Model railroad type example Sn3.5 is as follows, S (scale) being the prefix to n (narrow gauge) followed by 3.5 (prototype track gauge in feet), 3 and 1/2 feet, or 42 inches, which in Sn3.5 equates to actual modeling track width of .649 - .672 thousandths, 16.5 - 17.1 mm.
Definition of harmonic. 1  1a : overtone; especially : one whose vibration frequency is an integral multiple of that of the fundamentalb : a flutelike tone produced on a stringed instrument by touching a vibrating string at a nodal point.
Resonance is a condition in which a vibrating system responds with maximum amplitude to a periodic driving force.Mechanical systems will have a number of possible frequencies at which this occurs. These are the system's natural frequencies of vibration.t breaks. So, the resonance frequency is the frequency with which some external source must exert a force (swinging legs, sound wave, wind pushing a bridge, etc.) so that each application of the force adds constructively.