Resource: Sound

Sound is an everyday example of a longitudinal wave—a wave that has oscillations parallel to its direction of motion. A sound wave is produced by a vibration, which disturbs nearby molecules and causes them to vibrate. Their vibrations then disturb their neighboring molecules, causing them to move back and forth as well. The pushing and pulling apart of molecules produces alternating regions of high and low density, called compressions and rarefactions, respectively. Energy is transported through a medium—such as air—in these waves of varying density. Not surprisingly, longitudinal waves are also known as compression or pressure waves.

We typically experience sound waves traveling through air, but they can move through any medium, including liquids and solids. The speed of sound can vary, depending on the material through which the sound waves move. Under average conditions, the speed of sound through air is about 340 meters per second.

Other properties of sound waves include wavelength, frequency, period, and amplitude. Wavelength is the distance traveled by one complete cycle of the wave pattern, for example, from one compression to the next. The period of the wave is the time it takes for the wave to complete one cycle. Frequency, measured in units called hertz (Hz), is the number of vibrations or pressure disturbances that pass a point in one second. In a sound wave, the higher the frequency, the higher the pitch. Amplitude is related to how much energy the wave is carrying. The greater the amplitude, the louder the sound.

The meeting of two sound waves can produce an interesting phenomenon called interference. When two waves meet, the behavior of the molecules within the medium will be the net effect of both of the waves. For example, if a compression of one wave meets a compression of another wave, the net result will be greater compression. This is an example of constructive interference. However, if a compression of one wave meets a rarefaction of another wave, the net result will be less compression. This is an example of destructive interference. In some cases of destructive interference, the net result of two sound waves meeting could be no sound at all! When designing a room in which sound quality is important—such as an auditorium or concert hall—it is important to take into account the effects of interference. Bad design, because of improper positioning of speakers or the reflection of sound waves off walls and ceiling panels, could result in areas of destructive interference and thus an unhappy audience member.

To learn more about sound waves, check out Sound Waves Underwater: True or False and Sound and Solids: Visualizing Vibrations.

To learn more about how sound travels, check out Sound and Solids: Listening Stick.

To learn more about pitch, check out Experimenting with a Glass Xylophone and Pitch: Super Sounding Drums.

To learn more about how sound can be used as a tool, check out Sound Waves Underwater: The Loch Ness Monster and Sound Waves Underwater: Experiment with Sonar.

• How are sound waves graphically represented in this simulation? How are amplitude and frequency displayed?
• Why does the pitch of a sound change when you change the frequency?
• How does changing the position of the listener affect the sound heard from a single source? from two sources?
• Why does varying the air pressure affect the sound waves?