Part 1: This clinic shows how to measure the continuous (RMS) sound level of your Diesel Horn with a Sound Level Meter. There is a proper way to report your Meter results to others.
Part 2: Measuring the true (RMS) voltage driven by a DCC Sound Decoder into a speaker system. Then calculating the wattage of the measured (RMS) voltage based on the impedance of your speaker(s) (4, 8, 16, or 32 ohms)