186,000 miles per second). By knowing the speeds ofthese waves and the time it takes them to return as anecho, you can measure distance.Voice echo has been used to measure distanceacross canyons and the distance of icebergs from ships,as shown in figure 7-22. If it requires 6 seconds for asound wave to reach an iceberg and return, the totaldistance traveled by the wave is 6,600 feet. The actualdistance to the iceberg is only 3,300 feet. It requiresonly one-half the time, or 3 seconds, for the sound toreach the iceberg. Therefore, the iceberg is 1,100 × 3 or3,300 feet away. Mathematically, the distance to theobject is one-half the product of the velocity multipliedby the time in seconds. In this case, the velocity (1,100)is multiplied by the time in seconds (6). This divided by2 equals 3,300 feet—the distance to the object.Radar measures the distance to an object in muchthe same manner as the echo. (See fig. 7-23.) However,7-17SOUND TRAVELS 1100 FEET PER SECOND6 SECONDS TOICEBERG AND BACK3 SECONDS OUT3 SECONDS BACKDISTANCE3X1100=3300 FEETANF0722Figure 7-22.—Using voice echo to measure distance.EMITTING PULSETRANSMIT TIMETARGET AT-20MICROSECONDSNEXT TRANSMITPULSE1.6 MILESANF0723Figure 7-23.—Radar pulse detection.

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