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5 Pressure Waves

Sound waves are also called pressure waves because they increase and decrease in pressure as they move through a medium. An area where the pressure is greatest and the medium is squeezed together is called compression. An area of decreased pressure, where the medium is stretched out, is called rarefaction. Sound waves are initiated when a force (a push or pulling motion) is applied to a medium that can vibrate, meaning to move back and forth. Vibrating objects give surrounding air molecules a push, which starts a pressure wave that moves out in all directions like the ripples in a pond.

Blowing across the open mouth of an empty bottle starts a pressure wave in the bottle causing the air inside to vibrate. The air in the bottle will vibrate at a specific frequency, called its natural frequency, and the sound produced will be specific to that frequency. The height of the air column in an empty bottle is measured from the top of the bottle to its bottom. Adding water to the bottle will shorten the height of this air column. The sound waves bounce off the surface of the water in much the same way as they bounce off the bottom of the bottle. Thus, the sound heard depends on the height of the air column. As the height of the column decreases, the frequency of the sound produced increases. The highness or lowness of the sound, called the pitch, is directly related to the frequency of the sound.

FIG 1

See for Yourself

Materials

 6 empty water bottles, same size and shape

 water

What to Do

1 Support each empty bottle with your hand, and then blow across the top of each one.

2 Compare the sounds made by each bottle.

3 Change the height of the air column inside each bottle by adding varying amounts of water and compare the sounds that are produced.

What Happened?

The results of this experiment confirmed that as the height of a vibrating air column, which is closed at one end, decreases, the frequency of the sound produced increases. It also confirmed that the pitch of sound is directly related to the frequency of the sound. The lower the frequency, the lower the pitch, and vice versa. The empty bottle produces the lowest pitched sound because it has the air column with the greatest height; thus, the sound produced has the lowest frequency. By increasing the quantity of water added to the bottles, the height of the air column in each bottle was shortened, increasing the frequency of the sound waves, and so higher pitched sounds were produced.