I would also like to add I wish people kept the stock pipes on their dirt bikes too. Trails are being threatened by local land owners that are pissed off 'cause of the noise. I can't pretend I'm a little-goodie-two-shoes though. I own a KTM EXC 200 and even with the stock pipe on, 2-strokes are usually loud as heck. At the very least I take it easy while passing properties or residential area's instead of burning donuts in their backyard.
Trees have a wonderful ability to dampen noise, but this damping is proportional to f^1/3, where f is the frequency of the exhaust tone.
for example: a 200Hz exhaust note diminishes by 5.85dB/100m, whereas a 1200Hz note diminishes by 10.62dB/100m. (This doesn't give you an excuse to rev your engine really high to increase the exhaust note!
This is likely why some people have a greater issue with cruisers than sport bikes. Low notes penetrate surfaces, and are damped much less than high notes.
To put meaning to this, if you have a speaker that outputs 100dB, then you turn on another speaker right next to it, the sound level increases by 3dB to 103dB. (Simply doubling the sound energy increases the level by 3dB, so a difference in 4.77dB/100m is a BIG DEAL.)
Hope this sheds light on some of the mysteries of 'noise'
Source: Kinsler, Fundamentals of Acoustics