How Does Sound Sound?

     Listen: what do you hear right now? Dogs barking, kids playing, lawnmowers, music?  That's called sound, it's every where on earth. But what is it? Hearing is one of our natural senses, but not everyone knows just what it is or how it works.

      Sound is the transfer of energy in the form of waves. The waves travel by bumping air molecules together. The air molecules move back and forth, but the wave of energy moves outwards. The speed with which the waves travel through air depend solely on temperature. Waves will travel faster in warmer air. In general, at 68 F (20 C) sound travels at 1130 ft/sec (344.4 m/sec.) Sound travels slower in gases than in liquids and more slowly in liquids than solids.

       All sound waves have a specific amplitude and frequency. Amplitude is the height of a wave. Frequency is the number of waves that pass a point in a second. This is affected by how quickly the wave source is vibrating, which determines how close the waves are together. For sound, the higher the frequency, the higher the pitch of the sound. The lower the frequency, the lower the pitch of the sound.

      What we hear depends on the properties of the sound wave that enters our ears. A loud sound represents a high amplitude sound wave, one with lots of energy. Musicians use a letter code to describe the pitch of a musical sound. The musical scale begins with an interval called an octave. This is the interval between middle C and high C, for example. An increase in pitch of one octave represents a doubling of the frequency of the wave.

     When we speak, our words have a particular wave shape. Computers can imitate sounds, including human speech, by recreating the wave shape. For example, by reversing the wave shape, the sound is also reversed. A person can be made to speak backwards by simply reversing the sound waves. An echo is a sound we hear after it is reflected, meaning bounced back from an object.


    If you happen to be standing by a highway and a car going by blows its horn, you have no doubt noticed how the horn sounds suddenly lower just as the car passes you. This effect is called the Doppler effect. As the sound is moving toward you it is compressed, or pushed together. This makes you hear a higher frequency than the horn is actually producing. As the car passes you and the sound source begins to move away from you, the sound waves begin to spread out. This makes you hear a lower frequency than normal which the ear hears as a lower tone.

     Things can, of course, travel faster than sound. Some jets and the space shuttle do and they produce something called a sonic boom. What happens is that because they are traveling faster than sound, all those sound waves get compressed into one big compression wave. This wave intersects the ground at some distance behind the plane and when it touches the ground, we hear the sonic boom. Windows break, children cry and startled adults call the police to find out what caused the "explosion!"

     Because sound needs air molecules to "work" there is no sound in space. All those "sounds" you hear on your favorite science fiction show are just that, fiction! An explosion could never be "heard" in the vacuum of space. The astronauts can hear each other because there is air inside their helmets for breathing. The astronauts who stood on the moon and pounded rocks never heard any noise!

Copyright © 2001 Kathy A. Miles and Charles F. Peters II