Reduction of peak jet noise using jet refraction

The directivity pattern of sound generated by a subsonic jet exhibits a peak at angles between 15 and 45 degrees to the jet axis, decreasing to a minimum on the jet axis. Theoretical and experimental investigations have previously demonstrated that this minimum is due to refraction of sound by the jet flow. The present experimental study considers the effect of the introduction of a second jet of smaller exit area near to the main jet. It is shown that the small jet imposes its own heart-shaped directivity pattern on that of the first jet. Alteration of the angle between the two jets leads to a reduction in the peak sound level. Directivity patterns for unfiltered and filtered measurements are presented for changes in the jet velocities and temperature ratios. Calculations of the perceived noise level radiated by one and two jets of larger scale are made. The change in ground area effected by various noise levels are given for an aircraft taking-off using a number of alignments of auxiliary engines to alter the pattern of radiated sound.