Explanation of Sound Echoes by Ron Kurtus - Succeed in Understanding Physics. Key words: sonar, radar, measurement, velocity, wavelength, Doppler Effect, bats, moths, submarines, School for Champions. Copyright © Restrictions
by Ron Kurtus (revised 9 November 2005)
An echo is the sound you hear when you make a noise and the sound wave reflects off a distant object. Besides the novelty of hearing your words repeated, echoes can be used to estimate the distance of an object, its size, shape and velocity, as well as the velocity of sound itself. Special effects can be created with echoes reflecting off certain types of surfaces.
Questions you may have include:
- How can we hear an echo?
- How are things estimated with echoes?
- What sort of special effects can echoes cause?
This lesson will answer those questions.
Sound is a waveform made from vibrating matter. The sound wave travels through matter—especially air—in a straight line. When the wave hits a different material, some of it is reflected, absorbed and transmitted through the material. In the case of a sound wave in air hitting a solid wall, most of the sound is reflected.
If the wall is relatively flat, perpendicular to the source of the sound, and far enough away (but not too far), then you can hear the reflected waveform or echo. If the sound comes back in about 0.1 second or longer, you can readily distinguish the echo.
Since sound travels at approximately 1000 feet per second (or about 300 meters per second) and if the wall was 50 feet (or 15 meters away), the sound would return in 0.1 second. This can be seen from the relationship:
d = v*t or t = d/v
- d = the distance the sound wave traveled back and forth,
- v = velocity of sound, and
- t = the time it takes the sound to go back and forth.
t = 30 m / 300 m/s = 0.1 sec.
(Note that the distance was doubled to show the back and forth motion of the sound.)
That is enough time to be able to distinguish between the noises you made and the reflected sound.
Illustration of person and an echo
(You must have the Flash plug-in to see animation)
Echoes can be used to tell how far away an object is, how fast the object is moving, and even its shape.
By knowing the speed of sound and measuring the time it takes to hear the echo, you can calculate the distance of the object.
A sonar device sends out a sound and automatically calculates the distance of an object. Submarines use sonar to find objects under the water, including other submarines. The "ping" sound heard in a submarine comes from the sonar device sending out a sound wave under water.
Fishermen also use sonar to find schools of fish. Since this is an electronic device, the time it takes for the wave to return can be much less than the 0.1 second required to hear an echo. For example, if the speed of sound in water is 1500 meters per second and the fisherman's sonar device detects an echo in 0.02 seconds, the distance of the object under water will be d = v*t = 1500 meters per second * 0.02 seconds = 30 meters (back and forth).
That may mean a school of fish are 15 meters away.
When a wave bounces off a moving object, the frequency of the sound changes, according to the relative velocity of the object. (Velocity is the measurement of speed and direction.)
If the object is moving toward you, the frequency or pitch of the sound gets higher. When it is moving away, the pitch gets lower. The faster the object is moving, the greater the change in frequency or pitch. This is called the Doppler Effect.
You have probably experienced the Doppler Effect when you heard how the sound of an ambulance siren changes pitch as it passes by.
The Doppler Effect can be used to measure the velocity of an object by comparing the frequency of the sound sent out to the frequency of the sound reflected by in the echo. A sonar device is usually used to calculate the velocity of the object.
Note: Doppler radar works on a similar principle to measure the speed of storms in weather prediction, except that it uses echoes from electromagnetic waves.
Bats can find moths
Bats use echoes to find good tasting moths, while flying around at night. The bat sends a sharp click or chirping sound and then hears and processes any echoes off other objects in the area. Bats have large ears that are very sensitive to sounds in certain wavelengths.
Their brains are also able to process the sound of the echo coming off a flying moth to determine how far away it is, which direction and how fast it is flying, and the size of the moth. It continues to send out sound and receive echoes until it zeroes in on the moth and has a good meal.
Special echo effects
When sound reflects off a stepped grating, the echo can have interesting effects. The most common is a "chirping" or "pinging" sound.
Ancient people used these effects
Echoes from the great Mayan pyramid at Chichen Itza, Mexico sound like the quetzal bird that is found in the Mayan area. The shape of each riser on the pyramid's staircase measures over 10 inches, which is too steep for easy climbing but a perfect dimension to create the chirp.
Rock paintings from ancient Native Americans in Utah's Horseshow Canyon and Arizona's Hieroglyphic Canyon are mainly found at sites with good echoes. The placement of rocks in the areas also affects the quality of the echoes.
Some rock paintings in the French Caves of Font-de-Gaume and Lascaux have special echoes. Clap in front of a painting of horses and the echo sounds like thundering hoof-beats. But if you clap in front of a painting of a cat, almost no echo returns.
Walk by picked fence
If you walk by a picket fence that is near the sidewalk, you can hear the pinging echo of the noise you make. You can also sometimes hear this unusual effect near a flight of stairs.
Echoes are the reflection of sound from relatively flat object that is far enough away that you can discern the time difference. Echoes are used to measure distance, velocity, and the shape of objects. Echoes off gratings result in an unusual pinging sound.
You are a special person
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