Furuno MFD12 Operator's Manual page 171

7. Fish Finder
Surface noise
When the sea is rough or the vessel moves over a wake, surface noise can appear at
the top of the screen.
Air bubbles in the water
When the sea is rough or the vessel makes a quick turn, there can be blank spots in
the bottom echo. See the illustration. These blank spots are caused by air bubbles
which stop the movement of the sound wave. These air bubbles can occur with the
low-frequency ultrasonic waves.
False echo
When an ultrasonic pulse is transmitted, some energy leaves from each side of the
beam. This energy has the name "sidelobe". Echoes from sidelobes show on the dis-
play as false images like in the illustration shown below.
False
Image
7-28
Surface noise
Ultrasonic wave
blocked by air
bubbles
Mainlobe
Sidelobe