Sound Waves 101

shutterstock_582746284.jpg

In our modern world, we're constantly being stimulated with sound from the city, our technological devices, coworkers, and many other sources. Whether we're listening to an office presentation or a new single from our favorite band, we rely on sound waves to deliver this information to our eardrums. Sound is inseparable from our daily lives, but how exactly does it work? In this in-depth guide, we've explained how sound waves travel, why different waves have unique characteristics, and how the naked eye can perceive sound waves. 
 

What are Sound Waves?

When we hear a sound, what we're really experiencing is vibrating air. These vibrations travel through air molecules until they reach our ears, and we perceive them tonally. Essentially, sound is an energy wave moving through the air. As the sound wave moves farther away from its source, the vibrations become less intense and the sound gets quieter. For instance, if you shout someone's name in a room, they will most likely hear your vocal cord vibrations. However, if you shout someone's name across a busy city street, they may not hear it because the vibrations lose strength as they travel and mix with other street sounds.

So, what happens if you don't have air molecules for a sound to travel through (like in outer space)? Since sound waves need a medium to carry the vibrations, sound doesn't actually travel in outer space. Without air, there's nothing to vibrate. Simply put, when matter vibrates in an orderly fashion, it produces sound. 
 

What are the Different Types of Waves?

There are two main wave categories, which have their own special characteristics: 

Longitudinal Waves
When sound waves move through the air, they are longitudinal. Essentially, the vibrations are traveling parallel to the larger sound wave, like when a sports stadium does "the Wave". The longitudinal wave has both a direction and an amplitude, which indicates the vibration strength as it travels. Longitudinal waves can travel through gases and liquids. 

Transverse Waves
Unlike longitudinal waves, transverse waves vibrate at a 90-degree angle (i.e. perpendicular) to the overall wave direction. For instance, a vibrating guitar string or trampoline creates a transverse wave. Electromagnetic waves (including radio) are transverse as well. A transverse wave requires solid material, like a string or a drum, for the wave to propagate. 
 

How Do Sound Waves Travel?

Sound can travel through any object, whether it's solid, liquid, or gas. This happens with mechanical waves, which send energy from one place to another by creating vibrations. These vibrations can travel through particles in a solid or liquid, or even move through disparate particles in a gas.  

Let's look at a speaker as an example. When you play digital music on a speaker, the speaker cone vibrates in and out rapidly, according to the song's playback data. These vibrations interact with nearby air particles, sending a longitudinal wave outward from the speaker cone. More and more adjacent air particles vibrate against each other, sending the wave into the room, where it eventually reaches your ears. The air particles are the "medium" for sound waves to travel, while the speaker can be thought of as the original "disturbance".  

Sound can also travel between mediums, like when you hit a snare drum. The drum head vibrates, sending wave energy through the drum, as well as through the air. This movement between mediums is known as "transmission". If there isn't at least one medium for the vibrations to interact with, the sound waves won't be able to reach your eardrums. 
 

Can You See Sound Waves?

When you hit a drum, you can usually see the vibrations. That's one way of "seeing sound". Those vibrations are actually sound waves moving through the drum surface and interacting with the air. Likewise, when a plane flies near your home, it can cause a sonic boom that sends deep vibrations through your furniture and nearby objects. You can actually see the results of these intense sound waves moving in real time. 

However, most sound that we experience day-to-day is just traveling through the air, which is essentially invisible. If you're playing a song on the stereo at a normal volume or chatting with a friend, air molecules are the primary medium for those sound waves. That means you won't be able to see the sound. Even when we can see sound waves, they're often moving so fast that it looks like a blur to the naked eye. For instance, when you pluck a guitar string, you can see it vibrating as the sound travels outward, but it happens so fast that the string looks blurry. Our eyes can't fully process the tremendous speed of sound waves.

Now that you understand sound basics, take your knowledge to the next level by learning about DSP in audio.