Sonar Index

Simulation of Sidescan Transducer Arrays

Page: 12,  3

Simulation af a damaged and a bad Sidescan Sonar Transducer

Now we can also have a look at how the beampattern is altered in a transducer with one dead piezoceramic element or where the elements are mounted with a too large space in between.
The first figure again is our perfect sidescan sonar transducer we already saw on page 1 and its beampattern at 50kHz. The array is 610mm long, consisting of 6 inividual piezo-elements measuring 100mm x 25mm at the side facing the water, the elements are spaced at 2mm.

sidescan sonar transducer
Figure 1a: Sidescan Sonar Transducer Array, 610mm x 25mm

Sidescan Transducer: Polar Plot
Figure 1b: polar plot for the calculated horizontal energy distribution of the sidescan transducer in Fig.1a working at 50kHz.
89% of the emitted energy is concentrated in the central lobe.

What happens to the beampattern if one transducer element is broken as in Fig.4a, for instance by a broken electrical connection or because it loosened from the backing bar?
broken sidescan sonar transducer
Figure 4a: Sidescan Sonar Transducer with a broken element

polar plot of broken sidescan sonar transducer
Figure 4b: polar plot for the sidescan transducer from Fig.4a

As can be seen in the polar plot, the main change, as to be expected, is in stronger sidelobes. With our original transducer concentrating 89% of the emitted energy in the central lobe, this drops to 78% for the transducer with one missing element.

When building a sidescan sonar transducer, one should not space the individual elements more than 1/4 wavelength (statement of
Sture Hultqvist in the Geotech Sidescan Sonar Forum). But what happens if we space them more than 1/4 wavelength, that is 7.5mm for 50kHz? The next simulation for our 610mm transducer, this time with the individual elements spaced 0.5 wavelength from each other, gives an answer: Again the difference is in the sidelobes, as can be seen in Fig.5b. Because of the spacing, more energy is lost in the sidelobes, and the percentage of energy concentrated in the central lobe falls to 77%.

sidescan transducer with too large space between elements
Figure 5a: 610mm long Sidescan Sonar Transducer with 15mm (i.e. 1/2 wavelength for 50kHz) space in between the individual elements

polar plot of spaced sidescan sonar transducer
Figure 5b: polar plot for the sidescan transducer with the elements spaced 0.5 wavelength

What we can also see as a result of these simulations is, that both, the slightly damaged as well as the "bad" sidescan sonar transducer with the large spacing of the elements, would both still be considerably better than the optimized fishfinder sidescan sonar transducer.