Gone Fishing

Fish finder, Depth finder, Sonar

Some explanation of terms used in fish finder literature along with a collection of tutorials and information that can be found on the internet.

Speed of sound in water:

Speed of sound in water is approximately 4800 feet per second. Speed of sound in the water varies with salinity, temperature and depth. Depictions of salinity and temperature are shown in the adjourning graph and for varying depths, speed sound increase .056 ft/sec for every foot of depth. Industry has settled on this number as being an adequate compromise to cover all variables.

   Lowrance Tutorial Sonar     Chart tutorial    

Arches:

Arches are a product of cone angle size and the movement of the sonar cone over an object caused by boat forward speed. The manner in which arches are created is that the forward edge of the cone strikes the fish or object first as the boat is moving forward. The edge of the cone is at a greater distance than when the fish or object is directly underneath the cone. The return signal traces a response that starts at a deeper depth and gradually decreases in depth until the center of cone is reached and then the process is reverse, from a shallower to a deeper depth. The best arches are created when the object is driven over is directly in the center of the cone with wide cone angle. As the object is off-set to the side of the cone, the arch will have less definition.   

Large arches are generated with a large cone angle (i.e. 45 degrees) because the edge of the cone is much further in distance than 10-degree cone. With an object at a depth of 30 ft and cone of 45 degrees (-3dB), the arch will be 29 inches high and if the cone angle is extrapolate to -10dB point, the arch would be 149 inches or 12 ft. in height as seen on the fishfinder's display.

The -3 dB versus the -10 dB cone size specification debate:

Most manufactures specify the -3 dB points for the cone size; others specify the -10 dB points. There seems to be a movement of manufactures in adopting the -10 dB points as the cone size. In most cases the cone is effective to -10 dB points, with the exceptions of the deepest waters.  In other words, there will be a response that is visible with today's fishfinders that have multilevel gray scales and color machines to a return signals that correspond to the -10 dB points. The -3 dB, half power point, has been a longstanding standard measurement point for electrical performance. There is no set rule that one has to use the -3 dB point for specifying the cone size. There is an approximate relationship between the -3 dB to the -10 dB points for cone size, that is approximately 1:2 ratio. The cone size at the -10 dB points will be approximately twice as large as the -3 dB points. The -10 dB response closely defines the maximum width of the acoustic cone.

The ability to view arches which are produced from a narrow cone size (i.e. 10 degrees, -3 dB) are dependent on acoustic and pixel resolution, and zoom scale of the particular fishfinders. An object at a depth 30 ft. and cone of 10 degrees (-3dB), the arch will be 5.6 inches in height; this assumes that there is a response to the -10 dB points.  To see arches with a narrow cone angle like 10 degrees, the fishfinder needs to have a high definition zoom scales of like (10 feet). Good acoustic resolution is also needed to discern the small change in depth.  Acoustic resolution in the range of 1 inch would provide a decent arch along with a high pixel count to display the high acoustic resolution using a zoom scale of 10 ft.

Another interesting thing that can be determined from arches is the relative angle of the cone entering the water. If the front and rear sides of the arches are the same length, than the transducer is parallel to the bottom. If the front side of the arch is longer, the transducer is tilted forward. The opposite is true for a rear-tilted transducer.

Cone angle:

The polar graph, is a portrayal of a typical transducer response, showing a -3 dB cone width of approximately 16 degrees and a - 10 dB cone width of approximately 36 degrees. The side lobes are will attenuated to a -20 dB level. The sidelobes of other transducer of various cone widths can be higher or lower from this depiction. The smaller the cone angles, the more power that will be concentrated in a smaller area for a given output power level, thereby providing greater depth penetration.    Cone Angle      Airmar

Frequency: 

Depth penetration is inversely proportional to the frequency of the acoustic signal. For the same output power level, a 50 kHz transducer will penetrate deeper than a transducer operating at 200 kHz.

Acoustic Resolution:

Acoustic resolution is inversely proportional to cone angle and pulse width. A Short pulse width provides the ability to discern objects that are spaced close together in height in the water column. Some fishfinder can resolve objects separated in height by as little as 0.75 inches, which would equate to pulse of about 2 cycles of 200 kHz. The Furuno, FCV600L, color machine, has a maximum acoustic resolution (target) of 7.2 inches and a minimum of 178 inches depending on the depth range scale selected. Some Pinpoint machines have an acoustic resolution (target) of 1 in and some Bottom Line units have a resolution of 0.75. Most manufactures do not disclose the pulse width or resolution. The smaller the cone angle, the less area that is being illuminate, produces the best resolution because there is less area that is being integrated into the return pulse.

There are several wide coverage techniques to locate fish. One is using a side looking sonar; another is using multibeam or scanning sonars and lastly wide beam sonar.

Interphase                                       Scanning

Bottom Line                                     Side finding, Narrow and wide beam

Humminbird                                     Multibeam, Narrow and wide beam, Side Scan

Lowrance                                         Narrow and wide beam

Raymarine                                       Narrow and wide beam

Vexilar                                              Narrow and wide beam

Furuno                                             Narrow and wide beam

Garmin                                             Narrow and wide beam

Viewing Resolution:

Viewing resolution is defined by the characteristics of a particular display. The manufacture will give the number of vertical and horizontal pixels for a particular fishfinder. Now you will need to determine the vertical distance in inches and the vertical number of pixels. Now do the same for the horizontal axis. This will provide vertical and horizontal pixels per inch. Multiply the two, which gives the density, pixels per square inch. The more density, the more potential viewing resolution.

Viewing Resolution is directly proportional to the number of vertical pixels and the horizontal shift speed. The following table describes the visual resolution for fishfinder depths scales due to pixel count that has 240 vertical pixels. The next table provides the feet cover for a particular boat speed and the feet travel, (elapsed time) that it take to traverse the display screen, for an update rate of 8 times per second and for a 400 pixel horizontal display.

The point is that vertical pixel count, pixel/in, is not the sole determination of resolution of the fishfinder. Target, (acoustic resolution), zoom type, cone angle size, gray scale or color all play a role as to acceptability of a machine for one’s needs.

Minimum displayed water depth:  

Minimum displayed water depth is dependent on the sonar pulse decay time. The sonar pulse is very short in duration,( pulse width), but it take a relative long time for the pulse to decay to a level that is less than the sensitivity of the transducer in receive mode. The fishfinder blanks the receiver during this time therefore; it does not start to collect return echoes until the blanking is removed. The Bottom Line NCC 6500 has a minimum displayed water depth of 1.6 feet.

P-P vs. RMS power of acoustic signals:

The transducer converts electrical to acoustic energy by displacing the water in a positive and negative motion about a static pressure that exits in the fluid and would propagating at the speed of sound  in the medium. P-P power of the acoustic pulse is what does the work and the RMS value is a mathematical calculation extrapolated from the measured P-P power. The mathematical power relationship between P-P to RMS is 8:1 ratio in a CW (continuous wave) situation. When comparing output power of one fishfinders versus another; always compare P-P to P-P or RMS to RMS.

 Bottom Line LCC 6500 Sonar Specification: