[OldTownPilot]

Alright, The RPM of the engine is limited by not wanting supersonic relative airflow at the fan blade tips. Assuming that the airframes have the same forward velocity, the larger engine diameter has to spin slower. This internal rotation speed determines the frequency of the sound. The faster it spins, the higher pitched it is. Alot of people say an engine it 'big' when it just has a large N1 fan or bypass fan. Thrust is made by high velocity air that exits the turbine section. This bypass layer increases the amount of area that the shearing of the air takes place, a la a shallower shear gradient. This shearing produces the 'sharp' noise that can be VERY loud. Also with a larger fan area you get a higher mass flow rate through the engine, and to produce the same amount of force the average exit velocity of the air can be a lot lower, thus also reducing noise.