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I'm not sure but I think both EMD and GE have gone to two smaller turbos instead of one big turbo. Also, higher frequencies of sound are more directional and travel faster, while lower frequencies are non-directional and travel slower, so over distance the individual components of a sound "spread out". Sound waves slow down as they travel, so the faster waves catch up and pile up giving the effect of fading in and out. Individual waves also become longer with distance, expanding the relative scale of time within the sound; given enough distance you can hear the pulses of individual exhaust strokes. The higher frequencies remain more constant, while the lower frequencies will drop below hearing. The next time you hear a train from a long distance, think about it. Possibly why the EMD has more bass is because it is a two stroke of uniflow design. Air enters through ports in the bottom of the cylinder when the piston is passing BDC, and will exhaust from all four valves in the cylinder head during the exhaust portion of the piston's stroke. A two stroke's exhaust timing is also earlier than a four stroke's, while there is still pressure in the cylinder, the "pop" helps exhaust scavenging. In comparison, a four stroke engine exhausts through only two valves (w/4 valves per cyl.) and at or near atmospheric pressure. With half the valve area and no "pop", it's no problem because with a whole stroke for exhausting gases, there is plenty of time. A good analogy would be to cover the open bottom of a drum, and then drill two holes, listen to the drum and then drill two more holes and listen to the difference.
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