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

To truly understand hearing we must first understand how sound works, and this week’s post dives right back into the physics of sound.  If you remember our last Science Series discussion, you will recall that sound is defined as a vibration that produces a wave.  Today, we will discuss the wave produced: the sound wave.

If we drew a sound wave on a piece of paper, it would look like a smooth up and down graph, similar to the lines produced by a swinging pendulum.  Sound waves are made up of two parts: compression and rarefaction, which are both produced by the movement of molecules.  While the molecules being moved can be almost anything, for today’s discussion let’s assume that they are air.   Compression is the point in the sound wave where the air molecules are closest together, and most densely populated.  If we were to draw the molecules, they would look bunched up together, like ants at the top of an ant hill.  This is the point in the sound wave where, if we could freeze time, the molecules would look their most plentiful.

Rarefaction is the point in the sound wave where the air molecules are farthest apart, and at their least densely populated.  At the point of rarefaction, if we drew the molecules, they would look very far apart, like ants at the bottom of an ant hill.  If would could freeze time, the air molecules would look very far apart, and would appear fewer. As the wave cycles through compression and rarefaction, the air molecules continue to move closer together and farther apart, like waves in the ocean.  Those waves continue through the air until they can reach the ear to be heard by you, the listener.  The properties of the sound wave determine the frequency (or pitch) and intensity (or volume) of the sound.  Stay tuned to future Science Series posts for further discussion on frequency and intensity.

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