Not being knowledgeable about the technical aspects of steam whistles, I can't answer your question. But you might find it interesting to know that when I was a kid in the mid-50's living next to the SP main through Hayward, CA, where the last of SP steam came through regularly, most engineers on Cab Forwards rarely whistled for the grade crossings, using the air horn instead. I expect that is because it was probably more effective in getting traffic to stop. There were very few crossing gates in our area at that time...just the typical flashers with bell.