I know that most if not new GEVOs are equipped with the latest version of the K5HLL, or simply 3rd Generation K5HL. But couldn't a railroad order a 1st generation K5HL or 2nd generation K5HL?
Nathan does not differentiate between generations of horns, that is something only railfans do. And to be quite honest most railroads nowdays could care less as long as some sound at the right decibels come out. They all sound like garbage anyways.
Quick answer, it's a way of measuring the volume of sound.
Long answer: Decibels are a ratio between a given quantity and a reference. They're logarithmic, and are a tenth of a "bel," so called because of Bell Telephone. In our case, maximum dB for a train horn is 110 dB, which I assume is dBSPL. dBSPL's reference is 20 micro Pascals, as sound is a pressure wave. I really don't feel like writing out the equation to convert dB SPL into Pascals, but it comes out to =20log10(110/2x10^-5).
