If you had to describe a specific sound to a friend, what words would you use? If you can't think of any words, close your eyes and listen to the people around you talk. Are you able to tell who is talking without opening your eyes? How? What are the differences between the sounds that different people make?

      Perhaps you thought of describing a sound with the words loud or soft; high-pitched or low-pitched. These words describe, or characterize, how we perceive sounds. Scientists, on the other hand, describe sounds with characteristics that can be measured using instruments. We can relate characteristics that scientists measure to the words we use to describe the sounds we hear. When we talk about loud or soft, scientists talk about the intensity, or amplitude, of the sound. When we talk about the pitch of a sound, scientists use the word frequency.

      Our ears are not sensitive to some of the characteristics that scientists use to describe sound. One important characteristic that we cannot hear is the wavelength of the sound. However, you can see the wavelength of a water wave. It is the distance from the top of one wave to the top of the next wave. You cannot see the wavelength of a sound wave, but scientists can measure it using special instruments.

      Because sound travels in a wave, we can relate the words that scientists use to describe sound to a picture of a wave.
 

      This is probably what you think of when you imagine a wave. It is important to remember, though, that in a sound wave, the particles are moving back and forth as if they were being pushed and pulled. They are not moving up and down as this picture shows. However, we can use a picture like this to show the change in pressure as a sound wave moves through. In the picture below you can see what the particles are doing - when they are being squashed together (compressed under high pressure) - the wave goes up, and when they are being pulled apart (expanded under low pressure) - the wave goes down.