What is sound? Sound is a longitudinal pressure wave caused by the vibration of molecules. Um, what? In this article, we will be going over what exactly sound is as well as taking a deep dive into its most interesting properties.
We take sound for granted. There is no place on Earth without sound. However, the vast majority of the universe has never experienced it! Plus, while sound seems to be a constant in our world, it took thousands of years for humans to even begin to understand it.
Sound is a wave. More specifically, it is a longitudinal wave. But what is a longitudinal wave? There are two types of waves, transverse and longitudinal waves. Transverse waves such as light waves oscillate perpendicular to the direction of movement (simulation of a transverse wave). Ever seen how your favorite slinky oscillates in that back and forth motion? Well, it turns out that longitudinal waves are like slinkies; they compress and expand in the direction of travel. Sound waves are pressure waves, which are created by the vibration of particles and thus require a medium (matter) to travel. This means that sound waves cannot exist in a total vacuum (a space with no matter).
Longitudinal waves vs transverse waves (Source)
Wait, but don’t all molecules vibrate (collide with each other)? Yes, all molecules in the universe are constantly moving and colliding. This phenomenon is known as Brownian motion, which causes brown noise, an extremely quiet sound. As Brownian motion is random, brown noise sounds similar to the fuzzy static from a radio. Audible brown noise is caused by air molecules colliding with your eardrums. Most people can’t hear brown noise because the average human ear is not sensitive enough to detect it.
Ever eavesdropped on a conversation in another room? Don’t worry, we’ve all done it; it’s practically inevitable! But how is it that sound can travel through walls? Sound can pass through any substance. In fact, the denser the medium, the faster sound moves. A sound wave is a vibration of matter: thus, the closer molecules are to each other, the less time it takes for them to collide. This is why if you keep your ear to the ground, which has molecules closer together, you can hear footsteps better (or that conversation next door if you keep your ear to the wall). But what effect does the speed of sound have on your hearing? Sound loses energy as it travels due to friction. So the faster sound travels, the more energy it has when you hear it. Sound will also lose some energy when it changes mediums, which we call and observe as refraction.
If you read our previous article about chromatics, you have already heard of Doppler shift. For those of you who haven’t, Doppler shift is a phenomenon that causes sound and other waves to be higher-pitched as they move towards you and lower-pitched as they move away from you. But what happens when an object starts moving faster and faster until it breaks the sound barrier (exceeds the speed of sound)?
Since sound waves are pressure waves, even the slightest of movements creates some sound, just not always enough to be audible. As an object is moving, pressure waves are generated, and they move out of the way at the speed of sound. As an object gains speed, the pressure waves have less time to get out of the way. After an object reaches and passes the speed of sound, the pressure waves combine and form a shockwave behind the object. This shockwave is known as a sonic boom: an example of a sonic boom can be found at the beginning of this article! In contrast to common belief, a sonic boom is not a singular shockwave. A sonic boom can be heard behind a supersonic object so long as the object maintains its speed.
Sound is an essential part of our daily lives as well as an intrinsic part of the universe. Living without sound is like experiencing the world in two dimensions, never fully grasping the beauty of the universe. Now you understand what sound is and some of its quirks, but how do we, humans, perceive sound? I’ll leave you with that, till next time.
Post By: Armaan G.
Photo credits: Ensign John Gay, U.S. Navy, Public domain, via Wikimedia Commons
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