Characteristics of the decay of jet noise in the free field are discussed, and crude estimates of the free-field spectrum of jet noise are made from available experimental data. The theory of far-field noise decay in the presence of specular ground reflections is expanded. By using the decay theory and spectrum estimates, decay characteristics at constant height above a reflecting plane are computed for various values of ground impedance and receiver band-pass width. The effects on decay of spectrum shape and nonideal filtering are studied.