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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 about echoes are:
Hearing echoes
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
where
-
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.
Using echoes
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. When you hear the "ping"
sound when in a submarine, that is the sonar 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.
Doppler Radar works on a similar
principle to measure the speed of storms in weather
prediction.
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.
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.
If you walk by a picked 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 is 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. |
