Welcome to the first installment of “The World Through Sound.” In this series of monthly articles on the Acoustics Today web site, we will look at interesting phenomena that you might not have noticed and show how these concepts relate to acoustics, sometimes in surprising ways. For non-scientists (and even scientists unfamiliar with acoustics), many of these terms and their effects will be unfamiliar. For acousticians, while the terms may be familiar, some of the perspectives might be new and could provide you with ideas on how to explain these concepts to non-acousticians you meet in the future.
Not too long ago, a video was circulating on the internet showing a Slinky being dropped from a tall building. But rather than dropping the whole Slinky coiled up in a lump, it was first extended over the edge and dropped from the top. What happens next might surprise you, so watch the video and then read on.
I know that when I first saw that video, I thought the result was pretty amazing. Instead of falling all together, the top of the Slinky falls first, with the bottom seemingly suspended in midair! Today we are going to talk about how this surprising effect applies to more than just Slinky’s being dropped, and is directly related to the concept of “sound speed.”
Sound speed is one of those concepts that are ever present in the field of acoustics, so much so that many of us have come to take them for granted. As the name implies, sound speed is the speed at which sound moves through a medium (e.g., air, water, the ground). Without a continuous medium, like in the vacuum of space, sound speed does not exist, but what constitutes a suitable acoustic medium is a topic for a future article. You can find basic information about sound speed, like its value for different materials, in physics books and on Wikipedia. But what if I were to tell you that the speed of sound applies to more than just sound? What if I were to say that sound speed mediates every mechanical interaction in our lives?
Let us again consider the Slinky. What if, instead of hanging the Slinky and dropping it, a child decided to attach the Slinky to a toy? What would happen when the child pulled on the end of the Slinky? Would the toy on the other end move right away, or would there be some sort of delay, like the delay seen when a Slinky is dropped? To answer this question, I tested this exact scenario; pulling a toy fire truck using a metal Slinky (the life of a scientist is hard sometimes!). Watch the clip below to find out what happened!
Much like the Slinky drop experiment, there is a noticeable delay between the point where the Slinky is pulled and the toy fire truck begins to move. You can actually watch as the motion of the pull travels down the length of the spring. This disturbance travels as what’s called a “longitudinal wave,” the same type of wave as sound. And the speed that the wave travels in the Slinky can be appropriately described as the “sound speed” of the Slinky. Yet, the pull on the Slinky is not what would be normally described as sound, so what does this mean? It means that the speed of sound is the speed of mechanical information, the speed that change travels through a material.
To see the power of this interpretation of sound speed, consider a common question asked of physicists that goes something like this: “What if I had a very long pole, and I used that pole to poke something very far away? If the pole were long enough, and my push fast enough, could that motion travel faster than the speed of light?” Setting aside the question of how one would manufacture or move such an incredibly long poking stick, the question still remains as to how long it would take for the ‘push’ to be felt at the other end. The answer, it turns out, has less to do with breaking the speed of light and more to do with adhering to the speed of sound. Just like with the Slinky before, the motion of the rod would travel like a wave, and the speed of that wave is the speed of sound.
To understand why this is so, it helps to think of how this poke happens on an atomic scale. When you use the giant rod to poke, you begin by applying a force at one end of the pole, accelerating the atoms at that end and causing them to move. Those atoms, in turn, push against the atoms next to them through their atomic bonds. Those atoms must then accelerate, in turn pushing on the next atoms, and so on in a great chain reaction that continues all the way to the end of the pole. This longitudinal wave looks much like the one moving through the Slinky, quickly moving through the rod to its eventual destination. And just like with the Slinky, the sound speed of the rod is what determines the speed that force propagates.
Sound speed, then, is the speed of mechanical information. Every push, pull, tap, and turn that happens to an object moves through that object at the speed of sound. The same thing happens to a lead pipe as to the Slinky, albeit many hundreds of times faster. If you were to hang a lead pipe by one end and suddenly release it, it would take some non-zero fraction of a second for the bottom of the pipe to begin falling. If you were to push or pull the pipe on one end, it would take time for the other end to even move. Ultimately, the lead pipe is just a less springy version of the Slinky.
Throughout this discussion, while I have said a lot about sound speed, I have not said much about sound itself. The reason why is that sound is not really very different from other forms of mechanical action. The time taken for a push or pull to move through an object is the same as the time it takes for a sound to travel. This is because sound, as we normally think of it, is little more than a series of tiny mechanical pushes and pulls, so sound obeys the same rules as the actions described above. Thus, if you were to put your ear up to the very long rod described above while someone tapped on the other end you would notice a delay between when you saw the tap being performed and when you heard it (since the speed of light is so much greater than the speed of sound).
The world viewed through this lens of sound speed is one that’s far less rigid than we normally imagine. Instead of solid objects moving, touching, and sliding, the world is really made of deformable objects bending, twisting, and warping. In the world of sound speed, everything takes just a little bit of time.
I hope this discussion of sound speed has given you something interesting to think about. Perhaps in the future, you will consider how mechanical forces move through the world with a definite speed. The next “World Through Sound” will look a little more at the concept of time by talking about frequency. We will then bring back in sound speed, and talk about wavelength using one of the most important relationships in all of wave physics!
Andrew “Pi” Pyzdek is a PhD candidate in the Penn State Graduate Program in Acoustics. Andrew’s research interests include array signal processing and underwater acoustics, with a focus on sparse sensor arrays and the coprime array geometry. Andrew also volunteers his time doing acoustics outreach and education as a panelist and moderator on the popular AskScience subreddit and by curating interesting acoustics news for a general audience at ListenToThisNoise.com.
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