Sonar Technology Noise Rejection

Noise rejection is a signal processing feature built into fish finders that filters out unwanted interference from your sonar display. Electrical noise from trolling motors, bilge pumps, engine ignition systems, and other onboard electronics can create scattered dots, lines, or bands across your screen that obscure the actual sonar returns. Noise rejection algorithms identify and suppress these artifacts.

Most fish finders offer adjustable noise rejection levels, typically labeled as off, low, medium, and high. Running too little rejection lets interference clutter the display. Running too much can filter out weak but genuine returns — like a small fish or subtle structure change — along with the noise. The goal is to find the setting that eliminates obvious electrical interference without losing real targets.

Common noise sources include electrical interference from the boat's wiring, propeller cavitation creating air bubbles across the transducer face, turbulence from the hull at high speeds, and interference from other sonar units operating nearby on the same frequency. Each source produces a distinct pattern on screen that becomes recognizable with experience.

Beyond the built-in noise rejection setting, you can reduce noise at the source. Keep transducer wiring away from power cables and ignition wires. Use a dedicated battery or a filtered power cable for your electronics. Ensure your transducer is mounted in clean water flow with minimal turbulence. These installation practices do more for image quality than any software filter can accomplish on its own.