If we think of a wave spreading out from a rock that is thrown into a pond, the further from the source, the bigger the circle formed by the wave. As the circle gets bigger, its total length (circumference) also gets bigger.
Spreading loss occurs because the total amount of energy in a wave remains the same as it spreads out from a source. When the circle of a surface wave gets bigger the energy spreads to fill it. Therefore, the energy per unit length of the wave must get smaller. The height of the surface wave (amplitude) decreases as the energy per unit length of the wave crest gets smaller.
You can see something similar to spreading loss when you blow bubbles with chewing gum. Have you ever watched the bubble as it grows bigger? How does it change? Just as the total amount of energy in a sound wave doesnÕt change as it spreads out, the total amount of chewing gum doesnÕt change as the bubble gets bigger. This means that as the bubble grows, the walls of the bubble must get thinner and thinner. The thickness of the gum is similar to the amplitude of the sound wave. Just as the gum gets thinner as the bubble gets bigger, the amplitude of the sound wave decreases as it spreads out.