The EK80 transmits high energy sound wave pulses into the sea. A flat bottom reflects the transmitted wave as if it was a mirror. The propagating energy is spread over a larger and larger area as it travels down to the bottom and up again. The energy is spread over a four times larger area every time the travel distance doubles.

When a sound pulse is travelling from the transducer through the water it will undergo geometrical spreading.

A large school of fish reflects sound waves similarly. This type of spreading is referred to as square-law or 20 log TVG (Time Variable Gain) spreading.

When a school of fish covers the whole acoustic beam cross section, the received echo will increase with range simply because the acoustic beam cross section increases with range.

In addition to the geometrical spreading, sound energy is absorbed in water. Absorption is higher in salt than in fresh water and increases with the operating frequency. The received echo must also be compensated according to the absorption.