Why are earlier High-Bypass Jet Engines so loud? The JT9D is so loud, but the BR715 is so quiet. What makes a Jet Engine so quiet/loud. Look at the JT8D-9, and the JT8D-209, The -209 is much quieter than the basic JT8D... Why?
It depends on how much of the air is bypassed. On the newer generation of engines-RB211/CFM56 etc the majority of air that bypasses the core of the engine whereas with the older JT8D most of the air went through the engine core and so created more noise. The 737-200 compared to the 737-300 engines produce very little bypass.
On the early jets most of the noise was generated by the high temp & shearing action on the surrounding air as the gas stream came out of the jet pipe. The thrust was achieved by accelerating a small mass of air to high velocity.
Todays high by-pass engines move a large mass of air at low temp & velocity, hence low noise.