Dr. Emily Martinson, Au.D., Ph.D.

Now that we have defined sound and discussed sound waves, frequency, and intensity, we can start to think more in depth about how sound travels. As a refresher, sound is a vibration that passes through a medium before reaching our ear so that we can hear (more about the ear later). Today, we will be talking about mediums. Among psychics, a medium is a person who communicates with the dead through spirits.

For our purposes, however, a medium can be defined as a substance through which a vibration travels. Today we are assuming that this vibration is a sound wave. A medium must have mass to transmit the sound vibration. Air is a perfect example. Although we often forget that we are surrounded by air, air molecules surround us at all times. Those molecules vibrate in response to the vibration of the sound source. As the air molecules vibrate, the vibration eventually reaches our ears, allowing us to hear. Interestingly, air is not the only medium that sound waves can travel through.

Have you ever been underwater and heard sound? Whether that’s music when underwater at the pool or waves when underwater at the beach, sound can travel through water as well. Other substances can transmit sound, too, including metal. The density of the medium, or how closely packed together the molecules are, determines how well the sound wave is transmitted. Sounds travels faster through water than air, as the water molecules are much closer together than air molecules. In the absence of a medium, sound waves cannot travel at all. But if we are surrounded by air at all times, how it is possible to ever not have a medium? The absence of a medium occurs in two situations: in space, and in a man-made vacuum. Earth’s atmosphere provides us with a perfect medium through air. In space outside of the atmosphere, however, air molecules are not present. That means that while a sound source could be producing a vibration, you would not hear it as it cannot travel to your ear. Scientists can also artificially create a vacuum for experiments. This involves removing all of the air from an isolated container. Luckily, here on Earth we have plenty of air, both for breathing, and for listening to the sounds around us!

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