Fig. 11.2.

Sender and receiver. The magpie on the left is sending a message. The large circle represents the power at which it was sent. That power can be diminished in various ways. Distance alone makes the amplitude of the sound diminish (called attenuation). Other factors can cause further attenuation or even distortion (grey squiggles). These may be invisible obstructions, such as sounds created by wind, updrafts, rain and fog, and a range of visible obstructions such as trees and shrubs, refracting the sound. The recipient of the sound may receive a signal that is much diminished (smaller circle), possibly distorted or interrupted (cf Kaplan7).