Navman Datahelm 8120 Installation And Operation Manual page 34

Strength of echoes
The colours indicate differences in the
strength of the echo. The strength varies with
several factors, such as the:
• Size of the fish, school of fish or other
object.
• Depth of the fish or object.
• Location of the fish or object. (The area
covered by the ultrasonic pulse is a rough
cone shape and the echoes are strongest
in the middle.)
• Clarity of water. Particles or air in the water
reduce the strength of the echo.
• Composition or density of the object or
bottom.
Bottom types
Mud, weed and sandy bottoms tend to
weaken and scatter the sonar pulse, resulting
in a weak echo. Hard, rocky or coral bottoms
reflect the pulse, resulting in a strong echo.
Frequency and cone width
Water
Depth
25
50
100
150
200
300
400
600
800
1000
34
Cone width
at 50 kHz
20
40
80
45° cone
130
170
250
330
500
660
830
Planing hulls at speed produce air bubbles
and turbulent water that bombard the
transducer. The resulting ultrasonic noise
may be picked up by the transducer and
obscure the real echoes.
Cone width
at 200 kHz
5
10
20
11°
30
cone
40
60
80
120
150
190
DANGER
CAUTION
8120 Installation and Operation Manual