BIG WAVE, LITTLE WAVE
FISHFINDERS SEND OUT A “PING” via the transducer, which acts as both a speaker and a microphone. The ping travels through the water at around 3,300 mph, depending on temperature, until it hits something, then reflects back to the transducer, which relates the information to the fishfinder, which crunches a few time-difference numbers, and translates everything into a nice, neat picture on an
LCD screen. While all transducers shout, then listen, they send out their signals in differing frequencies, which travel through the water in
unique ways. Think of these frequencies like waves you see on the water’s surface. Let’s first understand the common, middle-of-the-road
200-kHz frequency waves utilized by standard sonar. Imagine standing on the shore of a perfectly still pond, with absolutely no waves. Now
throw a rock into it. The waves created by the rock travel over the water’s surface until they hit the bank, and get reflected back. But if they
hit a tree floating out in front of the shoreline, they get reflected back a tiny bit sooner – which is what happens when sonar waves hit a fish,
hovering just off the bottom.
Now, throw a little pebble into the pond. It makes much smaller waves, which don’t travel nearly as far as the ones the rock made. But
they’re so diminutive that they get reflected back by the tiniest things in the pond: a leaf, a twig, maybe even a bug that’s landed on the
water’s surface. These waves are how very-high-frequency sonar waves act when traveling through water, and why units utilizing those high
455/800-kHz beams can produce an extremely detailed picture.
Finally, throw a boulder in the pond. This creates low-frequency waves similar to the 50-kHz beams used to maximize depth penetration. These big waves
may travel for miles, rolling right over small items without reflecting back. In
other words, the detail level they deliver is very low.
Other factors affect how sonar peers through the depths. Cone angle, for
example, dictates how wide the sonar beams will be as they travel farther from
your boat. And power levels affect just how much “oomph” your transducer
has when it shouts out. Most of the differences between the modern fishfinders revolve around the use of frequency. So think back to those pebbles, rocks,
and boulders, and the different waves created by each.
Two Images Of A Tree: Sonar “pings” using shorter wavelengths of
sound allows for the bottom to be read in great detail. Right: Lower
frequency pings travel farther, and can provide clues about bottom
composition, but not with the same resolution.
HIGH FREQUENCY MID FREQUENCY