During the past three decades, there has been a growing concern over the in­ crease in noise pollution that comes as a direct result of the increased volume of automobile traffic, high-speed trains, and larger aircraft. Additional sources of noise are commonly found in air handling equipment (such as fans and pro­ pellers) and a variety of machinery used in construction and manufacturing. A vast majority of these noise sources are the result of a given systems aero­ acoustic response, or sound generated by the interaction of a flow field with the given structure. While barriers are commonly used to shield communities from highway and train noise, and absorption materials are used to shield machinery noise, there is no way to shield communities near major airports from the noise gen­ erated by low-flying aircraft. Tens of millions of people worldwide are affected by this airport noise problem. In densely populated Europe, up to 15 % of the total population is strongly influenced by airport noise. Since the volume of air traffic will continue to grow, so too will the problem and the number of people involved. It is not surprising that many countries and communities have taken legal action to preserve the quality of life in these areas. As a result, the airlines, airports, manufacturers and governments are working together to set new standards for aircraft noise reduction. In order to establish realistic goals, the generation and propagation of acoustic sources must be better understood.