Sonar Principles:- In part one I concentrated on the head unit s display size, resolution and brightness thus highlighting how important it is to understand these features before actually buying a Sonar, GPS or Combo unit. In this article I m going to stay with the head unit discussing power, target separation and frequency. The reason for this is because the head unit is the source, much like an amplifier for your Hi-fi sound system at home. In fact there are many similarities that can be drawn between Sonar and a Hi-fi, not surprising though as they are both producing sound waves. I am not going to go into the whole science and technology aspect of sonar, ultimately just flaunting my Google search, copy and paste skills. But rather, will attempt to put to pen, or keyboard in this case, what all that technology actually means to the angler. If the finer aspects of sonar appeal to you, I strongly suggest that you download and read Luke Morris s article Deciphering Sonar Charts available on the net. A typical specification sheet would look like this:- SONAR / SOUNDER Sonar Output Power Sonar Frequency Transducer Type Max 250W/32W RMS actual; 30,000/3,750 RMS analog equivalent 50/200 or 83/200 khz Choice of dual frequency Skimmer SIDE / DOWN SCAN SOUNDER Sonar Frequency Sonar Output Power Transducer Type 455kHz and 800kHz khz RMS: 500W, MAX: 4000W 3 X Dual Crystal for Left, Down & Right Power:- Power is the strength behind the sound wave and is measured in Watts, and is either displayed as RMS, Max or both. RMS is the average sustainable power output whereas Max is the peak output it is capable of for a very short period of time. Broadband power however is a bit of a grey area that I cannot find too many facts other than a bunch of opinions and an extract from the following paper:- Innovative Power Amplifier and Transducer Technologies for High Frequency, Broadband Sonar Arrays:- The purpose of the High Frequency Broadband Array Program, sponsored by the Office of Naval Research of the US Navy, was to explore the use of the most advanced electronic and transducer technologies together with modern software techniques in a sonar array. Although the specific frequency ranges cannot be disclosed, the frequency is higher than most human hearing and the bandwidth is multi-octave. One design goal was a compact, scalable, complete system in a box that could be contained in an enclosure on the outside of a vessel with minimum number of hull penetrations. M.P Andre The easiest way to explain this technology would be to compare the conventional high power sonar to a mid 1970 s loud V8 American muscle car. Broadband sonar on the other hand can be compared to that of a modern high spec tuner car, capable of mind numbing speeds with motors a fraction of the size of the old V8. The acoustically perfect and near stealth like ping of the Lowrance HDS Broadband system literally comes into a world of it s own when drop shotting directly below the transducer to big bass holding on deep structure, or covering large areas looking for deep isolated structure. Side and Down Scan sonar needs a bit more power however to pump those very high frequency sound waves as far as possible to produce those amazing life like bottom and structure features.
Target Separation is another term that pops up quite frequently on a sonar specification data sheet. Target separation is the distance between two objects within the sonar cone or beam, which a transducer and display can identify as two separate targets. The smaller the target separation, the clearer your readings will be. The higher the frequency, the better the target separation will be, as can be seen in the image of the 200 khz diagram below. In contrast, the wide arches created by the 83 khz echoes from the stump have effectively wiped out everything else. Cones & Beams:- A cone is exactly as the name implies, it is just the angle of the cone that varies with the different frequencies such as the 20 of the 200 khz and 60 of the 83 khz. The high frequency beams of the 455 and 800 khz frequencies are more like a wide Chinese fan. The returns from the high frequency beams can be described as thin slices of the bottom making up a very detailed image with exceptional target separation, particularly with the 800 khz frequency. Transducer Properties:- The 200 khz transducer has to be the most common and well known transducer frequency, and if compared to the sound system in your car or home, they would be the mid-range speakers. But if you want that deep power or crystal clear sharpness, like that of your sound system s sub woofer and tweeters, you are going to need not only a dual frequency transducer, but two transducers each with dual frequency properties. 200 khz:- Sound wave shape Cone Cone Angle 20 Depth to coverage ratio 4:1 Pros 1. Good target separation 2. The smaller coverage area also means that whatever you see on your display is directly beneath the boat. This is important for accurate capturing of waypoints and logging data for DrDepth.
Cons 1. The small coverage area is not effective at all when it comes to finding new structure or locating fish quickly. When looking from the side it is easy to see that the boat passed over an old river channel drop-off with a fish at the top of the drop off, a stump with a fish holding on it, a bait ball with predators nearby, and a brushpile with fish holding in and around it. But when looking directly down the path of the narrow cone, you will notice that the angler would never have even known that he had missed two of the best points of interest in that entire area. 83 khz:- Sound wave shape Cone Cone Angle 60 Depth to coverage ratio 1:1 Pros 1. Four times the coverage area of the 20 cone makes the 83 khz a highly efficient fish and structure finding frequency. Cons 1. Poor target separation. 2. Due to such a large coverage area, accuracy is sacrificed when taking waypoints from returns on the sonar. 3. Loss of bottom detail when logging for DrDepth. 455 / 800 khz:- Sound wave shape Beam Beam Angle (fore to aft) 2 Beam Angle (DownScan) 48 Beam Angle (SideScan Left) 60
Beam Angle (SideScan Right) 60 Pros 1. Exceptional target separation offering high definition 3D like imagery of the bottom created by the contrast in bottom, structure and sonic shadows created. 2. Very wide coverage area from side to side up to more than 400ft, making it very easy to find unique bottom contours, structure and shoaling fish. 3. The Lowrance StructureScan (LSS-1) has a very unique transducer in that it has a third pair of crystals in the transducer offering true High Definition directly down looking high frequency sonar. This takes all the interpretation out of conventional down looking sonar with its crisp detail of individual branches of trees and brushpiles, and even grass / weed beds. 4. Monitor amazing bottom detail and depth at speeds of over 70kph with Lowrance s true DownScan. 5. Create mosaic overlays on your 3D bathymetry charts with DrDepth SideScan
Cons 1. Isolated fish are sometimes difficult to spot as they are represented by merely a single dot, or drowned out entirely by the strong side returns of the bottom. This is why having a split screen between DownScan and 83 khz is in most cases the most powerful hunting combination offering the definition of the 800 khz beam and the fish finding ability of the 83 khz cone side by side. 2. Very slow speeds are required to get a near 1:1 ratio on the high frequency returns, sometimes as low as only 1.5kph. This means that once you have spotted a point of interest (POI) you will have to take a second pass on it while out of gear and just drifting slowly over it. For any enquiries please contact me on 0825582719, or email me on fishton@fishtec.co.za, or visit my webpage www.fishtec.co.za.
Innovative Power Amplifier and Transducer Technologies for High Frequency, Broadband Sonar Arrays:- By K.R Erikson, G. Zipfel, S.C Butler, G.S. Edelson and E.M Will The purpose of the High Frequency Broadband Array Program, sponsored by the Office of Naval Research of the US Navy, was to explore the use of the most advanced electronic and transducer technologies together with modern software techniques in a sonar array. Although the specific frequency ranges cannot be disclosed, the frequency is higher than most human hearing and the bandwidth is multi-octave. One design goal was a compact, scalable, complete system in a box that could be contained in an enclosure on the outside of a vessel with minimum number of hull penetrations. M.P Andre