Guest Writer, Astrophysicist, Megan Force -The Science of Sound

Some Questions for Megan Force...

What is Sound? 

Sound is a wave.  A sound wave travels from one place to another by compression (squeezing) and rarefaction (thinning out). Sound waves radiate out from a source and travel through the air to our ears, which receive the crests and troughs of the waves and turn them into something our brains recognize. How often the wave compresses and rarefies defines the frequency of the sound. Human ears are tuned to only receive certain frequencies of sound, just as human eyes only receive certain frequencies of light. Just as the air around and above us is filled with invisible light waves, such as radio and x-rays, it is also filled with sound waves unheard by humans, travelling through the air, water, and surfaces around us. Sound waves are a type of pressure wave because the air is varying in pressure as the sound travels through it. 

Where Does Sound Come From? 

Sound is energy. Someone or something creates motion, or makes an impact, and sound is produced. The law of Conservation of Energy tells us that the amount of energy around us remains constant, while the bits and pieces of energy are constantly moving and being transformed. When you eat an apple, you increase the energy in your body by taking in calories (units of energy). When you expend that energy by strumming on a guitar string, the potential energy you’ve taken from the apple is converted into the kinetic energy of your hand, which then goes into the strings, to make them vibrate. The energy in the strings then radiates out in the form of sound waves.

How Are Different Sounds Created? 

Sound comes from action. The wavelength of sounds we can hear is much larger than that of the light we can see. You can hear your friend around the corner, even if you can’t see them, because the sound wave is traveling around the wall, where the light wave doesn’t. Because air is being squeezed and thinned at a particular rate due to a particular sound, an object which is producing sound and moving at the same time will sound different if it is moving toward you or away from you. In one direction it’s pushing the waves closer together (the waves come more often, or higher frequency), and in the other direction the waves are being pulled farther apart (lower frequency). This is known as the Doppler Effect, which you can detect whenever you hear the sound change from a train as it passes by you.

How Does Sound Travel? 

Sound requires a medium. Sound travels through the air at 343 meters per second. It travels faster in water, at 1,493 meters per second and faster still in diamond, at 12,000 meters per second. The pressure waves created by a sound which is generated from an action become slower or faster depending on what they are traveling through. If there is no medium for sound waves to travel through, there is nothing to compress and rarify, so there is no sound. A vacuum is devoid of matter, and hence in the vacuum of space there is no sound (“in space, no one can hear you scream”). Light waves propagate by variations in electromagnetic radiation rather than in the pressure of matter. Thus, we can see the stars in the sky because light waves can travel through this vacuum, yet we cannot hear the stars because their sound is lost in the emptiness.  

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Megan Force is a physical science analyst and astronomer. She has studied R Corona Borealis stars at optical and infrared wavelengths with the Indian Institute of Astrophysics, and she has published research on pulsar radio astronomy with colleagues at the University of Vermont.