Humminbird 161 - Manual (Page 8)

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How Sonar Works
Sonar technology is based on sound waves. The 100 Series TM Fishing System uses
sonar to locate and define structure, bottom contour and composition, as well as
depth directly below the transducer.
Your 100 Series TM Fishing System sends a sound wave signal and determines distance
by measuring the time between the transmission of the sound wave and when the
sound wave is reflected off of an object; it then uses the reflected signal to interpret
location, size, and composition of an object.
Sonar is very fast. A sound wave can travel from the surface to a depth of 240 ft
(70 m) and back again in less than 1/4 of a second. It is unlikely that your boat can
"outrun" this sonar signal.
SONAR is an acronym for SOund and NAvigation
Ranging. Sonar utilizes precision sound pulses or
"pings" which are emitted into the water in a
teardrop-shaped beam.
The sound pulses "echo" back from objects in the
water such as the bottom, fish and other
submerged objects. The returned echoes are
displayed on the LCD screen. Each time a new
echo is received, the old echoes are moved
across the LCD, creating a scrolling effect.
When all the echoes are viewed side by side, an
easy to interpret "graph" of the bottom, fish and
structure appears.