The frequency response specifications on these speakers are all complete fantasy. Nearly all of them start to roll off at 900hz, and are down at least 5db by 500hz. At the high end, any response above 10khz is wasted - actually, probably anything above 8khz is wasted. I do look for the lowest resonant frequency I can find - sometimes this will be 450 or 500hz, sometimes it is as high as 850-900hz. The resonant frequency gives you a decent idea of when the response will fall off a cliff, but by the time a speaker is in an enclosure and the sound bounces around the locomotive or tender shell, it's all a moot point anyway. I also look for a decent maximum SPL (90db is good; more is better).
After a couple of years of chasing specifications on these speakers, I concluded that it just doesn't matter. Basically they will all sound pretty much the same after installation as long as you use a proper sealed enclosure with the correct minimum internal volume: 500 cubic mm is my low point; I shoot for 750 - 1000, though. It is not that hard to accomplish: a 6mm-high box (not including the bottom) around a 12 x 8mm speaker gives you an internal volume of 12 x 8 x 6 = 576 and that same box around any larger speaker gets you more internal volume. More generally is better until you pass 1000, and then the benefits of larger internal volume fall off drastically.
John C.
