Jet noise of subsonic aircraft: An aeroengine manufacturer’s perspective

Abstract

These papers trace the history of jet noise reduction from early pure jets to modern engines with high bypass ratios. Choice of engine cycle is important in reducing jet noise, but there is also the need to balance the requirements of cost, weight, and specific fuel consumption, in addition to minimizing other engine noise sources. Some techniques for reducing jet noise are illustrated. Forced mixers on long-cowl engines are effective in reducing low-frequency jet noise, but can cause an increase in high frequencies (especially in flight). For larger aircraft with more powerful engines, where the noise problem is most severe, installation issues dictate somewhat against long-cowl configurations. The current challenge is therefore to achieve similar noise reductions to forced mixers but on short-cowl engines. In the future, the requirement for further noise reductions might result in much more novel aeroplane and engine configurations. Jet noise experiments can easily give misleading results unless the facilities and rigs are designed to avoid contamination by excess noise and to provide correct simulation of boundary layers and other aerodynamic parameters (including flight simulation). Fortunately, testing at reduced scale is acceptable for noise. Advanced measurement techniques (like source location) can be very beneficial in aiding understanding.