Humans have subjective hearing… not everyone thinks tha same about loudness of a noise. Humans find tonal noise the most annoying… and these occur in short sharp “blasts”… as in… whenever a propeller blade passes through a specific place of air. For instance, if you were stood on top of a propeller engine cowling, you would be most irritated by the noise from the propeller as it pased infront of your face. This can be considered the same for a fan… many blades in a duct, and also turbines.
For low bypass engines, the jet velocity is large, so the noise is large from here. For High bypass engines, there are lower jet velocities so less noise from here… however, high bypass engines have larger fans, so there is more noise from the fan in high bypass than low bypass engines.
If you were on the ground, then you would notice a low bypass engine more than a high bypass engine, this is because the noise from the back end of the engine in a low bypass engine propagates further before it decays to a normal level.
If you were on the ground and a high bypass engine flew overhead, then the sound you would notice mostly comes from the fan. This is the noise which propagates the furthest before decaying to a normal level. And because the fan produces tonal noise, it is this which annoys humas the most.
For the Boeing 777, the fan noise is going to be larger than a lower bypass engine… assuming the same sound absorption within the engine… this is because of the huge fan.
Overall… the high bypass engines throw out less noise than a low bypass engine. This is due to the air velocity being lower at the back end of the engine for a high bypass engine.
Thats my basic input.
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