Sonar Frequency

Frequency is measured in Hertz (HZ) where 1 HZ is equal to one cycle (pulse) per second. When talking fish finder frequencies, low frequency waves pulse at about 50 KHZ or 50 thousand cycles per second.

Fish finders operate at variations of two frequencies: Low frequency 50 KHZ and high frequency 200 KHZ. Variations on the low end might be 83 KHz and on the high frequency end 192 KHZ. The number by itself isn’t critical, but it’s important to understand low frequency and high frequency differences.

According to Lowrance® LCX104C user manuals 80% of fishing needs can be meet by a 200 KHZ unit using a 12-degree cone angle (beam). The other 20% of usage is in saltwater from 300 – 500 feet or more, and by downrigger users who want to know where their baits are located. Presumably, because it is the Lowrance default angle, the optimal cone angle is 20 degrees on single frequency 200 KHZ units.

Low frequency operation allows more energy transfer with each transmitted acoustic wave and enables the wave to travel longer distances before being reflected or absorbed by the surroundings. High frequency acoustic waves travel shorter distances but produce more precise readings on a fish finder.

To demonstrate the concept in every day terms, think about high (powered) volume music played in a car. While sitting in an adjacent car you may hear the low frequency bass tones, but not the higher frequency and higher fidelity tones. Higher frequency tones are more frequent, more varied, and more dispersed and do not travel as far because each wave has less energy.

Fish finder electronics deliver voltage to a transducer (commonly mounted on the transom of the boat). The transducer converts the pulses to an acoustic waveform, which travels through the water at a specified frequency. It then collects reflected sound waves.

Saltwater absorbs acoustic energy faster than freshwater. Adding to the problem, objects such as plankton and small debris suspend more easily in saltwater and absorb even more energy. Low frequency fish finders are usually better in saltwater using wide cone angles at low frequency. This means they can look at more area and volume under the boat. Saltwater fishermen find this feature useful to track down riggers and see targets in a wider area.

Freshwater boaters not having to contend with excessive signal absorption should be fine with the more popular 200 KHZ high frequency readings. High frequency operation provides better target definition.

Frequency alone does not determine ultimate results, cone angle and power output also play a significant role in finding fish.

Crystals are specially tuned to certain frequencies and must be matched exactly with the fish finder device. 192 KHZ crystals don’t work with 200 KHZ devices.