How Does Fish Finder Work: Display Unit, Transducers, and Sonar

Fish Finder Guides

What is a fish finder and what does it do?

A fish finder is a device that uses sonar technology to detect fish, underwater structures, and the bottom. A fish finder generally has two components – a display unit and a transducer mounted on the boat.

fish finder display unit with transducer

The transducer acts as the transmitter and emits sound waves (also called SONAR) into the water, which bounce off objects and are received by the transducer. The fish finder display unit then analyzes these sound waves and displays them on the screen providing plenty of information such as:

  • Fish that appear as arches on 2D sonar
  • Bait Fish
  • Bottom depth
  • Bottom type (i.e. muddy or rocky) and transitions from hard to soft or vice versa
  • Fish holding structures like weeds, trees, brush piles, etc.
  • Water temperature

This information can help you choose the best bait and fishing spot and catch more fish.

What is a Transducer in a fish finder?

The transducer is mounted on the hull or transom of the boat and connected via cable with the fish finder head unit. Fish finder head unit also acts as a transmitter that sends electric pulses to the transducer. The transducer converts electric pulses into sound waves with the help of the piezoceramic element inside it.

transducer sonar beam and somar image side by side

The sound waves travel through the water, bounce off objects (fish, bottom, etc.), and return to the transducer. The transducer then converts the returning sound waves into electrical form. The returned signal depth and intensity are displayed as a Sonar image on the fish finder display unit.

The frequency of the sound waves emitted by the transducer affects how deep it can penetrate the water. Lower frequencies penetrate deeper than higher frequencies but provide less detail.

How Does Fish Finder measure the depth of Water?

The fish finder measures the time it takes for the sound waves to return to the transducer. By knowing the speed of sound in water and the time it takes for the sound wave to return, the fish finder can calculate the depth of water.

How Does a fish finder detect different structures and bottom hardness?

A fish finder uses the technology called Sonar (sound navigation and ranging) to detect fish, obstacles, and the bottom of lakes and oceans. The sound waves bounce off objects in the water and return to the fish finder. The fish finder then uses the returned impulse time and strength to create a sonar image.

Elite FS CHIRP Sonar

Soft structures like weeds, mud, and vegetation reflect the sound with less intensity. The returning signal will be weak because the mud absorbs most of the sound waves.

Hard structures like rocks and logs reflect more sound waves to the fish finder. The returned signal will be strong because the hard structure has absorbed few sound waves.

Based on this information fish finder creates a sonar image on the screen with different colors. Colors represent the intensity of the returned signal. You can read our article on how to read a fish finder for a detailed guide on interpreting sonar images.

How Does a fish finder Detect Fish?

fish air bladder illustration

The air bladder of the fish reflects the sound waves. The bigger the fish, the bigger the air bladder, and the stronger the return signal. A fish finder uses this information and displays the fish as arches on the sonar image.

fish under sonar beam illustration

Keep in mind a fish is represented as an arch in the best-case scenario when the fish is directly under the sonar cone, and the boat is also moving at a slow speed.

How Different Frequencies Work in Fish Finder

sonar frequency beam depth illustration

Traditionally, there are three main types of frequencies used in fish finder transducers: Low (50KHz), Medium (83KHz), and High(200KHz). Each has its own advantages and disadvantages, making it more suitable for certain conditions than others.

Low-frequency transducers use long waves to penetrate deep into the water. They are great for detecting large objects at greater depths (1000 feet plus) but struggle to pick up smaller targets closer to the surface.

Medium frequency transducers strike a balance between depth and target detection. They can reach down to around 300 feet (91 m) and also pick up smaller targets such as baitfish.

High-frequency transducers use short waves that don’t travel as far down into the water but can pick up very small targets at close range.

How Different Types of Sonar Work in Fish Finder

Traditional Fixed Frequency Sonar

Traditional sonar transmits an individual pulse and then measures the time it takes for the signal to bounce back. The speed of sound in water is nearly constant, so by measuring the time it takes for the sound to return, fixed frequency sonar can estimate both the depth of the water and the distance to any fish or other objects in the water.

Fixed frequency sonar is still used in many fish finders today because it is very simple and inexpensive to implement. However, there are some drawbacks. One is that since traditional sonar only emits a single pulse at a time, it can take several seconds to get a complete picture of what’s below. This can make it tough to find fast-moving targets like fish.

In addition, traditional sonar can be easily confused with dissolved minerals and other particles in saltwater, which can cause false readings.

Chirp Sonar

To overcome the limitations of traditional sonar, many modern fish finders use chirp sonar. Like traditional sonar, chirp sonar transmits a continuous range of frequencies rather than just a single frequency. By sweeping through a range of frequencies, chirp sonar can get a much more complete picture of what’s happening under the boat in a shorter time. This is especially useful for finding fast-moving targets like fish.

In addition, because chirp sonar transmits a range of frequencies, it is less likely to be confused with dissolved minerals and other particles in saltwater. This results in more accurate readings and fewer false positives.

Side Imaging Sonar

In addition to traditional sonar and chirp sonar, many fish finders also feature side imaging sonar. Side imaging sonar operates at a higher frequency (455/800 kHz) and creates picture-like images of the bottom.

Side imaging explained

However, side imaging scans to the left and right of the boat rather than scanning directly underneath the boat.

This creates a much wider view of the area around the boat, which can be very useful for finding fish or other underwater structures. In addition, since side imaging sonar scans a wider area, it can cover more ground in a shorter amount of time, which can be helpful when trying to find fish in a large body of water.

Down Imaging Sonar

tree branch visible in down imaging

Down imaging sonar is similar to side imaging sonar in that it operates at high frequencies and creates picture-like images of the bottom. However, rather than scanning to the left and right of the boat, down imaging sonar scans directly beneath the boat. This provides a detailed view of the area immediately below the boat, which can be very useful for finding fish or other underwater structures.