The Noise Shielding Effects of Multiple Jets

A jet located between a sound source and an observer can shield the sound from reaching the observer. In a flyby event of a multiple-engine aircraft, this phenomenon affects the sideline noise levels. The jet shielding effects were investigated experimentally and analytically. The model test included up to four nozzles with various placement arrangements and jet gas conditions. A simple model of a point source in an ambient flow outside a jet of constant flow was analyzed. The analytical results showed agreement with test data. The shielding effects depend upon nozzle placement, jet condition, frequency and angle. Both the analytical and the experimental results showed that the jet shielding effect can extend into regions where all the jets are visible to the observer. A shielding effect prediction procedure was developed. The procedure can be used to study the engine placement and power setting arrangements that provide the most effective shielding of aircraft sideline noise.