Every waking second of our lives, we hear sound waves everywhere. From the hum of a fan to the clicking of your keyboard, sound waves permeate our lives. But how do they travel throughout the world around us? To answer this question, we are going to engage in a thought experiment.
 
Let’s say we have a bar of steel that reaches the 384,402-kilometer distance between the floor of the Earth and the floor of the  Moon overhead. Let's be real, that's a very long bar that could never feasibly exist. but for the sake of this thoughts experiments let’s say it does and, that we are going to suspend all these factors.

So, let’s pose this question:

  1. if we were to raise the Earthside of the bar by one meter, what would happen to the far Moon end of the bar?

  2. Would it move a meter instantly? 

  3. Would there be a couple of seconds delay?

  4. Possibly an hour delay?

  5. Maybe a much longer delay?

        As it turns out, the moon end of the bar would move one meter 18 hours and 24 minutes after we moved the Earth end of the bar. This is a massive delay and it is due to the fact that when we are pushing on the bar on earth, we are not pushing on the entire 384,000-kilometer-long bar at once, but rather we are just pushing on this localized group of atoms here. 

These atoms at the Earth end of the bar push on their adjacent atoms, and then to the next group of atoms up the bar and so on, all the way until this pushing reaches the moon. This pushing can also be thought of as bouncing of atoms and it does not travel up the bar instantaneously but rather it takes time for one group of atoms to push or bounce into the next group of atoms. Specifically, in steel, this rate of bouncing and reacting to the adjacent atoms travels up the bar at a rate of 5.8 kilometers per second.
soundwave and equalizer bars with the human ear. 3d rendering illustration with copy space. Sense of hearing, sound, and music graphic concepts.

This speed is considered the speed of sound in steel and it varies depending on the material. If the bar were made of oak, the 384,000-kilometer-long delay would be just under 27 hours. That’s a long time, but eventually, the motion would get to the moon.

The point is that although materials like a steel bar, or a wooden table look to be a coherent single object and thus one would expect the entirety of the object to move instantly all at once; in reality forces and movements takes time to travel throughout the many atoms in an object.

This traveling of movement or propagation can be thought of as a chain reaction of atoms pushing or bouncing into one another. The speed of this propagation is called the speed of sound, and it is dependent on the material or medium that the motion passes through.

It is also dependent on the temperature of that particular medium. Now, back to the thought experiment, consider what may happen if we move the steel bar differently. On Earth, let’s move the bar up, and then after 10 minutes, we move the bar back down. What will happen on the Moon? Well, similar to our previous experiment, after 18 hours, the bar will move up and then 10 minutes after that, it will move down.

      Now, let’s say we move the bar faster specifically up and down 100x a second for a full minute and then stop?

    Well again, this shaking-like motion will start at the Earth end of the bar, make its way up the bar at a rate of 5.8 kilometers per second, and after the 18-hour delay, the moon end of the bar will move in the exact same shaking motion for 1 minute. 

In essence forces and motions in the material do not catch up to one another, even if the forces are in the opposite direction, but rather once a motion is started, it continues up the bar at a constant speed. The propagation of this motion is called a wave. In this thought experiment, the steel bar represents the air around us, and the chain reaction of bouncing atoms up the steel bar represents the propagation of a sound wave. 

 

There are five key concepts shown in this thought experiment that will help you better understand sound waves and waves in general. The first is that sound waves are bouncing off air atoms or particles into one another. Second, waves can travel very far, however, the atoms themselves only travel a short distance.
Conceptual image about human earing test. Digital illustration.
Third, waves travel at the speed of sound and these speeds are material dependent. Fourth, sequential waves in the same medium travel at the speed of sound, and thus one wave does not catch up to the following wave. And finally, fifth, the coup de grace of concepts a sound wave isn’t really an object like a steel bar, a wooden table, or atoms of air but rather, a sound wave is the propagation of force or motion. 

When we hear something, we are in essence hearing the motion that an object makes, or how kind of that object is applying a force to the air particles around it thereby causing those air particles to bounce around and that bouncing is picked up by your ears and perceived as sound. 

Please be sure to comment, like, and tell your friends and family about what you learned! This article is about how sound waves travel, and what are sound waves, what are the different types of waves how do your ears perceive sound, what are frequency and loudness, if the sound is the movement of air particles, then what is wind isn’t that also the movement of air particles, and what is the structure of a smartphone speaker?

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