From what I have read and re-read a dozen times trying to wrap my brain around this issue, I have discovered that DB levels (or claimed DB levels) means nothing as stated by manufacturers.   
The first problem is how the manufacturer has measured for a DB level.  There appears not to be a standardized testing method used; thus stated (advertised) DB levels are not consistent among the various products.  Testing has been found to have been done at varying distances from the source; thus nullifying any meaningful comparison between two products.   
Also, the "note" of the horn(s) can make a difference in how well it is heard by the human ear.  It is also known that a more dissonant sound, typically provided with the use of two horn reeds, is more noticeable than a truer tone/note despite any fact of any difference in actual DB levels (to a degree). 
Personally, I would like a loud horn sounding exactly like car tires screeching to a halt on hot pavement.   I suspect it would get people's attention if needed.