Sonar Index

Simulation of Sidescan Transducer Arrays

Page: 1,  2,  3

Figure 3a shows the sound projecting plane of an array of fishfinder transducers how they are typically set up by amateur sidescan makers to build a sidescan sonar. Since each transducer with 44mm diameter is enclosed in a housing with perhaps 60mm diameter the transducers are regularly spaced at this distance as a minimum.

180mm array
Figure 2a: Transducer Array, 180mm long

polar plot: 180mm array
Figure 2b: calculated polar plot for the 180mm transducer array of Fig. 2a working at 50kHz.
58% of the emitted energy is concentrated in the central lobe that is 6.3° wide.

soundfield 180mm array

Figure 2c: two-dimensional energy distribution (soundfield) of the transducer in Fig.2a Fieldsize: 120° x 120°, 25dB dynamic from white to black

As to be expected the soundpattern looks much more "sidescan sonar like", but in comparison to the soundpattern of the real sidescan transducer of Fig. 1a  we discern some disadvantages of such a setup: The central lobe is much broader than the one in Fig. 1b and very strong sidelobes arise. Strong sidelobes may lead to ghost-pattern and a loss of contrast in the images and, what is even more important, less amount of energy is concentrated in the central lobe leading to a reduced range.
To obtain a narrower central lobe we could distribute the four transducer in a longer array, but as can be seen in the resulting Beampattern (Figures 3b and 3c) this leads to even more energy being lost in the sidelobes.

300mm array
Figure 3a: 300mm long transducer array

polar plot: 300mm array
Figure 3b: polar plot for 300mm transducer array of Fig. 3a working at 50kHz.
Only 35% of the emitted energy is concentrated in the 3.4° wide central lobe, 65% is spread over the sidelobes.

soundfield of 300mm transducer

Figure 3c: 2-D soundfield of the transducer in Fig.3a at 50kHz, Fieldsize: 120° x 120°, 25dB dynamic

Although the transducer is far from being perfect we could build a fishfinder-array this way or the way in Fig. 2a and try how it performs, but as I will show we can do it much better with little changes.
From the optical analogon of the diffraction grating we know that we have to space the diffracting structures very regularly to concentrate as much energy as possible in the sidelobes (called first, second, ... order in optics); only in these sidelobes the desired wavelength dispersion occurs. Transferred to our transducer this means that, if we are forced to use more or less widely spaced individual transducers in a row, it is not a good idea to space them regularly. An irregularly spaced array should help to suppress the sidelobes. I ran some simulations to check this idea.

irregularly spaced 300mm array
Figure 3a: Irregularly spaced transducer array, 300mm long

polar plot: 300mm modified array
Figure 3b: polar plot for 300mm irregularly spaced transducer array of Fig. 3a working at 50kHz.
72% of the emitted energy is concentrated in the 3.4° wide central lobe.

soundfield of modified 300mm array

Figure 3c: 2-D soundfield of the irregularly spaced transducer in Fig.3a Fieldsize: 120° x 120°, dynamik of 25 dB

The array displayed in Fig.3a is, with the outer transducers spaced at 300mm, as long as the one in Fig.2a, but the two inner transducers are arranged in a way that there are no regular distances. If we investigate the simulated beampattern we find that the central lobe is as small as for the regularly spaced 300mm array, but the sidelobes are much weaker. The energy concentrated in the central lobe with 72% is twice as high as for the evenly spaced 300mm array that was 35%, and it is even considerably higher than for the 180 mm array that gave 58%! By simply spacing the transducers irregularly we only have advantages, a narrower and stronger central lobe and weaker sidelobes. This undoubtedly will lead to a transducer that gives a better horizontal resolution, stronger signals, higher range and better contrast.
We will certainly never reach the contrast and range of a good sidescan transducer, since a considerable larger amount of energy is lost in the sidelobes of our array and we will always have a diffuse background from the sidelobes that lowers the contrast, but this is the price we have to pay for the advantage of a cheap system compared to a semi-professional or professiomal sidescan sonar system.
The 300mm array shown in Fig.3a is already optimized concerning the positions of the transducers. I ran a lot of simulations on the try and error principle to find this geometry to be the best. With the left transducer being the number one and the right the number four we have 1->4 = 300mm, 1->3 = 150mm, and 1->2 = 65mm, the numbers given are always measured center to center.This kind of Transducer Array is called a "Thinned Array".

Running the array at 200kHz
There is more good news concerning the arrays: if it is optimized for 50kHz it is also optimized for other frequencies like 200kHz. Figure 3d shows the beampattern and soundfield of the transducer from Fig. 3a running at 200kHz. What is also clearly visible is the narrow vertical width of the central lobe leading to a need of an exact adjustment of the downtild angle; but even then the narrow vertical beam may lead to restrictions in the imaged portion of the seafloor.

300mm modified array at 200khz
Figure 3d: 300mm transducer array of Fig. 3a working at 200kHz.
72% of the emitted energy is concentrated in the 0.9° wide central lobe. The inset shows the 2D soundfield at a fieldsize of 120°x120° and 25dB dynamic

Making it longer?
The wish for a narrow central lobe may lead to the idea to make the array even longer, but this always leads to less energy concentrated in the central lobe. An optimized transducer with 400mm length turned out to be one with the values
1->4 = 400mm, 1->3 = 205mm, and 1->2 = 95mm; the central lobe holds 55% and is 2.7° wide at the -3dB point. An optimized transducer of 500mm length gives a central lobe that, with 2.3° is nearly as narrow as the one of the perfect sidescan mentioned at the beginning, but it holds only 42% of the energy. The distance values are: 500mm, 290mm and 100mm. A way out of this dilemma could only be to use more transducer, for instance six as a first try.

Practical Tests
I am planning to build a system on this principle, I already have many components, but time is always a problem, as most of us will know. If I have any results I will publish them here.

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Simulation af a damaged and a "bad" Sidescan Sonar Transducer