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Cometeer offers pour-over quality coffee delivered in frozen, recyclable capsules. Cometeer is offering a special deal for the holidays when you click our link in the description and use our code. [ ♪ INTRO ] If you were standing right next to a volcano when it erupted, you might expect to hear the loudest sound of your life.

But oddly enough, you might hear nothing. In fact, when Mount St. Helens exploded in 1983, people heard the blast all over the Pacific Northwest, but people living just tens of kilometers away from the volcano didn’t hear it at all.

As weird as it sounds, there are stories like this going way back in history. And it’s all because of a phenomenon called a sound shadow that makes certain sounds completely inaudible, even when you’re really close to the source. This can cause some strange situations in the natural world, but we can also use it to our advantage with some clever engineering.

Now, sound shadows, also known as acoustic shadows, happen anytime something gets in the way of a sound wave and keeps it from getting to a listener. So it’s basically like a regular shadow, except instead of light getting blocked, it’s sound. For example, if you’ve ever seen those sound barriers along the sides of highways, those are there to create sound shadows.

They’re meant to muffle the roar of the highway that would otherwise spread to nearby neighborhoods. So the basic concept is pretty straightforward. But you don’t necessarily need a wall, or any physical object, for that matter, to get a sound shadow.

Other things can divert a sound wave too, like the wind or even just the air. And that’s where things get interesting. Some of the most famous instances of this happened during the American Civil War, when sound shadows are thought to have played a major role in several battles.

Radio communication wasn’t a thing back then, so generals relied a lot on the sounds of far-off battles to decide on their next move. But sometimes that strategy did not exactly work out. Like during the Battle of Seven Pines in 1862.

This battle took place near Richmond, Virginia, when Confederate forces attacked Union forces, trying to drive them back from the city. Three different generals coordinated the attack, and one of them, General Joseph Johnston, planned to send reinforcements as soon as he heard musket fire. He was waiting just 3 kilometers away, but hours went by and he didn’t hear anything.

Finally he sent one of his officers to investigate, and that’s how he found out the battle was in full force. By the time his reinforcements joined the fight, it had already been raging for hours. It’s impossible to say what would have happened if the reinforcements had arrived on time.

After two days and thousands of casualties, the battle ended with no clear winner. But researchers believe the sound shadow drastically affected the outcome. A few months later, almost the same thing happened to Union forces at the Battle of Iuka, in Mississippi.

And in 1865, Confederate general George Pickett was so oblivious to a raging battle just a couple kilometers away that he had no idea that his troops were being steamrolled as he ate his lunch. This kind of thing happened over and over again. And by piecing together clues about the weather and the landscape, historians have been able to get an idea why these sound shadows happened.

It turns out there were a few reasons. Sometimes the explanation was simple: There were hills in the way that absorbed the sound of battle. But in some cases, the sound of musket shots was deflected by the atmosphere.

Sometimes this happened on windy days. When wind is blowing, the air at higher altitudes moves faster than air right above the ground. So if a sound is traveling upwind, the top of the wave will get slowed down more than the bottom of the wave.

As a result, the whole wave will begin to curve upward, almost like it’s passing through a lens. And it’ll go right over the heads of anyone sitting on the ground. Meanwhile, if the sound is traveling downwind, the opposite happens, and the wave gets bent toward the ground.

That can make sounds extra loud if you happen to be standing right where the sound wave is striking the ground, but as the wave bounces off the ground, it creates a sound shadow underneath it. Meaning someone just a little farther away might not hear the sound at all. A similar thing can happen during an atmospheric phenomenon known as a temperature inversion, which happens whenever warm air sits above cool air.

This often happens on clear nights, when the ground cools down quickly and lowers the temperature of the air in contact with it. But it can also happen under other circumstances, like when cool air sinks into valleys and gets trapped by warm air on top. Since sound travels faster through warm air than cool air, a temperature inversion will make the top of the wave curve toward the ground, creating the same effect as a steady wind.

So thanks to physics, a little wind or some funny atmospheric conditions can carry away the sounds of an entire battlefield. Sound shadows don’t just mess with soldiers on the battlefield. The same phenomenon can create quiet zones around exploding volcanoes and bombs and other loud noises.

And sometimes, sound shadows also happen in the sea. As ships travel through the water, they typically make a lot of noise. You might expect marine animals, like whales and manatees, to hear them coming and get out of the way, but too often, they don’t.

And it’s not their fault. Researchers have learned that one big reason for this is sound shadows. Unfortunately, as ships churn through the water, a lot of the sound they make gets blocked or redirected.

Sometimes the hull of a ship just straight-up blocks the sound of its propellers. But other times, different layers of water with different temperatures bend sound waves, just like air does during a temperature inversion. When this happens, animals either don’t hear the ship at all, or they can’t tell where it’s coming from because the sound isn’t traveling in a straight line from the source.

Some might even hear the noise and then travel straight into the silent shadow zone in front of a ship, thinking it’s a safe place. Over the years, collisions between ships and marine mammals keep going up. And they’re a huge ecological problem, especially because many of these animals are endangered to begin with.

So whether it’s on land or underwater, sound shadows can wreak all kinds of havoc. But as scientists have learned more about them, they’ve also begun using their knowledge of sound shadows to solve problems. For instance, one team of researchers developed a device that sends out high-frequency sound waves into the sound shadows in front of boats so that manatees will be able to hear them.

They’ve also modeled the sound shadows around various ships so that they know where sound needs to be projected. It’s a pretty simple solution, but technologies like these could prevent collisions between ships and marine mammals, helping protect marine ecosystems. Here on land, we also sometimes want to eliminate sound shadows.

And a whole other group of people trying to do that are the acoustical engineers who design theaters and performance halls. In any space like this, where you have a bunch of seats arranged around a stage, the sound you hear as an audience member usually arrives two different ways: There’s the sound that comes directly from the performers themselves, and there’s the sound that bounces off the ceiling and walls. If the space is designed properly, those sounds will arrive close enough together to reinforce each other.

If it’s done wrong, you get an echo, but if it’s done right, it all comes to you at the same time and sounds amazing. That’s what makes the sound in a concert hall sound so powerful and full. But if you’re sitting behind a pillar, certain wavelengths of sound coming from the stage will be blocked, creating a partial sound shadow.

You’ll still hear some sound, but it won’t be the same full experience as it is for someone with a clear view of the stage. Likewise, if you’re sitting under a balcony, you might have a perfect view of the stage, but the sound bouncing off the ceiling will be blocked, creating a sound shadow around You. So as acoustical engineers design these spaces, they have to use geometry in a way that minimizes sound shadows as much as possible.

But outside of concert halls, we don’t always want to get rid of sound shadows. Sometimes we want to create them. Because today, there’s so much noise in our world.

There’s the roar of highways, the clanking of construction sites, the blaring of car horns. In lots of places it’s just constant. And all this noise isn’t just annoying.

It also takes a serious toll on our mental and physical well-being. So, just like the architects of concert halls use geometry and physics to minimize sound shadows, some urban planners are using the same principles to maximize them. One main way they can do this is by setting up physical barriers that sit between people’s homes and nearby highways or industrial areas.

If there’s room for it, a hill or green space full of trees can make an excellent barrier. Both of these will absorb a bunch of sound. Also nicer to look at than those walls, if you ask me.

But that’s not always possible, especially because most of the time urban planners aren’t designing an ideal community from scratch. They’re trying to improve something that already exists. And that’s where they can use walls, like those sound barriers alongside highways.

The cool thing about walls is they don’t just act like plain old shields that reflect or absorb sound. They also create sound shadows by diffracting, or bending, sound waves over them. The path a sound wave takes over a barrier depends on where it’s coming from and how high the barrier is.

And different wavelengths of sound diffract by different amounts. But overall, the barrier forces the sound wave to take a longer path, casting a sound shadow beneath the diffracted wave. So if you need to create a pocket of quiet near a noisy area, a wall can really help.

Now, all of these barriers can be more or less effective depending on the weather. Winds and temperature inversions can change the path of sound waves, which changes how they interact with barriers. But put in practice, they can go a long way toward creating some quiet in our noisy world.

In the past, sound shadows often caught people by surprise, like Civil War generals or witnesses of volcanic explosions. And today, they can still certainly throw us for a loop. But knowing how they work and where they show up in our lives can help us prevent the chaos they sometimes cause and put them to good use!

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