Sound Principles

Several of the body’s senses rely on energy signals received from locations remote from the body. Sound is one example. Our sense of hearing relies on sound, carried mostly by the medium of air. The Bible speaks of many sound-producing musical instruments, such as trumpets, flutes, harps, and cymbals. It also speaks of the sounds of wind, moving water, rainfall, and thunder, along with less pleasing sounds of chariots in battle and grinding millstones. The sense of hearing is one of God’s greatest gifts to humanity, but the means by which sound travels through the sound-carrying medium is unfamiliar to most.

First, we know that sound does not travel through a vacuum. It needs a medium such as air, although liquids and solids also carry sound. In a soundless room there are trillions of air molecules zigzagging around with kinetic energy, colliding with each other and with the objects in the room. Without a sound producer, however, not much else of interest is happening. The most common producer of sound in air is the vibration of a solid body such as a string, rod, bell, or the human vocal cords of our larynx. What happens in the air when vocal cords or other objects vibrate is fascinating.

The vocal cords of a typical man vibrate back and forth about 120 times per second during speech; the average woman’s cords vibrate 210 times, the average child’s 300 times. Try to envision a vocal cord vibrating outward just once. The surrounding free air molecules are slightly compressed for just an instant, creating a region of slightly greater molecular density. This is called a compression: the air pressure is slightly increased. The compression then starts traveling away while the vocal cord pops back to its original position. A low pressure area is created in this “pop-back” area because the air is slightly thinner. When the cord vibrates outward again, another compression is formed and starts to travel away. If we could visualize the situation we would see many areas of compression separated by areas of rarefaction all traveling away from the vocal cords.

If you have ever stretched a “Slinky” coil toy along the length of a table and given one end a series of quick tweaks, you could see the compressions traveling along the Slinky separated by the rarefactions. Comparing sound production in air to a Slinky is a strong analogy, except that Slinky compression waves are slowpokes compared with sound waves in air. Sound waves travel about 1100 feet per second, or about one mile in five seconds. During this Advent season, the sound of carols produced by voices, bells, and drummer boys will all be the result of air compression waves impacting your eardrums. Enjoy the celebration!

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