There’s one fact about sound that most movies that are set in space get very wrong. All those epic fight scenes with explosion sound effects are simply unrealistic. As you probably know — sound can’t exist in a vacuum. But why is that? To put it simply, sound waves do require a transmission medium.
Recently, I wrote about whether sound travels up or down in an apartment building, which got me thinking. I realized that, despite having posted about soundproofing everything from your home to your office cubicle, I’d never actually explained how sound waves move. So today, I’m going to fix that!
In this article, we’re going to find out all about how sound waves move through the various states of matter. And have you ever wondered what the speed of sound is? The answer actually depends on the medium that’s carrying it. But first, let’s talk about why space explosions would be so unsatisfying in real life.
Why Can’t Sound Travel Without a Medium?
If you were to walk away from this article with a single piece of knowledge, let it be that sound needs matter to travel. There has to be some molecular movement for it to travel. Naturally, the vast vacuum of space is devoid of matter. Therefore, there are no molecules for sound waves to hitch a ride on.
In fact, sounds are, by definition, vibrations moving through air or another medium. Different properties of various mediums make sounds change in different ways. Some transmission mediums allow the sound waves to spread faster, while others don’t let the sound reach its full potential. That’s why materials such as rubber — which I’ll talk about a bit later — have a vibration dampening effect.
Other than sound waves, there are also many others. In physics, waves are transmissions of energy — whether they’re sound or light, or even ocean waves. If you’re still thinking about the properties of outer space, you may have come to a realization just now. Light waves exist in a vacuum, so why can’t sound?
You see, these two are different types of energy waves. Sound waves and ocean waves are mechanical ones. Therefore, they need a medium to travel. Ocean waves are caused by energy shifting the water, while sound waves can travel through many mediums, including air and water.
On the other hand, light waves are electromagnetic ones which can indeed travel without the help of matter. Other than light, this privilege also belongs to x-rays and radio waves. In fact, we’ve been sending interstellar radio messages (or IRMs) since the early 60s, hoping to attract extraterrestrials.
But now that we know why sound can’t exist in a vacuum, let’s talk about the mediums it can use.
Which Mediums Do Sound Waves Use?
There are four fundamental states of matter: gas, liquid, solid, and plasma. Sound waves can travel through each them by using them as transmission mediums. Because plasma actually has to start out as gas in order to become plasma, I’m going to only consider the three states of matter we’re all familiar with.
In school, you might have learned about the states of matter using the example of water. In its natural state, it’s a liquid. However, when it gets cold, it can become solid ice. Or, if it boils, it’s released into the air as a gas.
Well, while I will take the time to explain how sound waves travel through each of these, I won’t stop at the three states of aggregation of water. When we’re talking about soundproofing, we’re going to care about other materials as well, particularly:
- AIR. I’ve said it before, and I’ll say it again — the air gaps around our doors and windows are the weak points through which noise slips into our rooms. That’s because the sound is being carried in with the air that comes into the room.
- WOOD. Most of our homes are built at least in part out of wood. If you want a little prelude, you can read this article about the acoustical properties of wood.
- CONCRETE. Of course, the other component of our homes is concrete, so we have to mention it too.
- GLASS is in our homes, on our cars, and everywhere around us. So learning how sound behaves around glass will be helpful for future soundproofing projects.
- Various METALS. Cars typically have iron, aluminum, steel, and copper parts, among others. Between automotive soundproofing and, say, steel doorways, learning what to expect of metals is crucial.
How Fast Does Sound Move?
All of the materials I’ve listed above interact with sound in different ways. Depending on the density of their molecules and the environmental conditions, the sound waves can be passed along faster or slower.
Typically, they move faster through solid matter. The reason for this is that the molecules making up solid matter are closer together. So they’re colliding at a faster rate, allowing them to transmit vibrations faster.
Now, the environmental conditions that can influence the speed of sound are also pretty easy to understand. Essentially, it all comes down to temperature, as sound tends to travel faster in higher temperatures. Altitude is also a factor, but only because the higher you go, the colder it gets. Therefore, sound travels more slowly in greater altitudes.
Other than that, other environmental conditions, such as air pressure and humidity, seem to play no part in the speed of sound. Now let’s see some specific numbers on the speed of sound, as it’s carried by various mediums.
As I’ve already said, the main thing that factors into the speed of sound as it’s traveling through gas is the temperature. So even when dealing with something as simple as air, the speed of sound is changeable.
For example, when we’re dealing with a pleasant 70 degrees Fahrenheit, the speed of sound is going to be 767 miles per hour. That’s about 343 meters per second at 20 degrees Celsius. However, when the temperature rises to 104 degrees Fahrenheit, or 40 degrees Celsius, the story changes. In those conditions, the speed of sounds is closer to 795 miles per hour (355 meters per second).
Because the air molecules are so far apart from each other, the sound waves can’t vibrate them at more substantial speeds. Conversely, sound traveling through liquid or solid matter is much faster since the molecules are more densely packed.
So since liquids are generally denser than gas, it’s no wonder that water can carry sound faster than air. That’s also why sounds are actually louder underwater — even though we perceive them as being muffled.
Furthermore, the temperature affects the speed of sound, even in water. At 70 degrees Fahrenheit, sound travels at 3,319 miles per hour (1,484 meters per second) under water. So the deeper you go, the colder it gets, slowing the sound waves.
Still, the speed of sound in liquid is fairly unpredictable. After all, we’d have to take into account all of the minor changes and shifts in water that happen during the day and over the seasons. Not to mention its natural movements if we’re talking about sea or ocean water.
But then, talking about the speed of sound as it’s moving through water isn’t really going to help us soundproof anything. So now, let’s turn to the final piece of the puzzle: how sound behaves when going through solid matter.
As you know, when sound travels through solid matter, we get what’s known as impact noise. This is the thing that makes your house shake as a bus rounds the corner towards it. Essentially, the sound waves that are originating from the motor of the bus are traveling through the tires into the asphalt below. Then, they move toward your house, where the vibrations pass through the brick, wood, and concrete to you.
There are a few key characteristics about the medium itself that figure into the speed of sound. The medium’s density, compressibility, and the rigidity all play a part in this equation.
On a specific example, iron is a better sound conductor than wood, as the speed of sound traveling through iron is 11,453 miles per hour (5,120 meters per second). Aluminum is an even greater conductor, with speeds measuring over 14 thousand miles per hour, and even copper is in the above 10 thousand range. Even glass is similarly conducive.
However, wood still allows sound to travel faster than air. After all, sound waves move through wood at 8,860 miles per hour (3,960 meters per second). So keep that in mind the next time I recommend resilient channels to separate your drywall from the wooden studs inside your walls.
On the other hand, there are also some solids that are decidedly bad conductors (which is, after all, what we want when we’re soundproofing). Because of the density and mass of rubber, it’s an excellent anti-vibration material. The speed of sound passing through it actually drops to around 134 miles per hour (60 meters per second). That’s less than half of the speed of sound going through air!
So the next time you wonder why I recommend rubber products so often — that’s why!
On Sound Traveling Through Different Mediums
Hopefully, I’ve managed to clear some things up with this little PSA. The next time you’re watching Star Wars, feel free to mute the movie during the battle sequences. At least, you can mute it if you want a more authentic experience.
All joking aside, though, knowing how sound travels is very important. After all, it will help you determine how to approach any of the soundproofing projects you’re contemplating. Even if you don’t worry about the sound conductivity of the various mediums, I’m sure you’ll have an easy time soundproofing anything that needs it. My guides are, as always, at your disposal.