Echo

An echo is a scientific phenomenon in which a reflection of a sound arrives at the listener's ear after the original sound. It generally appears to be a softer version of the original sound and, depending on the environment, may repeat several times. Echoes generally result from surfaces that are mostly flat, smooth, and perpendicular to the ground, as a wall is to a floor. In addition, some surfaces reflect sound more readily than others, and louder sounds are more likely to create an echo. This is why echoes are not heard all the time.

Some animals, such as bats and dolphins, use sound reflections to help them find their way; this is called echolocation. Since the early twentieth century, humankind has learned how to apply echoes in similar ways to help with detecting and measuring objects that cannot easily be observed, such as those under the ground or in water. Other applications include medical technology such as ultrasound.

Background

Echoes are a natural phenomenon. Everything that makes a sound could conceivably make an echo under the right circumstances. The word echo is a Middle English word derived from French or Latin that in turn came from the Greek word ecko meaning "a sound."

The idea of applying this word to a sound reflection originated in Greek mythology. Like many myths, the story of Echo and Narcissus provides an explanation for a natural event people could not understand at the time. According to the myth, Echo was a beautiful nymph, or spirit in the form of a young girl. She and the leader of the Olympian gods, Zeus, were having an affair. Zeus's wife, Hera, tried to catch them in the act; however, every time she came near, Echo would start a conversation that distracted Hera while Zeus slipped away. When Hera realized what Echo was doing, she put a curse on her that prevented Echo from saying any more than the last few words spoken by another person. Echo then fell in love with Narcissus and started following him, though she was unable to tell him how she felt. When he got lost and yelled out to his companions, Echo was only able to repeat his words, "Let's come together." However, Narcissus was not interested in Echo; he was in love with himself, which caused his death. Echo pined away for her lost love until she died, leaving her voice behind to repeat back what others say.

Overview

Echoes are the result of sound waves, which are created whenever an object vibrates. Sound waves are patterns of disturbance caused by energy traveling through a medium, usually air. For instance, the vibration of air over the vocal cords in the throat creates the sounds made by humans; the vibration of air against a brass or woodwind instrument or the vibration of strings on a guitar or violin creates music; and the electrically enhanced vibration of the parts in a loudspeaker helps amplify and project sound.

These sound waves can pass through gases, such as the earth's atmosphere, as well as through solid objects such as the ground and liquids such as water. They do not travel through vacuums, or areas that are empty of gases, liquids, and solids. Sound waves are longitudinal, which means the sound vibrations go back and forth in the same direction in which the sound was projected. For instance, if a person is standing facing a long hallway and yells, the person's voice will travel down the hallway. If those sound waves encounter a relatively flat, smooth surface as they are travelling, they will bounce back or reflect to the person who made them. This reflection is an echo.

Some surfaces are better at creating echoes than others. Large, flat, smooth surfaces are more likely to produce an echo than a rough or textured surface; this is the reason that places that want to control echoes to improve sound quality—such as movie theaters—use textured walls and curtains to deaden the sound waves. It is also the reason sounds such as footsteps will seem loud and echo in an empty room but quieter in the same room with furniture; the sound waves are interrupted before they reach the smooth, flat walls, dampening the echo effect. It is also the reason some natural sites are known for their echoes, such as Echo Crater National Park in Idaho, where a pit crater provides the ideal conditions for sound to bounce back in an echo.

The conditions needed for the type of echo heard in canyons and craters include enough distance between the sound source and the reflective surface, the right kind of surface to bounce the sound, and sufficient volume. Sound waves travel fast enough that the optimal distance for creating an echo is at least 250 feet (75 m) between the sound source and the reflection point. The reflecting surface needs to be large enough and flat and smooth enough for the sound wave to bounce off of it; if several similar surfaces are near each other, such as the walls of a large valley or two adjacent walls in a large room, the echo wave will bounce from one to the other and repeat as each reflection reaches the listener's ears. Finally, the original sound needs to be loud enough for it to reach the reflecting surface and have enough energy to return, such as a loud shout or a thunderclap.

The power of the echo can be harnessed to help see things that are not otherwise visible. Bats and dolphins use echolocation to direct their movements, intentionally sending out sound waves that bounce back. The animals interpret the waves to avoid objects. Beginning in the 1920s, humans have adapted this ability to use in echo-sounding, which helped ships' crews determine the shape of underwater surfaces and the depth of the water below them. Arctic explorers are also able to use it to determine the thickness of ice. Sound navigation ranging—sonar—technology is used in many marine applications in the twenty-first century. By sending out sounds called pings and listening for the echo of their return, seamen and researchers can learn a great deal about the shape and size of things under the water. Sound echoes are also used in metal detectors and other devices that identify solid objects buried in the ground and in medical equipment used to view the inner workings of the body.

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