Tuesday, August 5, 2008

Sound Ranging

The speed of light or of radio wave transmission is so rapid that it is regarded as instantaneous in navigation (161,800 + nautical miles per second). The speed of sound through the atmosphere varies slightly with its density (temperature and barometric pressure), but, for this purposes, it may be considered to be 1,118 feet per second, or 0.1838816 nautical miles per second. If you're doing mental calculations, regard it as just over 5 seconds per mile.

Flash-Bang "Flash-bang" is a term borrowed from the marines who locate enemy artillery by timing the interval between the "flash" of flame from a cannon's mouth and the arrival of the "bang" at their position. Any time you see the production of a sound (the flash of lightning, or the puff of smoke from a starting gun), and then measure the time taken for that sound to reach your position, you may determine the range, the distance off, of the sound producing object. When two yachts cruise in company, they can keep track of the distance between them if one sounds its foghorn and simultaneously waves toward the other. If a navigational aid such as a lighthouse synchronizes its foghorn and its radiowave transmissions and you time the interval between their receptions at your location, you will get a fair distance off.

Distance off (nautical miles) = 0.1839 x seconds between flash and bang.

Time between flash and bang (seconds) x 0.1839 = (distance off in nautical miles)

"Dog-Bark" Where cliffs or large buildings on the water's edge reflect sound emitted from your vessel in the form of an echo, another method of sound ranging is possible and it requires no one else's help. The story goes that in the old days, downeast skippers of trading schooners found their way through fog with the echoes of the ship's dog's bark reflected from the cliffs, hence the term "dog-bark navigation."

The technique is simple-just emit a loud sound on board (the short blast of your horn, a hammer against something sturdy, while simultaneously starting your stopwatch. When you hear your echo, stop the watch. The time obtained will give you your distance away from the echo producing wall. In this case, the sound must travel twice: once from you to the cliff, and then back again. So it takes twice the time as in flash-bang ranging.

Distance off (nautical miles) = 0.0919 x seconds to return echo to ship.

Time between generating sound and hearing echo (seconds) x 0.0919 = distance off in nautical miles.