Why are propeller planes so loud if they're so far away?

Most of the time propeller planes do not have exhaust mufflers. this makes the engine extremely loud. and, since the airplane is on the ground, the earth acts as “bouncing board” for the airplanes loud engine. this bouncing effect makes the engines noise seem louder as it also bounces off of nearby objects, like a wall. in the air, propeller airplanes are not this loud, since they are away from you, away from the ground and in open space.

cavitation?

The human brain finds certain frequncies more annoying than others so if if it is technically quieter than sometheing else dB wise, the brain thinks it sounds louder.

Propeller aeroplanes do not usually have mufflers, as John mentioned above, but they also have much larger capacity engines than trucks and buses. The engine is not the main source of noise though, the propeller itself is. Since planes rely on moving air past the engine to generate propulsion, they will always be somewhat noisy. A propeller plane at cruising speed may be moving 500km/h forward, but the tips of the propeller blades will be moving much faster. If you have ever seen an aerobatics show, when the plane is on high thrust it becomes even louder as the tips of the propeller blades break the sound barrier – even if the plane itself is far from that speed.

It should also be noted that the noise is a low frequency, and low frequency sounds can travel much further than the high pitched ones.

Another factor that hasn’t been mentioned is that the sound travels directly from the aircraft to the listener—it’s not broken up by obstructions & reflections such as an engine at ground level would be, at those distances.