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Well, I strongly disagree with the sentence "Turbojet and turbofan engine designs have helped by increasing bypass ratios, thereby improving efficiency and dramatically reducing sound". That is simply not true.

I live near an airport and I know the sounds of all the airplanes that buzz around here. Take for example the GE90 used by the Boeing 777 airplane. This is a high-bypass turbofan aircraft engine and it is much louder than than other engines. The high-bypass reduces the rumbling of the engines a bit and certainly not dramatically. Unfortunately it generates and extremly loud high-pitched whistling sound, making the noise even more unbearable than those of engines with a lower bypass value.

The Boeing 737-800, which uses CFM56 engines (high-bypass ratio, you guessed it) is *extremely* loud. It is one of the most annoying planes that are in use. It is even louder than some 4 engine planes.The noise is much louder than the one Airbus A320 classic engine planes. It is the loudest small plane that I know of.

The same holds true for the much-praised Airbus "neo" engines (PW1100G-JM and LEAP-1A). They are a bit less-noisy than the "ceo" engines but also have this extremely annoying whistling sound which make them even harder to bear, especially shortly after take off.

Even the Airbus 350 with it's Trent XWB with a "reduced acoustic mode scattering engine duct system (RAMSES)", a quieting engine and "zero splice" is quite loud.

I know of only two planes that are dramatically less noisy: Boeing 787 and Bombardier CS-100/-300 (now called Airbus 220). The Boeing 787 uses GEnx-1B and Rolls-Royce Trent 1000 engines. The CS-100/-300 uses PW-1500G engines. All of them are also high-bypass engines. So what is the difference? The B787 uses engine nacelles with chevrons. I do not know what makes the PW-1500G less noisy.

All-in-all it is not the high-bypass ratio that makes plane engines less noisy. That is simply not true.