In this topic you will learn about how surfaces affect sound.

A sound wave does not act the same way when it hits a hard, smooth surface as when it hits a soft, textured surface. When a sound wave hits a surface, some of its energy bounces off the surface. The bouncing of a sound wave off a surface is called reflection. Not all of the sound wave reflects off the surface. Some of the wave's energy enters the surface, and part of the sound disappears. The disappearance of a sound wave into a surface is absorption.

When a sound wave hits a hard, smooth surface much of the sound wave's energy is reflected. A reflected sound wave is called an echo. If the echo is strong enough, you can hear it.

Sound waves travel fast. In general, sound waves have a greater speed in a solid than in a liquid, and a greater speed in a liquid than in a gas. The speed depends on the molecules of the material.

Sonar, or sound navigation and ranging, uses sound waves to detect objects far away. A sonar technician sends out sound waves and then times how long those sound waves take to bounce off distant objects and return. Many animals find things around them with a form of sonar called echolocation. Echolocation is finding an object by using reflected sound.

Sounds change in pitch as they come closer to and then pass a person. As a sound approaches you, more sound waves reach your ear per second. The frequency of the pitch increases. The pitch of the sound moving toward you is higher. As the sound moves away from you, the waves spread apart. Fewer sound waves reach your ear. The frequency of the sound decreases and the pitch is lower. The change in frequency (and pitch) as a source of sound moves toward you, or away from you, is called the Doppler effect.

The difference you hear in two sounds of the same frequency and pitch is the quality of a sound. Quality depends on the vibrations that make the sound. When a string vibrates, for example, it vibrates at more than one frequency at a time. The lower frequency, at which the whole string vibrates, is called the fundamental frequency. Meanwhile, some sections of the string will vibrate at higher frequencies, called overtones. An overtone is one of the series of pitches that blend to give a sound its quality. Quality makes a sound unique.