Don't let the term fool you, "beam width" is not the width of the beam
of sound energy. The graph is showing energy (signal attenuation) versus
angle. So at 0 degrees the signal is strongest, about -10dB. As you move
out towards 15 degrees, the energy in the sound wave gets lower. But
then as you go from 15 to 20, the energy actually increases.
As Mark notes, and as the paper points out, "beamwidth" is used
illustratively. The caption of the figure uses the more common
technical term, propagration pattern. Similar patterns are used for many
types of radiators, like radio antennas. The peaks and valleys that make
the side lobes in the pattern are typical of antennas, microphones,
speakers, lasers, and other transducers -- basically anything that
The way to look at these is as if you're looking down on the earth, and
the transducer is parallel to the ground. Most sonars produce sound in
three dimensions, but you're only you're only looking at X and Y planes,
sort of a slice.
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