Train horn's true loudness? anybody know (read on)?

Question

For a train horn kit, I know they differ, but if you measured @ 4 inches away, anybody have concrete data, like a decibel meter, because 175 DB in the ads seems too extreme, but I may be wrong

Answer 1

There has recently been a ruling from the Federal Railroad Administration on that issue.  It is captured in the Code of Federal Regulations, 49CFR229.129(a), which says in part:

Each lead locomotive shall be equipped with a locomotive horn that produces a minimum sound level of 96 dB(A) and a maximum sound level of 110 dB(A) at 100 feet forward of the locomotive in its direction of travel.

So, 175 dB at 4 inches away may not be too far off.

Answer 2

There are a couple problems here.  Certain sound intensity meters have at least three scales.  They can measure actual sound power in decibels or sound power as perceived by the (normal) human ear, etc. which may depend upon frequency.  Sound power diminishes with distance in accordance with the inverse square law from a point source.  Although your horn may approximate a point source at a considerable distance it likely will not at only four inches.

Decibels are a logarithmic ratio of measured sound power to an arbitrarily selected soft sound level and require careful measurements and interpretation of results.  Perhaps the horn has a specified sound level (on a given scale?) at a given distance.