SYNOPSIS. Acoustic signals often mediate the mating process and are under selection through the action of female choice. Acoustic signalling requires relatively large amounts of energy input, but metabolic energy is coupled to acoustic energy inefficiently. Although not necessarily a cause and effect relationship, females often prefer signals with more energy. Females may prefer more intense calls, more complicated calls, or calls produced at a greater repetition rate. I discuss various evolutionary changes that could increase acoustic energy received by the female and examine how these changes are influenced by other factors inherent to communication systems: signal radiation, species recognition, sexual selection, the physiology of the receptor system, and environmental bioacoustics. I conclude that these factors constrain the ability of the animal to maximize energy received by the female. I then consider how two hypotheses, the good genes hypothesis and the runaway sexual selection hypothesis, attempt to explain the evolution of female choice for signals with greater energy content.