This little quirk about sound has worked its way into the public consciousness through the famous tagline for the movie Alien: “In space, no one can hear you scream.” Indeed, sound in the human hearing range cannot travel through the vacuum of space, meaning that many of your favorite space battles from Star Wars or Star Trek should have been a lot quieter than shown on film. A vacuum does not have any matter for sound to travel through, and as a result sound cannot exist. About the only “thing” that cannot support sound is the absence of matter: a vacuum. Materials can be pure, mixed evenly or unevenly, or arranged in unusual patterns, and still support sound. Liquids, solids, gasses, and even more obscure forms of matter like plasmas can support sound and act as acoustic media. This begs the question, though, what makes something an “acoustic medium”? In truth, pretty much anything can be an acoustic medium. Sound absorption is likewise determined by the medium, which is why some materials (like fiberglass) find regular use in soundproofing, while others do not. Sound speed, for example, is determined by the density and elasticity of the medium. Many of the features that are most important to sound are not determined by the sound wave itself, but by the medium through which the sound travels. The material that the sound travels through is what we call the “acoustic medium,” and without an appropriate medium, sound cannot travel at all! In truth, sound is little more than a disturbance passing through some material that carries energy, be it gaseous, liquid, or solid. Indeed, some ancient Greek natural philosophers believed that a word was a physical thing whose shape was altered by the mouth as it passed out into the world. It is fairly common to think of a sound as an object, like some sort of invisible projectile. We will also talk about how, while you may not be able to hear anything there, it’s not quite true that there is no sound in space. In this edition instead of talking about the sounds themselves we will discuss what sound moves through: the acoustic medium. Last time we discussed one of the most popular units for measuring sound, the decibel , and gave a few rules of thumb to help even a non-scientist make sense of decibel values. Welcome to the sixth installment of The World Through Sound.
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