Jamieson, Fausset and Brown on 1 Samuel 26:13-20

13-20. Then David ... stood on the top of an hill afar off ... and cried to the people--(See on Jud 9:7). The extraordinary purity and elasticity of the air in Palestine enable words to be distinctly heard that are addressed by a speaker from the top of one hill to people on that of another, from which it is separated by a deep intervening ravine. Hostile parties can thus speak to each other, while completely beyond the reach of each other's attack. It results from the peculiar features of the country in many of the mountain districts.