Uniden Clock - Manual (Page 16)

UNDERSTANDING SONAR
All depth sounders emit ultrasonic sound signals from the transducer into
the water located under your boat. These sound signals travel through
the water at a rate of approximately 4,800 feet per second (1500 meters
per second). The depth sounder transmits a signal and receives a
returning echo. The unit calculates the amount of time in microseconds
that elapsed while the signal traveled down to the bottom and returned
back to the transducer. This time is then converted to depth and
displayed on the screen.
It may help to understand these sound signals traveling between the
transducer and the bottom by imagining a ping-pong ball bouncing up
and down from the floor. The closer the ball is to the floor, the less time
it takes for it to return. The higher the ball is bounced, the longer it takes
to return. Bouncing the ball off a hard surface such as cement is the
same as bouncing a signal off a sandy or hard bottom. Bouncing this
same ball off carpeting creates a totally different effect because the ball
returns with less force. The same principle applies to an echo bouncing
off a muddy or grassy bottom, causing the echoes to be weaker.
AIR ECHOES
Air echoes can be caused by excessive turbulence under the face of the
transducer. Ultrasonic signals from a transducer will not penetrate air.
They react to air in the same manner as they react to a hard bottom
described above. Therefore, if your transducer is not mounted properly
and you are getting turbulence (air bubbles) under your transducer, you
may get false readings. This is simply because signals are being
returned by the turbulence and are never reaching the bottom.
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