SFX: baleen whale] This is because sound travels so well through water.

But no matter what a sound is traveling though, the lower pitches end up traveling farther than higher ones. You can test this yourself by putting something hollow over your ear, like an empty Pringles can. If you tap your fingers on the end of the can, it’ll sound something like this. [SFX: tapping Pringles can]

Now, if we stuff a bunch of paper towels into the can [SFX: ripping and stuffing paper towel in can], and tap on it again, it’ll sound more like this [SFX: tapping Pringles can (muffled)]

So, you notice how muffled that is? The low frequencies are still there, but most of the high ones are almost gone. This is because different frequencies have different wavelengths.

[music in]

High frequency sounds have short soundwaves. These tiny waves bounce around and lose a lot of energy in whatever they're travelling through. In this case, paper towels. Low frequency sounds have longer sound waves. This means they can travel farther without bumping into as many particles and losing energy.

This is why we use low frequencies anytime we need a sound to travel as far as possible, like a foghorn [SFX: foghorn]. Or like a sweeping tornado siren [SFX: tornado siren].

[music morphs into muffled music]

It’s also why your noisy neighbor’s music mostly sounds like bass.

All of the high frequencies are bouncing around inside their space, but the bass frequencies pass right through the walls and into your ears. [SFX: doorbell] When your neighbor opens the door for pizza delivery, [SFX: door open + music unmuffles] all of those high frequencies can escape.

Now, some materials are really good at absorbing sound, like those foam tiles you see in recording studios. [SFX: small room reverb] They absorb the sonic energy, so there’s no reverb bouncing back at you [SFX: reverb faded from VO].

Other times, the sound waves get reflected back, causing an echo [SFX: echo delay on VO].

[music in]

Bill: So you make a sound and the pressure wave moves through the air, bounces off some distant surface, [SFX: echo delay] a cliff, a valley, the stands on the other side of the field, and then bounces back toward you [SFX: echo delay on Bill].

When an echo happens, you have a single sound bouncing in many directions.

Bill: And so another amazing or really remarkable thing about pressure, or rather waves, is they can pass through each other.

When soundwaves meet, they can have all kinds of interactions.

Bill: If you're just really on it as a physicist, you can make the waves cancel out or add up.

[music out]

Let’s do an experiment. We’ll start by adding these soundwaves together.

In your left ear, you’ll hear a tone from a synthesizer [SFX: synth pad in left ear]. Now, here’s that same tone in your right ear [SFX: synth pad in right ear]. Imagine the soundwaves like squiggly lines that move up and down over and over. [SFX: pads in either ear tremulate] Right now, both lines are perfectly in sync, or “in phase” with each other. They’re moving up and down at the exact same time. [SFX: pads back to normal tone]

Now, if we bring these tones together in the middle [SFX: pan pads to center], they combine into a single sound that’s twice as loud. [SFX] Both tones are still the same volume on their own, but now, their soundwaves are added together. And this makes the sound louder.

But we can also do the opposite, and make a quieter sound by making one soundwave out of phase with the other one.

Bill: So imagine a wave going up, meeting another wave that's going down and they cancel out.

Here are those two tones again, but this time, one of them is the exact inverse of the other one [SFX: sine wave panned left, inverse sine wave panned right] If we go back to thinking of these as squiggly lines, we’ve got one moving up [SFX: tone tremulating in left ear] while the other moves down, and vice versa. [SFX: tone tremulating in right ear, alternating with left ear] Now, when we bring these tones together, they cancel each other out, and become completely silent. [SFX]

You can do this with any sound, even a voice. Here’s what it sounds like when we make the left [SFX: panned left] and the right channel [SFX: panned right] inverted with my voice. Oh, and by the way, if you don’t hear anything for the next 18 seconds, that’s because your system is in mono. Notice how weird this sounds? It almost sounds like I’ve entered the middle of your head. As long as we keep these two signals separated in the left and the right you can hear them. But as we bring them together in the middle [SFX: pan left and right channels to center], the soundwaves start to cancel each other out so I get quieter and quieter until you can’t hear me.

You might think we just lowered the volume on my voice until it was silent, but we didn’t. That volume stayed the same the entire time, but the sound waves canceled each other out.

This is essentially how noise canceling headphones work: They pick up the sound of the outside environment through a little microphone, [SFX: environmental ambience)] and then—super quickly—create a sound that’s the exact inverse of your environment, [SFX: inverse of environmental ambience, both moved to center to fade to silence] so it cancels it out. Amazingly, some vehicles are also designed to do the same thing.

[music in]

Bill: And so these modern airplanes and modern cars have noise canceling systems, where we detect... “we,” engineers who design these crazy things, measure the frequency of sound in the car and then create a sound that's exactly out of phase with it and so you can get the waves to cancel out to a large extent. It's amazing.

Whether you're in the air or on the ground, moving fast generates a lot of noise. But what happens when you go faster than sound itself?

Bill: That is an amazing thing and it wasn't until I worked on a fighter plane that I got a satisfactory answer to one of my old questions.

That’s coming up, after this.

[music out]

MIDROLL

[music in]

On a basic level, sound is vibration. When you hit a drum with a drumstick, [SFX: snare hit] the force makes the drum vibrate. This vibration disturbs the surrounding air molecules [SFX: vibration sound reverberating/cascading outwards from center to ears] so they bump into each other like tiny bumper cars. This chain reaction of vibrating molecules continues all the way into your ear, where it vibrates your ear drum, and makes you hear a sound [SFX: snare hit].

Another way to describe this chain reaction is a pressure wave, since the vibrations cause tiny changes in air pressure that your ear detects and processes as sound.

[music out]

Under normal conditions, the speed of sound is about 767 miles an hour, and it doesn't change based on loudness. So if a kitten meows ten feet to your left, at the same time that a cannon fires ten feet to your right, both sounds will reach your ears at the same time. [SFX: cat meow [left], cannon [right] together]

But the speed of sound is affected by what the sound is traveling through. For instance, sound travels over four times as fast in water as it does in air.

Bill: Sound travels extremely well through water [SFX: underwater sfx and treatment on voice] because it's so much denser than the air. In other words, molecules bump into each other more readily. There's much, much less empty space between them than there is in the air. And then the higher [SFX: wind, snow crunching, thinning Bill’s voice as we go up] you go in the atmosphere, the less air there is between you and outer space and we use the expression the air is thin, but it's molecules farther apart.

Another thing that affects the speed of sound is temperature.

[music in]

Bill: So when we measure a temperature with a thermometer, we're measuring the average kinetic energy of molecules, the average momentum of molecules. And so “kinetic” means “moving”. So if somebody's “kinetic,” he's bouncing around all the time.

Since warmer molecules bounce around faster than cold ones, it means they can transmit sound faster. So the speed of sound in Hawaii is slightly faster than the speed of sound in Iceland.

Bill: And everybody, it's cool to appreciate, or wonderful to appreciate, that pressure and temperature and the speed of sound are intimately related.

Bill: The warmer things are, that's a manifestation of molecules moving faster. [SFX: boiling speed ramp] The denser things are, that means the molecules are more massive [SFX: low heavy churning] and are able to bump into each other more readily because they're closer together.

Bill: And then the speed of sound, which is an expression we throw around all the time, “The speed of sound!” [SFX: fast whoosh] is the average speed of molecules. It’s amazing.

Bill: This business of pressure and temperature are [SFX: kissy sounds] intimately related. You might say they “osculate” if you're a Latin buff. They “kiss.”

[music out]

Most of the time, the speed of sound is as fast as those molecules ever go in nature. But sometimes they get pushed even faster.

[music in]

Bill: So if you push these molecules faster than they're naturally going, they will bump into other molecules [SFX: Impact slow down] and slow down immediately and this is what we call a shock wave. And a shock wave, the kind of wave that's coming off a fighter plane [SFX: fighter plane] or the end of a whip when you [SFX: whip] like that, you break the speed of sound, you push molecules faster than their natural tendency.

The sound generated by a shockwave is called a sonic boom. And while shock waves are incredibly powerful, they’re also tiny.

Bill: The shock wave is barely 100th of a millimeter, 10,000th of an inch thick. It's thinner than a piece of paper.

[music out]

Most sonic booms are caused by human inventions, but not all of them.

[SFX: pistol shrimp snapping ]

What you’re hearing right now is a series of underwater sonic booms made by pistol shrimp snapping their claws together. These creatures are less than an inch long, but the way they hunt is incredible. [SFX fade out] When a pistol shrimp snaps its claws, it shoots a kind of bubble bullet [SFX: underwater shot] that moves towards its prey at up to 62 miles an hour. When the bubble collapses [SFX: bubble collapse, suck-in sound], it creates a sonic boom [SFX: muffled boom into water bubbling] loud enough to stun or even kill its prey, and for a split second, it generates temperatures almost as hot as the sun [SFX: sun/heat sound].

All the heat generated by a shockwave can be a problem if you’re trying to be stealthy.

[music in]

Bill: I'll give you an example not quite from your everyday experience, but from your tax dollars’ experience. The B2 bomber, a very sexy, amazing plane. Don't use it! It's too expensive! Don't ever use it. Okay, but we do have it. Don’t ever use it, man! But it’s really cool.

Bill: Anyway, in order to achieve its remarkable top speed, the exhaust coming out of the engines is generally going right about the speed of sound, and so there's a shock wave produced in the exhaust of the engines. And… it gets very hot.

Bill: And what they did with the B2 bomber, they, we it... your tax dollars; they put the exhaust on top of the wing so that the bad guys have much more difficulty sensing its heat from the ground.

Bill: The heat is radiating into space rather than toward the ground.

Bill: It's cool. Or hot. Your tax dollars at work.

[music out]

So imagine you’re in a jet [SFX: jet interior and VO treated to sound like in fighter jet] flying faster than the speed of sound, and then for some reason you decided to shout out of the window [SFX: unlatch window, windy], in this case how fast is your voice travelling? [SFX: back VO sfx off as wind recedes, transition wind into music]

[music in]

Bill: Someone standing outside would hear it at the speed of sound. In other words, they'd hear it as fast as sound wave could move. But understand there are some practical problems with rolling the window down when you're going the speed of sound, because...

Bill: The pressure from outside would smash you.

When you roll down the window on the freeway [SFX: window roll down], you instantly feel the air blowing back at you. In order to get your voice to carry to the sidewalk, the pressure wave from your voice has to be stronger than the pressure of the air coming through the window. Now imagine going almost 14 times faster in a jet plane. Even if you could roll down a window and survive…

Bill: What pressure is inside your cockpit will have to be as high as the pressure outside, or it'll just blow into you and you won't be able to produce the sound wave. That's a cool physics question. But that's why fighter planes are made of stuff that can resist air. They're not made of paper at those high speeds.

[music out]

But of course, when you’re fifty thousand feet in the air, going fifteen hundred miles an hour, you’re a long way from the places your ears are supposed to be in.8 Our hearing developed as sort of an early warning system. Think about when you stay the night in an unfamiliar place, you’re hyper aware of every creak and noise [SFX]. And it’s not just us humans who are like this.

Bill: If you watch wolves and big cats, they can turn their ears toward the source of the sound [SFX: quickly pan night sounds and some rustling bushes to far right]. This is not rocket surgery. You can see this and so they're able to aim their detectors in a way that you and I don't very much. [SFX: night sounds out] So our ears and our hearing, not only are very important for our communication but they're also a source of warning, of early detection.

[music in: Custom Track]

With all of the distractions we have today, it’s tempting to shut out the sonic world more and more.

Bill: “If you could change something about your body, what would it be?” And many people have said they want ear lids like eyelids, you could close your ears to those kids on your lawn or whatever.

Bill: “Get off my lawn!”

These days, most of us don’t have to worry about a hungry lion sneaking up behind us [SFX: savannah, rustle, growl and breath in ear]. Still, our ears are always listening for important cues from our surroundings, whether it’s a fire alarm from 3 houses away [SFX: distant alarm], or distant thunder [SFX: thunder, footsteps on grass] when you’re walking in an open field.

Bill: If you could shut off the outside world, you might lose your early warning and this would be very problematic.

[music out]

So we shouldn’t be too quick to close off one of the most fundamental ways of understanding the world.

[music in]

Bill: It's really amazing that evolution, 3.7 billion years of life on Earth… produces these elegant systems that enable us to hear these tiny pressures.

Bill: The amount of pressure, the size of the pressure wave that we detect when we listen to a podcast is just amazingly small and because your ear has this spiral inside, it makes low-pressure sounds, or very weak, quiet sounds detectable, and at the same time not be overwhelmed by very loud sounds. And I didn't do it, but this is where I can imagine somebody decides to be a physician and then an audiologist because it's all so crazy cool.

These days, we can be the curators of our own sonic world. We’ve built buildings and made entire professions around creating and enjoying sound. We’ve created technology that helps us hear, and technology that reduces noise. And it’s all because of our understanding of sound at a physical level.

Bill: People talk about physics and they throw the word physics around. It really is amazing that we can understand all this and produce shock waves and understand the speed of sound and understand temperature and pressure. It's just freaking amazing. Physics is so compelling to me.

Whether it’s a relaxing bird call [SFX: Bird song] or a recording of a screaming actor [SFX: Wilhelm scream], when it comes down to it...

Bill: It's all physics, man. There's an old saying: “Everything happens for a reason, dude.” If that's true, the reason is usually physics.

[music in]

Twenty Thousand Hertz is hosted by me, Dallas Taylor, and produced out of the sound design studios of Defacto Sound. For a little sound design inspiration, follow Defacto Sound on Instagram.

This episode was written and produced by Jack Higgins, with help from Sam Schneble. It was story edited by Casey Emmerling. It was sound designed and mixed by Soren Begin and Jai Berger.

A huge thanks Bill Nye for taking the time to speak with us. Getting him on the show is honestly a dream come true. These days, Bill hosts a podcast called Science Rules, which you can find right here in your podcast player. If you’ve got a science question for Bill, you can send it his way by visiting “ask bill nye dot com.”

And if you want more Twenty Thousand Hertz in your life, there are plenty of ways to get it. You can find us on Facebook, Twitter, or on our subreddit, reddit dot com slash R slash twenty K.

Thanks for listening.

Bill: Carry on! Let's change the world.

[music out]