Right BATCO here is a theory from a thick civi pilot.
I think the C130 engines are single shaft or direct shaft engines.
This means that the whole lot spins at a constant speed all the time. The props will be in the region of 1500-2000rpm and the turbine will be at about 21000rpm.
A jet aircraft at idle will be at about 21000rpm.
Most of the noise off a turboprop is the props thrashing the air into submission and it has quiet a large angular section which it radiates out from as the fan isn't contained.
The jet fan is contained and has a more restricted cone of noise.
Now the frequency of the noise comes into play.
The props will be pumping out in the 25-35hz region and the Jet will be in the 350hz region.
Now the lower the frequency the better it can diffract round objects and reflect off them. So noise abatement walls etc won't be as effective as the sound will bend round them and also reflect off them with minimal power lost.
Also as well the lower the frequency the more efficient the air is of transporting it.
You can relate this to music with bass frequency's travelling further and being more surrounding than the higher frequency's sometimes you can hear only the bass with none of the rest of the music.
You will also get loud zones where the noise has been reflected off an object and an interference pattern has been set up so the noise seems to drop then get louder again. Again this is more likely to happen with low frequency's.
So I reckon its a mixture of the cone of noise being larger around the aircraft virtually 360 degrees. The frequency which its at being better at travelling. The frequency allowing better diffraction and reflection. And I presume most of the noise abatement constructions being designed for jet frequency's of noise.
As a suggestion they could look at dykes of earth with enough width to reflect upwards that frequency instead of diffracting round they need to be flat topped as well otherwise the sound will curl round over the top of them and the backside soft so as not to transmit.