186,000 miles per second). By knowing the speeds of
these waves and the time it takes them to return as an
echo, you can measure distance.
Voice echo has been used to measure distance
across canyons and the distance of icebergs from ships,
as shown in figure 7-22. If it requires 6 seconds for a
sound wave to reach an iceberg and return, the total
distance traveled by the wave is 6,600 feet. The actual
distance to the iceberg is only 3,300 feet. It requires
only one-half the time, or 3 seconds, for the sound to
reach the iceberg. Therefore, the iceberg is 1,100 × 3 or
3,300 feet away. Mathematically, the distance to the
object is one-half the product of the velocity multiplied
by the time in seconds. In this case, the velocity (1,100)
is multiplied by the time in seconds (6). This divided by
2 equals 3,300 feet—the distance to the object.
Radar measures the distance to an object in much
the same manner as the echo. (See fig. 7-23.) However,
7-17
SOUND TRAVELS 1100 FEET PER SECOND
6 SECONDS TO
ICEBERG AND BACK
3 SECONDS OUT
3 SECONDS BACK
DISTANCE
3X1100=3300 FEET
ANF0722
Figure 7-22.—Using voice echo to measure distance.
EMITTING PULSE
TRANSMIT TIME
TARGET AT
-20
MICROSECONDS
NEXT TRANSMIT
PULSE
1.6 MILES
ANF0723
Figure 7-23.—Radar pulse detection.
