That 1st Lowrance drawing is the patent depictions for the Lowrance SideScan patent application...
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The sound pulse of the individual beams (any brand)...do not have "sharp" edges that just
"cease to exist" after the advertised beam angle...
A sound pulse is strongest at it's very center axis line...and continuously decreases as it moves out from the main axis..
This "ever-decreasing" intensity of the sound pulse continues moving outward until 0db (absolute silence)...
The
"measured beam angle" is just where the specific brand measures this "ever-decreasing" sound pulse...
Here is the depiction of the right SI beam sound pulse (in its entirety from beginning to 0db absolute silence...)
(The black line 30° from horizontal is the right SI sound pulse "Main Axis"...)
Visualizing a mirrored matching left SI beam sound pulse to the left in the depiction would definetly show the left and right SI sound pulses overlapping at the bottom...in either/both of the 455kHz or the 800kHz sound pulse...
Rickie