sonar graph

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 generates waves.

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.

-- Gordon

The graph shows beam power vs. angle normalized to on-axis power - so 0 degrees is 1/1 or 0dB, and for example - the first lobes at about 22 degrees off center are about 28dB down in power.

Kevin